A preliminary study on gas metal arc welding - Based additive manufacturing of metal parts

Science & Technology Development Journal, 23(1):422-429 Open Access Full Text Article Research Article Advanced Technology Centre, Le Quy Don Technical University, 236 Hoang Quoc Viet, Bac Tu Liem, Hanoi, Vietnam Correspondence Van Thao Le, Advanced Technology Centre, Le Quy Don Technical University, 236 Hoang Quoc Viet, Bac Tu Liem, Hanoi, Vietnam Email: thaomta@gmail.com History  Received: 2019-10-03  Accepted: 2019-11-26  Published: 2019-02-16 DOI : 10.32508/stdj.v23i1.1714 Copyright © VNU-HCM Press. This is an open- access article distributed under the terms of the Creative Commons Attribution 4.0 International license. A preliminary study on gasmetal arc welding-based additive manufacturing of metal parts Van Thao Le* Use your smartphone to scan this QR code and download this article ABSTRACT Introduction: In the past three decades, additive manufacturing (AM), also known as 3D print- ing, has emerged as a promising technology, which allows the manufacture of complex parts by adding material layer upon layer. In comparison, with other metal-based AM technologies, gas metal arc welding-based additive manufacturing (GMAW-based AM) presents a high deposition rate and has the potential for producing medium and large metal components. To validate the technological performance of such a manufacturing process, the internal quality of manufactured parts needs to be analyzed, particularly in the cases of manufacturing the parts working in a crit- ical load-bearing condition. Therefore, this paper aims at investigating the internal quality (i.e., microstructures and mechanical properties) of components manufactured by the GMAW-based AM technology. Method: A gas metal arc welding robot was used to build a thin-walled com- ponent made of mild steel on a low-carbon substrate according to the AM principle. Thereafter, the specimens for observing microstructures and mechanical properties were extracted from the built thin-walled component. The microstructures of the specimen were observed by an optical microscope; the hardness was measured by a digital microhardness tester, and the tensile tests were carried out on a tensile test machine. Results: The results show that the GMAW-based AM- built thin-walled components possess an adequate microstructure that varies from the top to the bottom of the built component: lamellar structures with primary austenite dendrites in the upper zone; granular structure of ferrites with small regions of pearlites at grain boundaries in the middle zone, and equiaxed grains of ferrites in the lower zone. The hardness (ranged between 1643.46 HV to 1923.81 HV), yield strength (YS o f f set o f 0:2% ranged from 3402 MPa to 349.671.53 MPa), and ultimate tensile strength (UTS ranged from 4291 MPa to 4772 MPa) of the GMAW-based AM-built components were comparable to those of wrought mild steel. Conclusions: The results obtained in this study demonstrate that the GMAW-based AM-built components possess adequate microstructure and good mechanical properties for real applications. This allows us to confirm the feasibility of using a conventional gas metal arc welding robot for additive manufacturing or repairing/re-manufacturing of metal components. Key words: Additive manufacturing, Gas metal arc-based additive manufacturing, Mild steel, Microstructures, Mechanical properties INTRODUCTION In the past three decades, Additive manufacturing (AM), also known as 3D printing, has emerged as a promising solution for manufacturing complex ge- ometries and parts made of materials that are expen- sive and/or difficult tomachine, for example, titanium and nickel alloy1. In contrast tomachining processes, AM technology builds a solid part by addingmaterials layer-by-layer from a substrate without using any ad- ditional resources such as cutting tools, cooling fluid, and fixture system2. Nowadays, AM technologies - in particular, metal-based AM, have been efficiently ap- plied in different sectors, for example, aerospace, au- tomotive and biomedical engineering1,3–5. Metal-based AM systems can be categorized based on the material feedstock, energy source, and so on. Ac- cording to the heat source used in AM, metal-based AM technologies can be classified into laser-based, electron beam-based, and arc welding-based addi- tive manufacturing4. In comparison with laser-based and electron beam-based AM systems, welding-based additive manufacturing - also called wire arc ad- ditive manufacturing (WAAM) has demonstrated as a prospective solution for the manufacture of medium and large-dimensional metal parts 6. More- over, WAAM presents a higher deposition rate, lower equipment costs, and low production costs7–9. In WAAM systems, an industrial robot or a CNC ma- chine tool is normally used to provide accurate move- ments of welding torch during the build of compo- nents. The arc heat source used in a WAAM sys- tem can be gas metal arc welding (GMAW), gas tung- Cite this article : Le V T.Apreliminary studyongasmetal arcwelding-basedadditivemanufacturing of metal parts. Sci. Tech. Dev. J.; 23(1):422-429. 