See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/327154136 A review of the wire arc additive manufacturing of metals: properties, defects and quality improvement Article in Journal of Manufacturing Processes · August 2018 DOI: 10.1016/j.jmapro.2018.08.001 CITATIONS READS 364 5,468 1 author: Bintao Wu University of Wollongong 35 PUBLICATIONS 1,101 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: wire arc additive manufacturing of large component View project WAAM of Hastelloy C 276 View project All content following this page was uploaded by Bintao Wu on 14 July 2020. The user has requested enhancement of the downloaded file. Journal of Manufacturing Processes 35 (2018) 127–139 Contents lists available at ScienceDirect Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro A review of the wire arc additive manufacturing of metals: properties, defects and quality improvement T ⁎ Bintao Wua, Zengxi Pana, , Donghong Dingb, Dominic Cuiuria, Huijun Lia, Jing Xuc, John Norrisha a School of Mechanical, Materials, Mechatronic and Biomedical Engineering, University of Wollongong, Wollongong NSW 2522, Australia School of Mechatronic Engineering, Foshan University, Foshan, Guangdong 528000, China c School of Mechanical Engineering, Tsinghua University, Beijing 100084, China b A R T I C LE I N FO A B S T R A C T Keywords: Wire arc additive manufacturing (WAAM) Materials Defects Quality improvement Due to the feasibility of economically producing large-scale metal components with relatively high deposition rates, significant progress has been made in the understanding of the Wire Arc Additive Manufacturing (WAAM) process, as well as the microstructure and mechanical properties of the fabricated components. As WAAM has evolved, a wide range of materials have become associated with the process and its applications. This article reviews the emerging research on WAAM techniques and the commonly used metallic feedstock materials, and also provides a comprehensive over view of the metallurgical and material properties of the deposited parts. Common defects produced in WAAM components using different alloys are described, including deformation, porosity, and cracking. Methods for improving the fabrication quality of the additively manufactured components are discussed, taking into account the requirements of the various alloys. This paper concludes that the wide application of WAAM still presents many challenges, and these may need to be addressed in specific ways for different materials in order to achieve an operational system in an acceptable time frame. The integration of materials and manufacturing process to produce defect-free and structurally-sound deposited parts remains a crucial effort into the future. 1. Introduction In recent years, wire arc additive manufacturing (WAAM) has increasingly attracted attention from the industrial manufacturing sector due to its ability to create large metal components with high deposition rate, low equipment cost, high material utilization, and consequent environmental friendliness. The origin of the WAAM process can be traced back to the 1925s when Baker [1] proposed to use an electric arc as the heat source with filler wires as feedstock materials to deposit metal ornaments. Since then, consistent progress has been made on the development of this technology, particularly in the last 10 years. The WAAM technique bears various nomenclatures by different research institutions worldwide [2–24], as shown in Fig.1. Today, WAAM has become a promising fabrication process for various engineering materials such as titanium, aluminium, nickel alloy and steel. Compared to traditional subtractive manufacturing, the WAAM system can reduce fabrication time by 40–60% and post-machining time by 15–20% depending on the component size [25]. For instance, recent breakthroughs in WAAM technology have made it possible to fabricate aircraft landing gear ribs with a saving of approximately 78% in raw ⁎ material when compared with the traditional subtractive machining process [26]. Due to the highly complex nature of WAAM, many different aspects of the process need to be studied, including process development, material quality and performance, path design and programming, process modelling, process monitoring and online control [9]. Several WAAM review papers have been published by leaders in the field, covering state-of-the-art systems, design, usage, in-situ process monitoring, insitu metrology and in-process control and sensing [26–31]. Nevertheless, there is still a need for a systematic review of the properties of various WAAM-processed materials, the defects associated with different alloys, and a summary of current and future research directions that are aimed at quality improvements for the alloy classes of interest. This paper reviews the microstructure and mechanical properties of various metals, including titanium and its alloys, aluminum and its alloys, Ni-based alloy, steel and other intermetallic materials fabricated by the various WAAM processes. The common defects that have been found to occur for different materials are also summarized. The current methods for both in-process and post-process quality improvement and defect reduction are introduced. Finally, a discussion is given on Corresponding author. E-mail address: [email protected] (Z. Pan). https://doi.org/10.1016/j.jmapro.2018.08.001 Received 23 February 2018; Received in revised form 1 August 2018; Accepted 1 August 2018 Available online 08 August 2018 1526-6125/ © 2018 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved. Journal of Manufacturing Processes 35 (2018) 127–139 B. Wu et al. Fig. 1. The distribution of main WAAM research groups. first design uses an enclosed chamber to provide a good inert gas shielding environment, similar to laser Power-Bed Fusion (PBF) systems. The second design uses existing or specially designed local gas shielding mechanisms, with the robot positioned on a linear rail to increase the overall working envelop. It is capable of fabricating very large metal structures up to several a meters in dimension. Fig. 2 shows an example of this design of WAAM system, used for the research and development at the University of Wollongong (UOW). Fabricating a part using WAAM involves three main steps: process planning, deposition, and post processing. For a given CAD model, 3D slicing and programming software generates the desired robot motions and welding parameters for the deposition process, aimed at producing defect-free fabrication with high geometrical accuracy [22,23,33]. Based on the welding deposition model for the specific material being used to fabricate the component, the 3D slicing and programming software offer automated path planning and process optimization to avoid potential process-induced defects [34–37]. During fabrication, the robot and external axis provide accurate motion for the welding torch to build up the component in a layer-by-layer fashion. Advanced WAAM systems can be equipped with various sensors to measure welding signals [38], deposited bead geometry [39], metal transfer behaviour [40] and interpass temperature [32,41,42], thereby supporting in-process monitoring and control to achieve higher product quality. This is an area of current and future research interest, with the potential for significantly improving WAAM process performance. improving quality of WAAM fabricated parts through process selection, feedstock optimization, process monitoring and control and post-process, including proposals for future research directions. 2. Wire arc additive manufacturing (WAAM) systems 2.1. Classification of WAAM process Depending on the nature of the heat source, there are commonly three types of WAAM processes: Gas Metal Arc Welding (GMAW)-based [32], Gas Tungsten Arc Welding (GTAW)-based [2] and Plasma Arc Welding (PAW)-based [3]. As listed in Table 1, specific class of WAAM techniques exhibit specific features. The deposition rate of GMAWbased WAAM is 2–3 times higher than that of GTAW-based or PAWbased methods. However, the GMAW-based WAAM is less stable and generates more weld fume and spatter due to the electric current acting directly on the feedstock. The choice of WAAM technique directly influences the processing conditions and production rate for a target component. 2.2. Robotic WAAM system Most WAAM systems use an articulated industrial robot as the motion mechanism. Two different system designs are available. The Table 1 Comparison of various WAAM techniques. WAAM Energy source Features GTAW-based GTAW GMAW-based GMAW Non-consumable electrode; Separate wire feed process; Typical deposition rate: 1-2 kg/hour; Wire and torch rotation are needed; Consumable wire electrode; Typical deposition rate 3-4 kg/hour; Poor arc stability, spatter; Reciprocating consumable wire electrode; Typical deposition rate: 2-3 kg/hour; Low heat input process with zero spatter, high process tolerance; Two consumable wires electrodes; Typical deposition: 6-8 kg/hour; Easy mixing to control composition for intermetallic materials manufacturing ; Non-consumable electrode; Separate wire feed process; Typical deposition rate 2-4 kg/hour; Wire and torch rotation are needed; Cold metal transfer (CMT) Tandem GMAW PAW-based Plasma 3. Metals used in WAAM process WAAM processes use commercially available wires which are produced for the welding industry and available in spooled form and in a wide range of alloys as feedstock materials. Table 2 indicates the commonly used alloys and their various applications in WAAM. Manufacture of a structurally sound, defect free, reliable part requires an understanding of the available process options, their underlying physical processes, feedstock materials, process control methods and an appreciation of the causes of the various common defects and their remedies. This section reviews the metals that are commonly used in WAAM, with a particular emphasis on the microstructure and mechanical properties of the additively manufactured alloys. 3.1. Titanium alloys Titanium alloys have been widely studied for application of additive manufacturing in aerospace components due to their high strength-toweight ratio and inherently high material cost. There are increasing 128 Journal of Manufacturing Processes 35 (2018) 127–139 B. Wu et al. Fig. 2. WAAM system design concepts, University of Wollongong. build direction owing to thermal gradient [66], commonly seen in additively manufactured titanium alloy components. Table 3 summarizes the microstructure and mechanical property data (tensile strength, yield strength and elongation) of Ti6Al4V samples fabricated using various WAAM technologies [62,64,67–73]. The as-forged and as-cast minimum specifications from ASTM standards are also listed for comparison. As shown in Fig.4, the tensile property of asfabricated Ti6Al4V samples is close to that of wrought Ti6Al4V and exceeds that of cast Ti6Al4V as specified by ASTM standards. In addition, WAAM fabricated Ti6Al4V samples show anisotropic properties with lower strength and higher elongation values in the build direction (Z) compared to deposition direction (X), which is mainly attributed to the grain size of α lamellae and the orientation of the elongated prior β grains. Table 2 Metals typically used with WAAM process. Applications Aerospace Automotive Marine Corrosion resistance High temperature Tools and molds Alloys Ti-based Al-based Steel-based Ni-based Bimetal [26] – [49,50] [52,53] [56] – [43] [46] – – – – – [47] [51] – – [59,60] [44] – – [54] [57] – [45] [48] – [55] [58] – demands for more efficient and lower cost alternatives to the conventional subtractive manufacturing methods, which suffer very low fly-tobuy ratios for many component designs. There exists many business opportunities for the WAAM process, particularly for large-sized titanium components with complex structures [26]. It is well accepted that the microstructure of the product depends on its thermal history during the fabrication process. The distinctive WAAM thermal cycle, which involves repeated heating and cooling [61,62], produces meta-stable microstructures and inhomogeneous compositions in the fabricated part [63]. For example, Baufeld et al [62] investigated the microstructures of Ti6Al4V fabricated using a GTAW-based WAAM system, and found two distinctive regions on the as-built wall. In the bottom region, where alternating bands are perpendicular to the build direction, a basketwave Widmanstätten structure with α phase lamellae is present, while in the top region, where no such bands appear, needle-like precipitate is the main structure. Similar microstructural evolution has also been observed in the PAW-based process. Lin et al [64,65] reported a graded microstructure along the build direction and identified the martensite α’ structure, Widmanstätten structure and basket-wave structure from the bottom to the top region of the fabricated component, as shown in Fig.3. An epitaxial growth of β grains with discrete direction is also observed along the 3.2. Aluminum alloys and steel Although fabrication trials for many different series of aluminum alloys, including Al-Cu (2xxx) [76], Al-Si (4xxx) [77] and Al-Mg (5xxx) [40] have been successfully carried out, the commercial value of WAAM is mainly justifiable for large and complex thin-walled structures, since cost of manufacturing small and simple aluminum alloy components using conventional machining processes is low [78]. Using WAAM to fabricate steel is unpopular for the same reason although it is the most commonly used engineering material [79]. Another reason for the poor commercial application of WAAM in aluminum is that some series of aluminum alloys, such as Al 7xxx and 6xxx, are challenging to weld due to turbulent melt pool and weld defects, which frequently occur during the deposition process. In general, as-deposited additively manufactured aluminum alloy parts have inferior mechanical properties compared to those machined from billet material. In order to achieve higher tensile strength, most of the as-deposited aluminum parts undergo post-process heat treatment to refine the microstructure. Table 4 lists the yield strength (YS), ultimate tensile strength (UTS), and elongation of WAAM-fabricated 129 Journal of Manufacturing Processes 35 (2018) 127–139 B. Wu et al. Fig. 3. Optical microstructure of the deposited wall (from Lin, et al. [64]): (a) the bottom region; (b) the middle region; (c) the top region. Table 3 Mechanical properties of Ti6Al4V fabricated from various WAAM processes. Process Condition Microstructure YS [MPa] UTS s [MPa] EL [%] Reported by Cast Wrought GTAW / / AF / / Columnar prior β grains + Widmanstätten α/β 758 (min) 860 (min) / ASTM F1108 ASTM F1472 Baufeld et al. [13] AF α phase lamella basket weave structures lamellar structure / 860 930 929 965 939 HT (600 °C/4 h/ FC) HT (834 °C/2 h/ FC) AF Widmanstätten α/β + Columnar β grains / / >8 > 10 9 ± 1.2a 9 ± 1b 16 ± 3a 7.8 ± 2.3b 12.5 ± 2.5a 6 ± 3b 21 ± 2a 14 ± 2b 14.8 a 11.7 b 16.5 ± 2.7a 7.8 ± 2b 11.6 ± 2.4a 6.6 ± 2.6b 20.4 ± 1.8a 13.5 ± 2b / Prior columnar β + Martensite α’ Prior columnar β + martensite α’+ fine basket-weave structure 909 ± 13.6b 877 ± 18.5 b 988 ± 19.2b 968 ± 12.6 b 7 ± 0.5b 11.5 ± 0.5 b / 972 ± 41 lamellar structure / Plasma Pulsed-PAM PAM 977 ± 14 a 931 ± 19 971 ± 28 b Widmanstätten α + banded coarsened lamella α 803 ± 15 a 950 ± 21 / 861 ± 14 / 891 ± 16 918 ± 17 a 1033 ± 19 b a 892 ± 31 b HT (600 °C/2 h/ FC) AN(834 °C/2 h/ FC) AF 600 °C/840 °C AF AF a b b AF (min) (min) ± 41a ± 39b ± 24 a 1033 ± 32 937 ± 21 a 963 ± 22 b a 915 ± 14 976 ± 35 a b 981 ± 8 931 ± 17 a 962 ± 29 b / 856 ± 21 a 893 ± 24 b b b Baufeld et al. [67] Wang et al. [74] Brandl et al. [71] Martina et al. [75] Lin et al. [65] Lin et al. [64] AF: as fabricated, HT: heat treated, AN: annealed. a In build direction. b Orthogonal to build direction. The microstructure of WAAM fabricated Inconel 718 parts generally consists of large columnar grains with interdendritic boundaries delineated by small Laves phase precipitates and MC carbides [81]. Xu et al. [82] reported that columnar dendrite structures decorated with a large amount of Laves phase, MC carbides and Ni3Nb are also present in WAAM-fabricated Inconel 625 parts, as shown in Fig. 5. It is worth noting that the microstructure can be refined to smaller dendritic arm spacing, less niobium segregation and discontinuous Laves phase in the interdendritic regions using post-process heat treatments, which are beneficial to the mechanical properties. Table 5 lists the mechanical properties of several Nickel-based superalloys fabricated using the WAAM process. For GMAW-based WAAM-fabricated Inconel 718 alloy, the yield and ultimate tensile strength is 473 ± 6 MPa and 828 ± 8 MPa respectively. These values lie between the minimum values specified by ASTM for wrought and cast materials, whereas the elongation is much lower than the standards for both wrought and cast conditions. As for WAAM-fabricated Inconel 625 alloy, the YS, UTS and elongation all meet the requirement set by 2219aluminium alloy samples. Due to the uniform distribution of large diamond particles within the microstructure, the sample exhibits lower UTS and YS than that of the wrought part specified by ASTM standard. However, after heat treatment, significant improvement beyond ASTM standard can be observed in both strength and elongation as a consequence of the grain refinement [43]. 