422 Science & Technology Development Journal, 23(1):422-429 sten arc welding (GTAW), and plasma arc welding (PAW) 6,8. In terms of productivity, the deposition rate of GMAW-based AM is two or three times higher than that of GTAW- and PAW-based AM 10. That is why GMAW-based AM is more suitable than GTAW- and PAW-based AM for the manufacture of metal par ts with large dimensions. In the literature, much research has been carried out on WAAM. Most of the published works focused on observing the influence of process parameters on the geometry of built components8,11–13. For example, Xiong et al.8 investigated the influences of main pro- cess parameters (e.g., wire feed speed, travel speed, and inter-layer temperature) on surface roughness of thin-walled parts built by GMAW-based AM. In their work, a better understanding of the influentialmecha- nisms of the process parameters on the surface rough- ness was presented. The authors showed that the sur- face quality of thin-walled components could be im- proved by decreasing the inter-layer temperature. The increase of the wire feed speedwas associatedwith the increase of the surface roughness, and so on. On the other hand, not much research on the in- ternal quality of WAAM-built parts has been carried out. Suryakumar et al.14 investigated the effects of heating cycles on the tensile properties and the hard- ness of low-carbon steel produced by the GMAW- based AM process. The authors highlighted that ther- mal cycles have negligible effects on material proper- ties after around five deposited layers. Chen et al.15 studiedmicrostructures andmechanical properties of stainless steel 316L components manufactured by the GMAW-based AM process. They found that the ten- sile properties of GMAW-based AM-built 316L steel were comparable to those of wrought 316L. In fact, the internal quality of parts is a very important criterion, which allows us to demonstrate the techni- cal performance of the manufacturing process. Thus, a better understanding of microstructures and me- chanical properties of components manufactured by GMAW-basedAM is necessary for the production de- cision, especially for components that work in a crit- ical load-bearing condition. In addition, studies re- lated to AM technologies in Vietnam are very limited. Most of the 3D printers available in Vietnam are only capable of printing plastic and non-metallic materi- als. The main reason is that the investment costs for a metal-based AM system are still very expensive. To overcome this difficulty, the selection of an arc weld- ing system, which is readily available and has low costs of investment for the research on metal-based AM, is consistent in Vietnam. Therefore, the objective of this study is to investigate the internal quality of thin-walled partsmanufactured by the GMAW-based AM process. The results ob- tained in this study allow us to demonstrate the feasi- bility of using the GMAW robot for manufacturing or repairing/remanufacturing of metal components ac- cording to the AM principle. This paper is organized as follows: Section MATERI- ALSANDMETHODSdescribes thematerials and ex- perimental methods. In Section RESULTS, the main results on microstructures and mechanical properties of built materials are presented. Section DISCUS- SION is intended for conclusions and future work. MATERIALS - METHODS Materials In this study, the mild steel copper-coated welding wire (ER70S-6, supplied by Changzhou City Yunhe WeldingMaterial Company of China) with a diameter of 1.2 mm, and a low-carbon steel plate (SS400, man- ufactured by JawayMetal Company of China) with di- mensions of 250 mm in length, 100 mm in width, and 10 mm in thickness were used to build a thin-walled sample. Before depositing the first welding layer, the substrate surface was machined to remove oxide scale and rust. The chemical compositions of the welding wire and the substrate are shown in Table 1. The thin-walled sample was built according to the WAAMprocess by an industrial GMAWrobot (Pana- sonic TA-1400, provided by Panasonic Welding Sys- tem Company of Japan) (Figure 1a). In this system, the 6-axis robot (1) implements the movement of the welding torch (5) to deposit successive welding lay- ers from the substrate. The welding process was con- trolled by the welding power source (2). Building the thin-walled sample The welding process parameters used to build the thin-walled sample are shown in Table 2. These pa- rameters were chosen