3.3. Nickel-based superalloys Nickel-based superalloys are the second most popular material studied by the additive manufacturing research community after titanium alloys, mainly due to their high strengths at elevated temperatures and high fabrication cost using traditional methods. Nickel-based superalloys are widely applied in aerospace, aeronautical, petrochemical, chemical and marine industries due to their outstanding strength and oxidation resistance at temperatures above 550 °C. To date, various Nickel-based superalloys, including Inconel 718 and Inconel 625 alloy have been studied after WAAM processing. 130 Journal of Manufacturing Processes 35 (2018) 127–139 B. Wu et al. Fig. 4. Mechanical property comparison for Ti6Al4V parts fabricated using WAAM: (a) yield and ultimate strength, (b) elongation. example, severe oxidization for titanium alloys, porosity for aluminum alloys, poor surface roughness in steel as well as severe deformation and cracks in bimetal components have been found to typically occur. Table 7 lists the major defects that are commonly present in components fabricated using current WAAM techniques. The details of these common defects and their relationship to the materials will be discussed this section. ASTM for cast materials, and are slightly lower than those for wrought material. 3.4. Other metals Other metals have also been investigated for potential fabrication using WAAM, such as magnesium alloy AZ31 for automotive applications [87], Fe/Al intermetallic compounds [88,89] and Al/Ti [90,91] compounds, as well as bimetallic steel/nickel [92] and steel/bronze [93] parts for the aeronautic industry. The detailed mechanical properties of these materials fabricated using WAAM are listed in Table 6. Most of this research has focused on determining the microstructural and mechanical properties of samples taken from simple straight-walled structures, rather than developing a process to fabricate functional parts. Manufacturing intermetallic parts with accurate pre-designed composition still poses major challenges for the WAAM process. 4.1. Deformation and residual stress Like other additive manufacturing process, distortion and residual stress are inherent to the WAAM process and it is impossible to completely avoid its generation. The residual stress can lead to distortion of the part, loss of geometric tolerance, delamination of layers during deposition, as well as deterioration of fatigue performance and fracture resistance of the additively manufactured components. Therefore, control and minimization of deformation and residual stress is a key area if research. Various types of deformation appear in WAAM fabricated parts, including longitudinal and transvers shrinkage, bending distortion, angular distortion and rotational distortion [103]. The distortions are caused by thermal expansion and shrinkage of the part during repeated melting and cooling processes, which is particularly an issue for large thin walled structures [104]. Residual stress is the stress that remains in the material when all external loading forces are removed. If the residual stress is sufficiently high, it can be a critical influential factor in the mechanical properties and fatigue performance of the fabricated component. If the residual stress exceeds the local UTS of the material, cracking will take place, while if it is higher than the local YS but lower than UTS, warping or plastic deformation will occur [105]. Ding et al. [32] found that the residual stress uniformly distributes across the 4. Common defects in WAAM-fabricated component Although the mechanical properties of components fabricated by WAAM are in many cases comparable to those of their conventionally processed counterparts, there are however some AM processing defects that must be addressed for critical applications. Porosity, high residual stress levels, and cracking, must be avoided, particularly for parts exposed to extreme environments where these defects lead to failure modes such as high temperature fatigue. Defects in WAAM can occur for various reasons, such as poor programming strategy, unstable weld pool dynamics due to poor parameter setup, thermal deformation associated with heat accumulation [95], environmental influence (such as gas contamination) and other machine malfunctions. As shown in Fig. 6, certain materials tend to be vulnerable to specific defects. For Table 4 Tensile properties of WAAM-fabricated aluminum alloy (2xxx). Materials Process Condition Microstructure YS[MPa] UTS [MPa] EL [%] Reported by Al6.3Cu Wrought(2219) CMT T851 AF / Fine dendrites + equiaxed grains Homogeneous dispersed θ precipitates 390 262 264 458 466 > 4 15.8 ± 0.3a 18.6 ± 1.5b 13.6 ± 0.9a 14b ASTMB211M [80] Gu et al. [43] HT(T6) 267 128 133 305 333 AF: as fabricated, HT: heat treated. a In build direction. b Orthogonal to build direction. 131 ± ± ± ± 2a 5b 6a 6b ± ± ± ± 4a 2b 3a 3b Journal of Manufacturing Processes 35 (2018) 127–139 B. Wu et al. Fig. 5. The major phases appearing in the as-deposited microstructure of Inconel 625 alloy (from Xu et al. [83]). 4.2. Porosity WAAM deposited wall, and the residual stress in the preceding layer has little effect on the future layers. After release of clamping, however, the internal stress is redistributed with a much lower value at the top of integral part than at the interface to the substrate, resulting in bending distortion of the component. Path planning also involves the distortion and residual stress evolution in WAAM process [100]. If appropriate deposition path designs, it will help in the significant improvement in these defects, especially in large metal fabrication. A detailed overview of the residual stress origin would exceed the scope of this article. Among all WAAM engineering materials, bimetal components exhibit high levels of residual stress and deformation due to the material thermal expansion difference. Hence, accurate interpass temperature control is needed when bimetal materials are used. WAAM-fabricated Inconel alloy has comparatively lower residual stresses levels, but it is more susceptible to process defects such as delamination, buckling and warping, since its residual stress is usually higher than the yield stress [106]. Other comparatively softer materials, such as aluminum alloys, easily respond to deformation defects due to their high thermal expansion coefficients. A better understanding pf the effect of material characteristics in WAAM processing is needed for controlling residual stress and deformation during deposition. Deformation and residual stress are associated with many process parameters, such as welding current, welding voltage, feeding speed, ambient temperature, shielding gas flow rate, etc. There is still a lack of systematic methods for controlling defects through optimized selection or manipulation of these parameters. Fortunately, several post-process treatments that have been proven to effectively mitigate residual stress and deformation, and these will be discussed in Section 5. Porosity is another common defect in WAAM processing that needs to be minimized due to adverse effects on mechanical properties [107]. Firstly, porosity will lead to a component with low mechanical strength by damage from micro-cracks, and secondly, it usually brings low fatigue property to deposition via spatially with different size and shape distribution. In general, this type of defects are mainly classified as either raw material-induced [108] or process-induced [109]. The WAAM raw material, including as-received wire and substrate, often has a degree of surface contamination, such as moisture, grease and other hydrocarbon compounds that may be difficult to completely remove. These contaminants can be easily absorbed into the molten pool and subsequently generate porosity after solidification. Among common engineering materials, aluminum alloy is the most susceptible to this defect as the solubility of hydrogen in solid and liquid (0.036 against 0.69 cm3/100 g at a melting point of 660℃, respectively) is significantly different [110]. Even small amounts of dissolved hydrogen in the liquid state may exceed the limit of solubility after solidification, resulting in porosity [111]. Therefore, the cleanliness of raw materials is critical, especially for aluminum alloys. Process-induced porosity is usually non-spherical, and mainly caused by poor path planning or an unstable deposition process. When the deposition path is complex or the manufacturing process is changeable, insufficient fusion or spatter ejection is easily produced, creating gaps or voids in these influenced regions. To control porosity, the following methods can be adopted: (1) an AC GMAW-based process or CMT-PADV based process (cold metal transfer pulsed advanced, a controlled short-circuiting GMAW transfer process) is preferred, especially for aluminum; (2) the highest quality Table 5 Mechanical properties of various Ni-based superalloys using different WAAM processes. Materials Process Condition Microstructure YS [MPa] UTS [MPa] EL [%] Reported by IN 718 IN 625 GMAW Cast Wrought PPAM AF / / AF AF IC 473 ± 6 350 490 438 ± 38a 423 ± 22b 449 480 828 ± 8 710 855 721 ± 32a 718 ± 19b 726 771 28 ± 2 48 50 48.6 a 49.2 b 43 50 Baufeld. [81] ASM [83] ASM [83] Xu et al. [82] PPAW PPAW GTAW HT(980 °C /STA) 469 802 42 Wang et al. [85] CMT AF Nb precipitates + dendritic structure / / Laves phase + columnar dendrite structure Laves phase + MC carbides + δ-Ni3Nb Laves phase + NbC carbides Coarser Laves particles + Nb precipitates Nb,Mo precipitates + dendrite structure / 684 ± 23 722 ± 17 AF: as fabricated, HT: heat treated, IC: interpass cooling. a In build direction. b Orthogonal to build direction. 132 a b 40.13 ± 3.7 42.27 ± 2.4 Xu et al. [84] Xu et al. [84] a b Ding et al. [86] Journal of Manufacturing Processes 35 (2018) 127–139 B. Wu et al. Table 6 Mechanical properties of other metallic materials fabricated using WAAM process. Materials Ti/Al Process GTAW Condition Microstructure AF Interdendritic γ structures + fully lamellar colonies HT(1200 °C/24 h) Full γ microstructure 425 ± 15 471 ± 14 HT(1060 °C/24 h) Fully lamellar structure copper twinning Columnar Fe3Al grains austenite, δ-ferrite in low part; dendritic in up part the mixture of α-Fe and ε-Cu 487 ± 14a 486 ± 12a 63 ± 2.1a 847 ± 2b / 231 ± 2.5a 944 ± 19 b 565 ± 15 a 0.35 0.5b 1.1a 1.1b 0.30a 0.45b 63 ± 4.0a 3.2 ± 0.1 b / / 305a 48.5 Cu/Al Fe/Al Fe/Ni GTAW GTAW GMAW AF AF AF Fe/Cu GMAW AF YS[MPa] UTS[MPa] EL[%] a 424 ± 30 474 ± 17 b a b 488 ± 50 549 ± 23 a b a b 412 ± 11 472 ± 11 569 ± 12a 590 ± 4b a Reported by Ma et al. [24] Dong et al. [94] Shen et al. [88] Takeyuki et al. [92] Liu et al. [93] AF: as fabricated, HT: heat treated. a In build direction. b Orthogonal to build direction. solid between layers. Generally, this deficiency is visible and cannot be repaired by post-process treatment. In order to prevent this defect, preprocess treatment such as preheating of the substrate needs to be considered. Bimetal material combinations, such as Al/Cu, Al/Ti and Al/Fe, are quite susceptible to cracking and delamination when fabricated with the WAAM process. The dissimilar metals have large differences in their mutual solubility and chemical reactivity so that the intermetallic phase-equilibrium is freely broken, thus inducing crack growth along grain boundaries. Also, Inconel alloy readily generates solidification cracking issues due to the existence of liquid film at terminal solidification [101]. Both of these material types should receive particular attention to avoid cracking and delamination. To control crack defects, corresponding measures can be taken as follows: (1) Mixed wires and optimization of their compositions; (2) Decrease the cooling rate during the deposition process (3) Other measures to improve strength rather than solution treatment. shielding gas, tight gas seals, non-organic piping and short pipe lengths are highly recommended; (3) the wire and substrate surfaces are as clean as possible before fabrication; (4) high quality feedstock should be used; (5) the deposited bead shape needs to be optimized; (6) the thermal profile during processing should be monitored and controlled; (7) post-deposition treatment, such as interpass rolling can be applied. 4.3. Crack and delamination Similar to residual stress and deformation, cracking and delamination not only involves the thermal signature of the manufacturing process, but also relates to the material characteristics of the deposit. Ordinarily, the crack is categorised as either a solidification crack or grain boundary crack within the WAAM component [105]. The former type of crack depends mainly on the solidification nature of the material and is usually caused by the obstruction of solidified grain flow or high strain in the melt pool [112]. Grain boundary cracking often generates along the grain boundaries due to the differences between boundary morphology and potential precipitate formation or dissolution [113]. Delamination or separation of adjacent layers takes place due to incomplete melting or insufficient re-melting of the underlying 5. Current methods for quality improvement in the WAAM process Generally, WAAM parts require post-process treatment to improve Fig. 6. The correlation between materials and defects in WAAM processes. 133 Journal of Manufacturing Processes 35 (2018) 127–139 B. Wu et al. Table 7 Tendency of various defects in WAAM fabricated parts. Material Ti6Al4V H08Mn2Si steel Copper-coated steel ER4043 Al alloy AA2319 Al alloy 5356 Al alloy Inconel 625 Inconel 718 AZ31 Mg alloy Nickel-Al-Cu Steel–bronze bimetal Steel- nickel bimetal Intermetallic Fe/Al Intermetallic Al/Ti Intermetallic Al/Cu Process TIG Plasma CMT DCEP-GMAW DE-GMAW GMAW CMT VP-GTAW CMT CMT-PADV VP-GTAW PPAD GTAW GMAW PMIG CMT GMAW GMAW GTAW GTAW GTAW Defect or feature Ref. Porosity Cracking Delamination Oxidation Substrate adherence Surface finish No No No No low No High No High No No High No Medium No No No No High Low No No No No No No No No No No No Yes Yes No Yes No No No No Yes Yes No No No No No No No No No No No No No No Yes No No No No No No Yes Light No Light Light No Light Light No No No No No No No Light No No No Serious No Light Good Good Good Medium Good Good Good Good Good Good Good Good Good Good Medium Good Good Good Medium Good Poor Smooth Smooth Smooth Poor Waviness Medium-rough Smooth Medium-rough Smooth Smooth Smooth Smooth Smooth Smooth Medium-rough Smooth Smooth Medium-rough Medium-poor Rough Rough [13] [75] [96] [96] [18] [97] [77] [98] [99] [99] [100] [82] [101] [14] [87] [86] [93] [92] [89] [24] [102] Fig. 7. Schematic diagram of WAAM with cold rolling process [104]. respectively. In addition, post-process heat treatment plays an important role in grain refinement, especially for WAAM-fabricated aluminum and Inconel alloy [67]. The decision to use post-process heat treatment depends on the material alloying system and also the pre-heat treatment state. Generally, high carbon content materials must be heat treated, while a few materials can be damaged by this technique. Therefore, the utilization of post heat treatment process to WAAM component needs to consider the specific material and its application. material properties, reduce surface roughness and porosity, and remove residual stress and distortions. By appropriate application of post process, the majority of issues that influence deposition quality can be mitigated or eliminated. Presently, several post-process treatment technologies have been reported to improve part quality in the WAAM process. This section will review these post-process techniques, both their features and limitations. 