based on the recommendations of the manufacturer of welding wires and material properties. The distance between theGMAWtorch and thework- piece was 12 mm. The deposition was conducted at room temperature and without preheating the sub- strate. Once the deposition of a welding layer was finished, the welding torch is retracted to the begin- ning point for the deposition of the next layer with a dwell time of 60 seconds. The dwell time used be- tween two successive layers aims at cooling down the workpiece and transferring accumulated heat to the environment. The final cooling of the built thin wall 423 Science & Technology Development Journal, 23(1):422-429 Table 1: Chemical compositions of wire and substrate materials (in wt. %) 16 Element C Si Mn P S Al Ca Cu Fe Wire (ER70S-6) 0.04 0.92 0.45 0.011 0.015 - - 0.2 Balance Substrate (SS400) 0.05 0.037 0.46 0.013 0.002 0.044 0.0017 - Balance was carried in the calm air of the room. During the building process, a gas of 100% CO2 with a constant flow rate of 20 (L/min) was used for the shielding. The built thin- walled sample was presented in Figure 1b and c. Its dimensions are approximately 140 mm in length, 80 mm in height, and 4.5 mm in thickness. Microstructures observation and hardness measurement To observemicrostructures andmeasure the hardness of the built material, a specimen (MS, Figure 1c and d) was cut from the built thin-walled sample by us- ing a wire-cut Electrical DischargeMachining (EDM) machine (Aristech CW-10, supplied by Lien Sheng Mechanical & Electrical Company of Taiwan). Sub- sequently, the EDM-cut surface of this specimen was grinded and chemically etched. The microstructure of the specimen was observed using an optical micro- scope AXIO A2M of Carl Zeiss Company. The hard- ness was measured by a digital microhardness tester (Vicker FV-310 of Future-Tech Company) with a load of 5 kgf (49.05 N). Tensile tests For observing tensile properties of built materials, two groups of tensile specimens in the vertical (TSv1, TSv2, TSv3) and horizontal (TSh1, TSh2, TSh2) di- rection s were cut from the built thin-walled sample by using the wire-cu t EDMmachine (Figure 1c). Be- fore cutting these specimens, two side surfaces of the built thin wall were machined to obtain an effective width of the built thin-walled materials. The dimen- sions of the tensile specimens are shown in Figure 2. Figure 2: Dimensions of the tensile specimen. Their cross-section and length for examining tensile properties are 6 mm x 2mm and 20 mm, respectively. The tensile tests were conducted on the tensile test machine (INSTRON 3369 of Instron Company) with a crosshead displacement speed of 1.2 mm/min and at room temperature. RESULTS Microstructures Figure 3 presents the microstructure of the specimen MS observed in five zones: the upper zone, themiddle zone, the lower zone, the heat-affected zone (HAZ), and the substrate zone (as illustrated in Figure 1d). The upper zone (Figure 3a) presents lamellar struc- tures with primary austenite dendrites that distribute along the cooling direction - perpendicular to the sub- strate. In addition, the upper zone has a sudden high variation of thermal and a high-cooling rate because it contacts calm air at room temperature, thus resulting in three types of ferrite grains: allotriomorphic ferrite a , Widmanstatten ferrite aw, and acicular ferrite aa. The middle zone is mainly characterized by the gran- ular structure of ferrites with small regions of pearlites at grain boundaries (Figure 3b). In this zone, it was also found that there are two types of grains: granular grains in the overlapped zone with a relatively large size and equiaxed grains with a dense distribution in the non-overlapped zone. The reason is that the heat of molten pool, which forms the current deposited layer (e.g., layer i +1), reheats and re-melts the pre- viously built layer (e.g., layer i), resulting in the solid- state phase transformation in the overlapped zone, for example, grain growth, recrystallization, and phase transitions. On the other hand, the microstructures in the lower zone consist of equiaxed grains of ferrite, in which thin lamellae are distributed and coexisting with thin strips of pearlite (Figure 3c). The lower zone under- goes a slow er cooling rate when compared to the up- per zone, resulting in ferrite phases. It is also observed that the grain size in the lower zone is finer than that in the middle zone. The reason is that the value of the thermal shock of the lower zone is higher with respect to the middle zone. The lower zone (including about 4 first deposited layers) contacts the cold substrate, while the middle zone contacts the warm deposited layer18. In addition, the middle zone presents a ther- mal gradient lower than that of the lower zone19. 