5.1. Post-process heat treatment 5.2. Interpass cold rolling Post-process heat treatment is widely used in WAAM to reduce residual stress, enhance material strength and as a method of hardness control. The selection of a suitable heat treatment process depends on the target material, additive manufacturing methods, working temperature and heat treatment conditions. If the heat treatment state is set incorrectly, the probability of cracking will increase under mechanical loading, as the combination of existing residual stress with load stress exceeds the material’s design limitation. As summarized Tables 3–5, after heat treatment, the mechanical strength of WAAM-fabricated parts improved significantly, with increase of 4%, 78%, 5% and 17% being reported for titanium alloy, aluminum alloy, Nickel-based superalloys and intermetallic Ti/Al alloy, Rolling of the weld bead between each deposited layer has been shown to reduce residual stresses and distortion [114].Interpass cold rolling not only lowers residual stress, but also brings more homogeneous material properties. In the WAAM process, the thermal gradient with deposition layers and alternate re-heating and re-cooling process result in the target part having anisotropic microstructural evolution and mechanical properties. The cold rolling technique significantly reduces microstructural anisotropy through plastically deforming the deposition. Fig.7 shows the schematic diagram of an interpass cold rolling system developed at Cranfield University. A slotted 134 Journal of Manufacturing Processes 35 (2018) 127–139 B. Wu et al. Table 8 Material properties of component fabricated using WAAM with interpass cold rolling. Materials Condition Microstructure YS[MPa] UTS[MPa] EL[%] Hardness [HV] Ref. Ti6Al4V AF Widmanstätten α + Martensite 22.8 ± 0.2a 13.8 ± 1.2b 14.2 ± 1.3a 10.0 ± 0.5b 13.0 ± 1.0a 13.1 ± 1.8b 15.5 13 10.3 7.3 54 23 6 3 2 15.8 ± 0.3a 18.6 ± 1.5b 14.6 ± 0.4a 15 ± 1.4b 12 ± 0.6a 13.2 ± 0.2b 7.4 ± 0.6a 8.6 ± 0.4b [115,117,120,122] Grain size decrease from average 200 μm to 100μm 903 ± 3a 945 ± 6b 1022 ± 3a 1006 ± 5b 1078 ± 3a 1077 ± 18b 260 274 288 304 697 1034 1321 1062 1925 262 ± 4a 264 ± 2b 270 ± 7a 271 ± 9b 286 ± 3a 293 ± 6b 312 ± 8a 323 ± 9b / RL:50 807 ± 2a 853 ± 10b 916 ± 22a 911 ± 11b 993 ± 31a 1028 ± 5b 132 145 190 244 292 750 1239 1529 1900 128 ± 2a 133 ± 5b 142 ± 4a 146 ± 3b 188 ± 4a 195 ± 2b 241 ± 2a 249 ± 1b RL:75 Al 2319 AISI 301 L N Al-6.3Cu AF RL:15 RL:30 RL:45 AF CR:20 CR:40 CR:60 CR:80 AF RL:15 / Austenite phase transformed to α'-rnartensite + deformed austenite Fine dendrites + equiaxed grains With the increase load, the grain size is decreased with freeform deformation RL:30 RL:45 / / / / / / 220 345 456 522 610 68.3 [121] [116] [43] 77.4 90.1 102.3 AF: as-fabricated; RL: Rolling loads (KN); CR: Cold reduction (%). a In build direction. b Orthogonal to build direction. 5.3. Interpass cooling roller is used to refine the microstructure and enhance tensile strength in the longitudinal direction by supporting external force [115,116]. As shown in Table 8, both ultimate tensile strength and yield strength in the build direction were improved through interpass cold rolling, which contributes to more homogeneous material properties in the target component. Interpass cold rolling also can play a critical role in the healing of hydrogen porosity in WAAM-fabricated aluminum parts. Generally, high dislocation density is produced by the rolling process. These dislocation can act as preferential sites for atomic hydrogen absorption [117] and as well as ‘pipes’ for the hydrogen, allowing to diffuse to the surface [118]. Table 9 summarizes the outcomes documented in the literature, in terms of the pore incidence and size distribution in aluminum components fabricated using WAAM with interpass cold rolling. The porosities existing in as-fabricated component can be reduced or even eliminated when interpass cold rolling is applied [75]. This technique is only feasible for simple deposited parts, such as straight walls, due to the geometrical limitation of the rolling process. For more complex components with curves and corners, special flexible tooling need to be developed to achieve an effective rolling process, thus limiting the range of industrial application. Cold rolling technique will also reduce residual stress, but the ability to reduce overall part distortion is yet to be proven [26,119]. Recently, interpass cooling has been developed and evaluated at the University of Wollongong. Fig. 8 presents the schematic diagram of a WAAM system with interpass cooling. The moveable gas nozzle, which supplies argon, nitrogen or CO2 gas, is used to provide active, or forced, cooling on the fabricated part during and/or after deposition of each layer. Using such rapid cooling, the in-situ layer temperature and heat cycle can be controlled within a range to obtain the desired microstructure and mechanical properties. This process may also potentially reduce residual stress and distortion, although this aspect has not been investigated. An initial feasibility study shows promising results when using forced interpass cooling with compressed CO2 to fabricate Ti6Al4V thin-walled structures, as shown in Fig. 9 [122]. It was found that interpass cooling produces less surface oxidation, refined microstructure, improved hardness and enhanced strength. In addition, manufacturing efficiency is significantly improved due to the reduction of dwell time between deposited layers. More detailed research findings will be presented in future. Table 9 The distribution of porosity in aluminum part using WAAM with cold rolling. Materials Condition Layers Height (mm) Deformation (%) Number of pores (in area of 120 mm2) Pores diameter (mm) Reported by Al 2319 AF RL:15 RL:30 RL:45 AF RL:15 RL:30 RL:45 21 25 30 38 30 35 45 55 49.4 50.6 49.4 49.8 49.5 49.6 51 49.5 / 13.9 30.0 44.2 / 14.1 31.3 45.4 614 192 5 Elimination 454 336 11 Elimination 13.5 12.5 8.8 Elimination 25.1 13 9.6 Elimination Gu et al. [76] Al 5087 AF: as-fabricated; RL: Rolling loads (KN). 135 Journal of Manufacturing Processes 35 (2018) 127–139 B. Wu et al. Fig. 8. Schematic diagram of the combined WAAM gas cooling process. methods. An in-depth understanding of various materials, ideal process setup, in-process parameter control and post processing is essential for achieving such a goal. After a systematic review and analysis, a qualitybased framework aiming to achieve high-quality and defects-free WAAM process is proposed, as shown in Fig. 10. Three main aspects are considered: feedstock optimization, manufacturing process, and postprocess treatment. Selection of the most suitable welding WAAM process for the deposition material can ensure manufacturing process stability and contribute to reduction of defects. For example, if the CMT-PADV process is used for producing aluminum parts, porosity defects can be dramatically reduced when compared to other GMAW modes [98]. Moreover, integrated and reliable process monitoring and control systems are needed to maintain the stability of the process and ensure the quality of production. Usually, the bead geometry, interpass temperature, arc characteristics and metal transfer behaviour are included in process monitoring and control. Controlling the interpass temperature within a reasonable range is beneficial to microstructural evolution and the resulting mechanical properties. Further, regulating the arc characteristics and metal transfer behaviour in real time is helpful to process stability and avoidance of defects. Based on the process monitoring data that has been collected during deposition, one (or more) of several postprocess treatments can be selected to mitigate defects and improve mechanical performance. Considering the material characteristics, microstructural evolution and mechanical properties can also be optimized through new feedstock composition design. It is well known that different alloying elements have specific effects on material properties. By referring to the phase 5.4. Peening and ultrasonic impact treatment Peening and ultrasonic impact treatments (UIT) have been used in welding applications to reduce local residual stress and improve weld mechanical properties. Both techniques are cold mechanical treatments that impact the weld surface using high energy media to release tensile stress by imposing compressive stress at the treatment surface. Usually, the mechanical peening process produces compressive stresses at a limited depth below the component surface, such as around 1–2 mm in carbon steels [123]. Ultrasonic impact treatment produces grain refinement and randomizes orientation, thus contributing to mechanical strength improvement. It is reported that after ultrasonic impact treatment, the surface residual stress of WAAM-fabricated Ti6Al4V part can be reduced to 58% and the microhardness can be increased by 28% compared to the as-fabricated sample. Also, the surface-modified layers undergo plastic deformation with significant grain refinement and dense dislocations [124]. The ultrasonic impact treatment is limited by penetration depth, which is up to 60 μm below surface. Therefore, although both techniques are good post-process treatments, they have negligible effect on internal residual stresses of large metal part fabricated using WAAM. 6. Discussion Improving process stability, eliminating or decreasing deposition defects and producing components with high quality and mechanical performance have become major research focuses in making the WAAM process more competitive against other additive manufacturing Fig. 9. Mechanical properties of Ti6Al4V part produced by WAAM with interpass cooling using CO2 gas: (a) Hardness; (b) Tensile strength and elongation. 