424 Science & Technology Development Journal, 23(1):422-429 Figure 1: (a) Schema of the GMAW-based AM system17, (b) the built thin-walled sample, (c) the positions for cutting the specimens and (d) five zone s for observingmicrostructures andmeasuring the hardness on a cut surface of the specimen. Table 2: Process parameters used to build the thin-walled sample Process parameters Travel speed (mm/min) Welding current (A) Welding Voltage (V) The flow rate of shielding gas (L/min) Value 300 70 18 20 Figure 3d presents microstructures of the heat- affected zone (HAZ). It can be found that there is a microstructure transformation from austenite to martensite. The substrate zone presents a typical fer- rite/perlite bandedmicrostructures of the low-carbon steel obtained by the hot rolling process (Figure 3e). This type ofmicrostructure is opposite to the homoge- nous distribution of phases observed in the middle zone. Hardness Table 3 shows the results of hardness measurement in five zones of microstructure observation. For each zone, the hardness (HV) was measured at five posi- tions that distribute on the “centerline” of the cross- section from the top to the bottom of the specimen MS (Figure 1d). It is also noted that the reported hardness test results for eachmeasured position inTa- ble 3 are the average value of three different indenta- tion points on polished surfaces of the specimen MS. In the thin-walled part, the upper zone presents the highest hardness value, and the middle zone has the lowest hardness value. The average hardness value of 192  3.81 HV, 163.8  5.63 HV, and 175.8  2.77 HV was obtained in the upper zone, the middle zone, and the lower zone, respectively. The HAZ present a hardness value slightly lower than that of the sub- strate zone (222.6  2.70 HV versus. 224  3.52 HV, Table 3). Tensile properties Figure 4 shows the installation of a specimen on the tensile machine (Figure 4a), two examples of the bro- ken specimens after the tensile tests (Figure 4b), and typical engineering strain-stress curves (Figure 4c) 425 Science & Technology Development Journal, 23(1):422-429 Figure 3: Microstructure s of built materials observed in five zones. (a) the upper zone, (b) the middle zone, (c) the lower zone, (d) the heat- affected zone, and (e) the substrate zone. Table 3: Measurement of hardness (HV) in different zones of the specimenMS Measured zone Upper zone Middle zone Lower zone HAZ Substrate zone Position 1 197 167 171 221 225 Position 2 192 162 178 219 223 Hardness value (HV) Position 3 187 159 177 223 226 Position 4 190 165 176 224 222 Position 5 194 167 177 226 224 Average value (HV) 192 3.81 164 3.46 175.8 2.77 222.6 2.70 224 1.58 426 Science & Technology Development Journal, 23(1):422-429 obtained from the tensile tests of two tensile speci- mens in the vertical and horizontal directions (TSv1 and TSh1 specimens). The yield strength (YS, offset of 0.2%) and ultimate tensile strength (UTS) of all tensile specimens are given in Table 4. Table 4: YS and UTS values of vertical and horizontal specimens Tensile properties YS (offset of 0.2%, MPa) UTS (MPa) TSv1 348 477 TSv2 351 479 TSv3 350 475 Average value of vertical specimens 349.67 1.53 47 7 2 TSh1 338 428 TSh2 342 430 TSh3 340 429 Average value of horizontal specimens 340 2 429 1 ASTM A36 steel 20 250 400- 550 DISCUSSION Themicrostructures of GMAW-based AM-built thin- walled component vary from the top to the bottom of the built sample with different structures, i.e., lamel- lar structures with primary austenite dendrites in the upper zone; granular structure of ferrites with small regions of pearlites at grain boundaries in the middle zone, and equiaxed grains of ferrite in the lower zone. This microstructure formation is due to the reheating and remelting effect induced by successive layer depo- sitions and different cooling conditions in each zone. The microstructural characteristics of the built thin- walled component observed in this study are similar to those reported in the previous studies18,20,21. The microstructures of the built thin-walled parts could also be adjusted by using alternating cycles of cooling or rolling deposited layers6 . Moreover, the built sam- ple also presents a continued transition ofmicrostruc- tures in the interface zone between theweldedmateri- als and the substrate. This demonstrates a strongmet- allurgical bonding between the built materials and the substrate. Due to the variation of microstructures of built mate- rials from the upper to the lower zones, as observed in subsection 3.1, the hardness of built materials also changes in a consistent way. Due to the presence of the Widmanstatten structure in the upper zone (Figure 4a), the hardness of the upper zone is higher than that of the middle and lower zones (192  3.81 HV in comparison with 163.8 5.63 HV and 175.8 2.77 