136 Journal of Manufacturing Processes 35 (2018) 127–139 B. Wu et al. Fig. 10. A quality-based framework for the WAAM process. chemical metallurgical mechanisms in WAAM process to provide a guidance for the process optimization, improvement and control. The defects generated in WAAM-produced part are closely related to the target material characteristics and process parameters. The development of strategies or ancillary process to overcome defects generation are of prime importance. With the requirement of high quality WAAM part, the proposed quality-based framework will see a wide application in the future years. As WAAM matures as a commercial manufacturing process, development of a commercially available WAAM system for metal components is an interdisciplinary challenge, which integrates physical welding process development, materials science and thermo-mechanical engineering, and mechatronic and control system design. Although much research has been carried out over recent years in various areas, such as process planning, programming, and material study, a general purpose off-the-shelf WAAM system similar to commercially available powder-bed fusion systems is yet to be developed. Due to the variety of requirements of different engineering materials and the varying scale of fabrication, many different WAAM system designs are expected to be developed that will be optimized for particular applications, rather than a single system that is capable of addressing all of the possible problems. diagram, the desired deposition microstructures can be obtained via adding specific alloying elements in the feedstock and subsequent mechanical properties improved. Moreover, availability of wire mixings provides a potential possibility for building large functionally graded products for special applications. For example, twin-wire GTAW-based WAAM has been successfully developed to produce intermetallic graded materials [24,89]. The development of new powder cored wires, will offer exciting opportunities for fabricating target components with accurate metal composition. In summary, using new welding consumables brings a cost effective solution, which supports high deposition quality through obtaining the desired microstructures, lowering manufacturing costs by reducing or eliminating pre-weld cleaning and re-work, and providing safer working environments by reducing weld fumes. Another essential part of WAAM processing for most materials is post-process treatment, which is used to reduce residual stresses and distortion, refine microstructures, improve microhardness and enhance material strength. However, post-process technologies have their own limitations, for instance, peening and ultrasonic impact treatment only improve material property and reduce defects near the part surface, while extended heat treatment of certain materials promotes grain growth rather than grain refinement. Currently, most WAAM fabricated parts need to be post-process treated with a selective combination of technologies to reduce the defects and improve the product quality to the greatest extent possible. Acknowledgements This research was carried out at the Welding Engineering Research Group, University of Wollongong. The authors would like to acknowledge the National Natural Science Foundation of China for their financial support (Grant No. 51501034). 7. Conclusions A detailed review of recent technological developments in WAAM process has been presented, with a focus on microstructure, mechanical properties, process defects and post-process treatment. Through matching a knowledge of material characteristics with the performance features of particular WAAM techniques, a quality-based framework is proposed, for producing high-quality and defect-free components. In WAAM of metallic materials, the fundamental interrelationships between material composition and microstructure govern the material properties and fabrication quality. Since the WAAM process is an inherently non-equilibrium thermal process, it is challenging to predicate and control the microstructural evolution, which is responsible for the variation of mechanical properties in the deposited part. Further research attention should be paid on the study of underlying physical and References [1] Ralph B. Method of making decorative articles, in. Google Patents 1925. [2] Dickens P, Pridham M, Cobb R, Gibson I, Dixon G. Rapid prototyping using 3-D welding, in. DTIC Document 1992. [3] Spencer J, Dickens P, Wykes C. Rapid prototyping of metal parts by three-dimensional welding. Proc Inst Mech Eng Part B J Eng Manuf 1998;212:175–82. [4] Kovacevic R, Beardsley H. Process control of 3D welding as a droplet-based Rapid prototyping technique. Proc. Ofthe SFF Symposium, Univ. of Texas at Austin 1998:57–64. [5] Dwivedi R, Kovacevic R. Automated torch path planning using polygon subdivision for solid freeform fabrication based on welding. J Manuf Syst 2004;23:278–91. 137 Journal of Manufacturing Processes 35 (2018) 127–139 B. Wu et al. [38] Sequeira Almeida P. Process control and development in wire and arc additive manufacturing. Cranfield University; 2012. [39] Xiong J, Yin Z, Zhang W. Closed-loop control of variable layer width for thinwalled parts in wire and arc additive manufacturing. J Mater Process Technol 2016;233:100–6. [40] Geng H, Li J, Xiong J, Lin X, Zhang F. Optimization of wire feed for GTAW based additive manufacturing. J Mater Process Technol 2017;243:40–7. [41] Zhang S, Li J, Kou H, Yang J, Yang G, Wang J. Effects of thermal history on the microstructure evolution of Ti-6Al-4V during solidification. J Mater Process Technol 2016;227:281–7. [42] Denlinger ER, Heigel JC, Michaleris P, Palmer TA. Effect of inter-layer dwell time on distortion and residual stress in additive manufacturing of titanium and nickel alloys. J Mater Process Technol 2015;215:123–31. [43] Gu J, Ding J, Williams SW, Gu H, Bai J, Zhai Y, et al. The strengthening effect of inter-layer cold working and post-deposition heat treatment on the additively manufactured Al–6.3Cu alloy. Mater Sci Eng A 2016;651:18–26. [44] Uriondo A, Esperon-Miguez M, Perinpanayagam S. The present and future of additive manufacturing in the aerospace sector: a review of important aspects. Proc Inst Mech Eng G J Aerosp Eng 2015;229:2132–47. [45] Sing SL, An J, Yeong WY, Wiria FE. Laser and electron‐beam powder‐bed additive manufacturing of metallic implants: a review on processes, materials and designs. J Orthop Res 2016;34:369–85. [46] Murr LE, Gaytan S, Ceylan A, Martinez E, Martinez J, Hernandez D, et al. Characterization of titanium aluminide alloy components fabricated by additive manufacturing using electron beam melting. Acta Mater 2010;58:1887–94. [47] Guo N, Leu MC. Additive manufacturing: technology, applications and research needs. Front Mech Eng 2013;8:215–43. [48] Kainer KU. Metal matrix composites: custom-made materials for automotive and aerospace engineering. John Wiley & Sons; 2006. [49] Leyens C, Peters M. Titanium and titanium alloys: fundamentals and applications. John Wiley & Sons; 2003. [50] Kim TB, Yue S, Zhang Z, Jones E, Jones JR, Lee PD. Additive manufactured porous titanium structures: through-process quantification of pore and strut networks. J Mater Process Technol 2014;214:2706–15. [51] Wang R, Beck FH. New stainless steel without nickel or chromium for marine applications. Met. Prog. 1983;123:72. [52] Aziz-Kerrzo M, Conroy KG, Fenelon AM, Farrell ST, Breslin CB. Electrochemical studies on the stability and corrosion resistance of titanium-based implant materials. Biomaterials 2001;22:1531–9. [53] Wu B, Pan Z, Li S, Cuiuri D, Ding D, Li H. The anisotropic corrosion behaviour of wire arc additive manufactured Ti-6Al-4V alloy in 3.5% NaCl solution. Corros Sci 2018;137:176–83. [54] Lu G, Zangari G. Corrosion resistance of ternary Ni P based alloys in sulfuric acid solutions. Electrochim Acta 2002;47:2969–79. [55] Stoloff N, Liu C, Deevi S. Emerging applications of intermetallics. Intermetallics 2000;8:1313–20. [56] Varghese OK, Gong D, Paulose M, Grimes CA, Dickey EC. Crystallization and hightemperature structural stability of titanium oxide nanotube arrays. J Mater Res 2003;18:156–65. [57] Bewlay B, Jackson M, Subramanian P, Zhao J-C. A review of very-high-temperature Nb-silicide-based composites. Metall Mater Trans A 2003;34:2043–52. [58] Jackson M, Bewlay B, Rowe R, Skelly D, Lipsitt H. High-temperature refractory metal-intermetallic composites. JOM 1996;48:39–44. [59] Bissacco G, Hansen HN, De Chiffre L. Micromilling of hardened tool steel for mould making applications. J Mater Process Technol 2005;167:201–7. [60] Yan L. Wire and arc addictive manufacture (WAAM) reusable tooling investigation. 