HV, Table 3). Similarly, the lower zone charac- terized by lamellae structures results in higher hard- ness values than those in the middle zone. The HAZ hardness value is lower than that of the substrate zone because the metal in the upper HAZ was heated and partially melted by the heat of molten materials of the first layers, resulting in softening effects. Lastly, it is found that the hardness of the built materials is com- parable to that of wrought ASTMA36 steel (about 168 HV), which h as a similar chemical composition with ER70S-6 steel. From Figure 4, it is first found that the strain-stress curves of all specimens present an elastic region at the onset of load applications and followed by inhomo- geneous yielding at the elastic and plastic transition. This shows a typical behavior for mild steels22. Sec- ondly, the average values of YS and UTS are statis- tically different between the vertical and horizontal specimens (with p-value = 0.001 for YS and p-value  0 for UTS). The vertical specimens reveal higher values of UTS when compared to the UTS values of the horizontal specimens. As shown in Table 4, the UTS values of the vertical specimens ranged from 475 MPa to 479MPawith an average value of 477MPa and standard deviation of 2, whereas theUTS values ob- tained for the horizontal specimens ranged from 428 MPa to 430 MPa with an average value of 429 MPa and standard deviation of 1. Similarly, the YS values obtained from the vertical specimens are also higher than those of the horizontal specimens, 349.67 1.53 MPa in comparison with 340 2MPa (Table 4). This significant difference in terms of YS and UTS values between the vertical and horizontal specimens may be due to non-uniform microstructures of built ma- terials. Moreover, the YS values of these specimens are higher than that of wrought A36 steel (about 250 MPa). On the other hand, the UTS values of these specimens are in the value range of wrought ASTMA- 36 steel (400-550 MPa)20. These results demonstrate the good mechanical properties of the GMAW-based AM-built components. CONCLUSIONS This paper presents a preliminary study in our project on the use of an industrial GMAW robot for additive manufacturing or remanufacturing of metal parts. In this work, a thin-walled sample made of mild steel 427 Science & Technology Development Journal, 23(1):422-429 Figure 4: Tensile tests with two specimens TSv1 and TSh1: (a) Installation of the specimen on the tensile testmachine, (b) the broken specimens after the tensile tests, and (d) the engineering stress-strain curves. was built to investigate microstructures and mechan- ical properties. The results show that the microstruc- ture of built materials varies from the top to the bot- tom of built samples in four zones: the upper zone, themiddle zone, the lower zone, and the heat-affected zone of substrate materials. The upper zone of built materials presents the highest hardness value (192  3.81 HV versus 163.8  5.63 HV in the middle zone and 175.8  2.77 HV in the lower zone). There is also a significant difference in terms of YS and UTS between the vertical and horizontal specimens due to non-uniform microstructures of built materi- als. Moreover, the mechanical properties of the thin- walled component built by the GMAW-based AM process are comparable with those of parts manufac- tured by traditional processes such as forging andma- chining. Hence, the components built by the GMAW- based AM process are adequate for industrial appli- cations. This confirms the feasibility of using the GMAW robot for additive manufacturing of parts or repairing/remanufacturing of damaged components. In future works, we will focus on optimizing process parameters and evaluating the economic efficiency and environmental performance of theGMAW-based AM process. LIST OF ABBREVIATIONS AM: Additive Manufacturing EDM: Electrical Discharge Machining GMAW: Gas Metal Arc Welding GTAW: Gas Tungsten Arc Welding HAZ: Heat Affected Zone PAW: Plasma Arc Welding UTS: Ultimate Tensile Strength WAAM: Wire Arc Additive Manufacturing YS: Yield Strength COMPETING INTERESTS The author declares that this paper has no competing interests. ACKNOWLEDGMENT This research is funded by the Vietnam National Foundation for Science andTechnologyDevelopment (NAFOSTED) under grant number 107.99-2019.18. REFERENCES 1. Guo N, Leu M. Additive manufacturing: technology, applica- tions and research needs. Front Mech Eng. 2013;8:215–243. 2. Herzog D, Seyda V, Wycisk E, Emmelmann C. Additive manu- facturing of metals. Acta Mater. 2016;117:371–392. 3. Gardan J. Additive manufacturing technologies: state of the art and trends. Int J Prod Res. 2015;7543:1–15. 4. Frazier WE. Metal Additive Manufacturing: A Review. J Mater Eng Perform. 2014;23:1917–1928. 5. 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