2013. [61] Thijs L, Verhaeghe F, Craeghs T, Humbeeck JV, Kruth J-P. A study of the microstructural evolution during selective laser melting of Ti–6Al–4V. Acta Mater 2010;58:3303–12. [62] Baufeld B, Biest Ovd, Gault R. Microstructure of Ti-6Al-4V specimens produced by shaped metal deposition. Int J Mater Res 2009;100:1536–42. [63] Herzog D, Seyda V, Wycisk E, Emmelmann C. Additive manufacturing of metals. Acta Mater 2016;117:371–92. [64] Lin J, Lv Y, Liu Y, Sun Z, Wang K, Li Z, et al. Microstructural evolution and mechanical property of Ti-6Al-4V wall deposited by continuous plasma arc additive manufacturing without post heat treatment. J Mech Behav Biomed Mater 2017;69:19–29. [65] Lin JJ, Lv YH, Liu YX, Xu BS, Sun Z, Li ZG, et al. Microstructural evolution and mechanical properties of Ti-6Al-4V wall deposited by pulsed plasma arc additive manufacturing. Mater Des 2016;102:30–40. [66] Hirata Y. Pulsed arc welding. Weld Int 2003;17:98–115. [67] Baufeld B, Brandl E, Van der Biest O. Wire based additive layer manufacturing: comparison of microstructure and mechanical properties of Ti–6Al–4V components fabricated by laser-beam deposition and shaped metal deposition. J Mater Process Technol 2011;211:1146–58. [68] Wang F, Williams SW, Rush M. Morphology investigation on direct current pulsed gas tungsten arc welded additive layer manufactured Ti6Al4V alloy. 2011. [69] Brandl E, Greitemeier D. Microstructure of additive layer manufactured Ti–6Al–4V after exceptional post heat treatments. Mater Lett 2012;81:84–7. [70] Szost BA, Terzi S, Martina F, Boisselier D, Prytuliak A, Pirling T, et al. A comparative study of additive manufacturing techniques: residual stress and microstructural analysis of CLAD and WAAM printed Ti–6Al–4V components. Mater Des 2016;89:559–67. [71] Brandl E, Baufeld B, Leyens C, Gault R. Additive manufactured Ti-6Al-4V using welding wire: comparison of laser and arc beam deposition and evaluation with [6] Jandric Z, Labudovic M, Kovacevic R. Effect of heat sink on microstructure of three-dimensional parts built by welding-based deposition. Int J Mach Tools Manuf 2004;44:785–96. [7] Song YA, Park S, Choi D, Jee H. 3D welding and milling: part I–a direct approach for freeform fabrication of metallic prototypes. Int J Mach Tools Manuf 2005;45:1057–62. [8] Song YA, Park S, Chae S-W. 3D welding and milling: part II—optimization of the 3D welding process using an experimental design approach. Int J Mach Tools Manuf 2005;45:1063–9. [9] Zhang Y, Chen Y, Li P, Male AT. Weld deposition-based rapid prototyping: a preliminary study. J Mater Process Technol 2003;135:347–57. [10] Zhang YM, Li P, Chen Y, Male AT. Automated system for welding-based rapid prototyping. Mechatronics 2002;12:37–53. [11] Kwak YM, Doumanidis CC. Geometry regulation of material deposition in near-net shape manufacturing by thermally scanned welding. J Manuf Process 2002;4:28–41. [12] Katou M, Oh J, Miyamoto Y, Matsuura K, Kudoh M. Freeform fabrication of titanium metal and intermetallic alloys by three-dimensional micro welding. Mater Des 2007;28:2093–8. [13] Baufeld B, Biest OVd, Gault R. Additive manufacturing of Ti–6Al–4V components by shaped metal deposition: microstructure and mechanical properties. Mater Des 2010;31(Suppl 1):S106–11. [14] Clark D, Bache MR, Whittaker MT. Shaped metal deposition of a nickel alloy for aero engine applications. J Mater Process Technol 2008;203:439–48. [15] Merz R, Prinz F, Ramaswami K, Terk M, Weiss L. Shape deposition manufacturing, engineering design research center. Carnegie Mellon Univ. 1994. [16] Xiong J, Zhang G, Gao H, Wu L. Modeling of bead section profile and overlapping beads with experimental validation for robotic GMAW-based rapid manufacturing. Robot Comput Integr Manuf 2013;29:417–23. [17] Xiong J, Zhang G, Qiu Z, Li Y. Vision-sensing and bead width control of a singlebead multi-layer part: material and energy savings in GMAW-based rapid manufacturing. J Clean Prod 2013;41:82–8. [18] Yang D, He C, Zhang G. Forming characteristics of thin-wall steel parts by double electrode GMAW based additive manufacturing. J Mater Process Technol 2016;227:153–60. [19] Aiyiti W, Zhao W, Lu B, Tang Y. Investigation of the overlapping parameters of MPAW-based rapid prototyping. Rapid Prototyp J 2006;12:165–72. [20] Suryakumar S, Karunakaran KP, Bernard A, Chandrasekhar U, Raghavender N, Sharma D. Weld bead modeling and process optimization in Hybrid Layered Manufacturing. Comput Des 2011;43:331–44. [21] Ding D, Pan Z, Cuiuri D, Li H. A tool-path generation strategy for wire and arc additive manufacturing. Int J Adv Manuf Technol 2014;73:173–83. [22] Ding D, Pan Z, Cuiuri D, Li H. A practical path planning methodology for wire and arc additive manufacturing of thin-walled structures. Robot Comput Integr Manuf 2015;34:8–19. [23] Ding D, Pan Z, Cuiuri D, Li H, Larkin N. Adaptive path planning for wire-feed additive manufacturing using medial axis transformation. J Clean Prod 2016;133:942–52. [24] Ma Y, Cuiuri D, Li H, Pan Z, Shen C. The effect of postproduction heat treatment on γ-TiAl alloys produced by the GTAW-based additive manufacturing process. Mater Sci Eng A 2016;657:86–95. [25] Reduce costs and increase output with robotic welding, in, https://www.scottautomation.com/applications/metal-fabrication/welding/. [26] Williams SW, Martina F, Addison AC, Ding J, Pardal G, Colegrove P. Wire + arc additive manufacturing. Mater Sci Technol 2016;32:641–7. [27] Ding D, Pan Z, Cuiuri D, Li H. Wire-feed additive manufacturing of metal components: technologies, developments and future interests. Int J Adv Manuf Technol 2015;81:465–81. [28] Levy GN, Schindel R, Kruth JP. Rapid manufacturing and rapid tooling with layer manufacturing (LM) technologies, state of the art and tuture perspectives. CIRP Ann Manuf Technol 2003;52:589–609. [29] Frazier WE. Metal additive manufacturing: a review. J Mater Eng Perform 2014;23:1917–28. [30] Lewandowski JJ, Seifi M. Metal additive manufacturing: a review of mechanical properties. Annu Rev Mater Res 2016;46:151–86. [31] Gao W, Zhang Y, Ramanujan D, Ramani K, Chen Y, Williams CB, et al. The status, challenges, and future of additive manufacturing in engineering. Comput Des 2015;69:65–89. [32] Ding J, Colegrove P, Mehnen J, Ganguly S, Sequeira Almeida PM, et al. Thermomechanical analysis of Wire and Arc Additive Layer Manufacturing process on large multi-layer parts. Comput Mater Sci 2011;50:3315–22. [33] Ding D, Pan Z, Cuiuri D, Li H. A multi-bead overlapping model for robotic wire and arc additive manufacturing (WAAM). Robot Comput Integr Manuf 2015;31:101–10. [34] Ding D, Pan Z, Cuiuri D, Li H, van Duin S, et al. Bead modelling and implementation of adaptive MAT path in wire and arc additive manufacturing. Robot Comput Integr Manuf 2016;39:32–42. [35] Ding D, Shen C, Pan Z, Cuiuri D, Li H, Larkin N, et al. Towards an automated robotic arc-welding-based additive manufacturing system from CAD to finished part. Comput Des 2016;73:66–75. [36] Ding DH, Pan Z-X, Dominic C, Li H-J. Process planning strategy for wire and arc additive manufacturing. Robotic welding, intelligence and automation. Springer; 2015. p. 437–50. [37] Ding D, Pan Z, Cuiuri D, Li H, van Duin S. Advanced design for additive manufacturing: 3D slicing and 2D path planning. New trends in 3D printing. InTech; 2016. 138 Journal of Manufacturing Processes 35 (2018) 127–139 B. Wu et al. [99] Cong B, Ding J, Williams S. Effect of arc mode in cold metal transfer process on porosity of additively manufactured Al-6.3%Cu alloy. Int J Adv Manuf Technol 2015;76:1593–606. [100] Wang H, Kovacevic R. Variable polarity GTAW in rapid prototyping of aluminum parts. Proceedings of the 11th Annual Solid Freeform Fabrication Symposium 2000. [101] Tian Y, Ouyang B, Gontcharov A, Gauvin R, Lowden P, Brochu M. Microstructure evolution of Inconel 625 with 0.4 wt% boron modification during gas tungsten arc deposition. J Alloys Compd 2017;694:429–38. [102] Dong ZPB, Shen C, Ma Y, Li H. Fabrication of copper-rich Cu-Al alloy using the wire-arc additive manufacturing process. Metallurgical and Materials Transactions B: Process Metallurgy and Materials Processing Science 2017;48:3143–51. [103] Masubuchi K. Analysis of welded structures: residual stresses, distortion, and their consequences. Elsevier; 2013. [104] Colegrove PA, Coules HE, Fairman J, Martina F, Kashoob T, Mamash H, et al. Microstructure and residual stress improvement in wire and arc additively manufactured parts through high-pressure rolling. J Mater Process Technol 2013;213:1782–91. [105] Sames WJ, List F, Pannala S, Dehoff RR, Babu SS. The metallurgy and processing science of metal additive manufacturing. Int Mater Rev 2016;61:315–60. [106] Mukherjee T, Zhang W, DebRoy T. An improved prediction of residual stresses and distortion in additive manufacturing. Comput Mater Sci 2017;126:360–72. [107] Edwards P, O’Conner A, Ramulu M. Electron beam additive manufacturing of titanium components: properties and performance. J Manuf Sci Eng 2013;135:061016. [108] Busachi A, Erkoyuncu J, Colegrove P, Martina F, Ding J. Designing a WAAM based manufacturing system for defence applications. Procedia Cirp 2015;37:48–53. [109] Sames WJ, Medina F, Peter WH, Babu SS, Dehoff RR. Effect of process control and powder quality on inconel 718 produced using electron beam melting. 8th international symposium on superalloy 718 and derivatives. John Wiley & Sons, Inc.; 2014. p. 409–23. [110] Devletian JH, Wood WE. Factors affecting porosity in aluminum welds - a review. Welding Research Council; 1983. [111] Bai J, Ding H, Gu J, Wang X, Qiu H. Porosity evolution in additively manufactured aluminium alloy during high temperature exposure. IOP conference series: materials science and engineering. IOP Publishing; 2017. p. 012045. [112] What is hot cracking (solidification cracking), in, TWI Ltd, http://www.twi-global. com/technical-knowledge/faqs/material-faqs/faq-what-is-hot-cracking-solidification-cracking/, 2017. [113] Davis TA. The effect of process parameters on laser deposited Ti-6Al-4V, electronic theses and dissertations. 2004. Paper 319. [114] Colegrove PA, Donoghue J, Martina F, Gu J, Prangnell P, Hönnige J. Application of bulk deformation methods for microstructural and material property improvement and residual stress and distortion control in additively manufactured components. Scr Mater 2017;135:111–8. [115] Huang JX, Ye XN, Xu Z. Effect of cold rolling on microstructure and mechanical properties of AISI 301LN metastable austenitic stainless steels. J Iron Steel Res Int 2012;19:59–63. [116] Martina F, Colegrove PA, Williams SW, Meyer J. Microstructure of interpass rolled wire+ arc additive manufacturing Ti-6Al-4V components. Metall Mater Trans A 2015;46:6103–18. [117] Martina F, Roy MJ, Colegrove PA, Williams SW. Residual stress reduction in high pressure interpass rolled wire+ arc additive manufacturing Ti-6Al-4V components. Proc. 25th Int. Solid Freeform Fabrication Symp. 2014. p. 89–94. [118] Williams S. Wire + arc additive manufacture – current and future developments. 2017https://waammat.com/documents. [119] Residual stress reduction in high pressure interpass rolled wire þ arc additive manufacturing Ti–6Al–4V components. [120] Williams S. Wire + arc additive manufacture. 2017https://waammat.com/ documents. [121] Donoghue J, Antonysamy AA, Martina F, Colegrove PA, Williams SW, Prangnell PB. The effectiveness of combining rolling deformation with Wire–Arc Additive Manufacture on β-grain refinement and texture modification in Ti–6Al–4V. Mater Charact 2016;114:103–14. [122] Wu B, Pan Z, Ding D, Cuiuri D, Li H, Fei Z. The effects of forced interpass cooling on the material properties of wire arc additively manufactured Ti6Al4V alloy. J Mater Process Technol 2018;258:97–105. [123] Cheng X, Fisher JW, Prask HJ, Gnäupel-Herold T, Yen BT, et al. Residual stress modification by post-weld treatment and its beneficial effect on fatigue strength of welded structures. Int J Fatigue 2003;25:1259–69. [124] Li G, Qu S, Xie M, Li X. Effect of ultrasonic surface rolling at low temperatures on surface layer microstructure and properties of HIP Ti-6Al-4V alloy. Surf Coat Technol 2017;316:75–84. respect to aerospace material specifications. Phys Procedia 2010;5:595–606. [72] Zhang J, Zhang X, Wang X, Ding J, Traoré Y, Paddea S, et al. Crack path selection at the interface of wrought and wire + arc additive manufactured Ti–6Al–4V. Mater Des 2016;104:365–75. [73] Brandl E, Schoberth A, Leyens C. Morphology, microstructure, and hardness of titanium (Ti-6Al-4V) blocks deposited by wire-feed additive layer manufacturing (ALM). Mater Sci Eng A 2012;532:295–307. [74] Wang F, Williams S, Colegrove P, Antonysamy AA. Microstructure and mechanical properties of wire and arc additive manufactured Ti-6Al-4V. Metall Mater Trans A 2013;44:968–77. [75] Martina F, Mehnen J, Williams SW, Colegrove P, Wang F. Investigation of the benefits of plasma deposition for the additive layer manufacture of Ti–6Al–4V. J Mater Process Technol 2012;212:1377–86. [76] Gu J, Ding J, Williams SW, Gu H, Ma P, Zhai Y. The effect of inter-layer cold working and post-deposition heat treatment on porosity in additively manufactured aluminum alloys. J Mater Process Technol 2016;230:26–34. [77] Wang P, Hu S, Shen J, Liang Y. Characterization the contribution and limitation of the characteristic processing parameters in cold metal transfer deposition of an Al alloy. J Mater Process Technol 2017;245:122–33. [78] Brice C, Shenoy R, Kral M, Buchannan K. Precipitation behavior of aluminum alloy 2139 fabricated using additive manufacturing. Mater Sci Eng A 2015;648:9–14. [79] Agrawal BK. Introduction to engineering materials. New Delhi: Tata McGraw-Hill; 2007. [80] ASTM B. 221-standard specification for aluminum and aluminum-alloy extruded bars, rods, wire, profiles, and tubes. 2005. [81] Baufeld B. Mechanical properties of INCONEL 718 parts manufactured by shaped metal deposition (SMD). J Mater Eng Perform 2012;21:1416–21. [82] Xu F, Lv Y, Liu Y, Shu F, He P, Xu B. Microstructural evolution and mechanical properties of inconel 625 alloy during pulsed plasma arc deposition process. J Mater Sci Technol 2013;29:480–8. [83] Bauccio M. ASM metals reference book. ASM international; 1993. [84] Xu FJ, Lv YH, Xu BS, Liu YX, Shu FY, He P. Effect of deposition strategy on the microstructure and mechanical properties of Inconel 625 superalloy fabricated by pulsed plasma arc deposition. Mater Des 2013;45:446–55. [85] Wang JF, Sun QJ, Wang H, Liu JP, Feng JC. Effect of location on microstructure and mechanical properties of additive layer manufactured Inconel 625 using gas tungsten arc welding. Mater Sci Eng A 2016;676:395–405. [86] Ding D, Pan Z, van Duin S, Li H, Shen C. Fabricating superior NiAl bronze components through wire arc additive manufacturing. Materials 2016;9:652. [87] Guo J, Zhou Y, Liu C, Wu Q, Chen X, Lu J. Wire arc additive manufacturing of AZ31 magnesium alloy: grain refinement by adjusting pulse frequency. Materials 2016;9:823. [88] Shen C, Pan Z, Ma Y, Cuiuri D, Li H. Fabrication of iron-rich Fe–Al intermetallics using the wire-arc additive manufacturing process. Addit Manuf 2015;7:20–6. [89] Shen C, Pan Z, Cuiuri D, Dong B, Li H. In-depth study of the mechanical properties for Fe3Al based iron aluminide fabricated using the wire-arc additive manufacturing process. Mater Sci Eng A 2016;669:118–26. [90] Ma Y, Cuiuri D, Hoye N, Li H, Pan Z. Characterization of in-situ alloyed and additively manufactured titanium aluminides. Metallurgical and Materials Transactions B: Process Metallurgy and Materials Processing Science 2014;45:2299–303. [91] Ma Y, Cuiuri D, Shen C, Li H, Pan Z. Effect of interpass temperature on in-situ alloying and additive manufacturing of titanium aluminides using gas tungsten arc welding. Addit Manuf 2015;8:71–7. [92] Abe T, Sasahara H. Dissimilar metal deposition with a stainless steel and nickelbased alloy using wire and arc-based additive manufacturing. Precis Eng 2016;45:387–95. [93] Liu L, Zhuang Z, Liu F, Zhu M. Additive manufacturing of steel–bronze bimetal by shaped metal deposition: interface characteristics and tensile properties. Int J Adv Manuf Technol 2013;69:2131–7. [94] Dong B, Pan Z, Shen C, Ma Y, Li H. Fabrication of copper-rich Cu-Al alloy using the wire-arc additive manufacturing process. Metall Mater Trans B 2017. [95] Wu B, Ding D, Pan Z, Cuiuri D, Li H, Han J, et al. Effects of heat accumulation on the arc characteristics and metal transfer behavior in Wire Arc Additive Manufacturing of Ti6Al4V. J Mater Process Technol 2017;250:304–12. [96] Almeida P, Williams S. Innovative process model of Ti–6Al–4V additive layer manufacturing using cold metal transfer (CMT). Proceedings of the Twenty-First Annual International Solid Freeform Fabrication Symposium 2010. [97] Xiong J, Zhang G, Zhang W. Forming appearance analysis in multi-layer singlepass GMAW-based additive manufacturing. Int J Adv Manuf Technol 2015;80:1767–76. [98] Wang H, Jiang W, Ouyang J, Kovacevic R. Rapid prototyping of 4043 Al-alloy parts by VP-GTAW. J Mater Process Technol 2004;148:93–102. 139 View publication stats