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steel joints using a Zn-based filler wire
Optics and Laser Technology 122 (2020) 105882
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Full length article
Microstructure and mechanical properties of laser fusion welded Al/steel joints using a Zn-based filler wire
T
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Jiahao Sua, Jin Yanga,b, , Yulong Lic, Zhishui Yua,b, Jieshi Chena,b, Wanqin Zhaoa,b, ⁎ Hongbing Liua,b, , Caiwang Tand a
School of Materials Engineering, Shanghai University of Engineering Science, Shanghai 201620, China Shanghai Collaborative Innovation Center of Laser Advanced Manufacturing Technology, Shanghai 201620, China c Key Lab for Robot and Welding Automation of Jiangxi Province, Mechanical and Electrical Engineering School, Nanchang University, Nanchang 330031, China d State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China b
H I GH L IG H T S
fusion welding of Al alloy to steel using Zn-based filler wire was performed. • Laser FeAl IMC was formed at the interface, which enhanced the fracture load. • New fracture load first increased and then decreased with the rising laser power. • The • The fracture modes of the joints changed with the laser power.
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser fusion welding Dissimilar metals Intermetallic compounds Microhardness Fracture load
Laser fusion welding technique was used for lap joining of Q235 steel to AA5052 aluminum alloy using a fluxcored Zn-22Al filler wire. The influence of laser power on the microstructure and mechanical properties of laser Al/steel joints was investigated. A fusion welded region and a brazed region were formed at the fusion zone/steel interface: Fe2Al5−xZnx and FeZn10 were formed at the both regions, while a new FeAl IMCs with much lower hardness and brittleness was observed in the welded region. The phase constitutions at the both regions were unvaried with the change of laser power, while the phase morphologies were slightly changed. The tensile-shear testing results showed that the joint fracture load first increased then decreased with the rising laser power and the maximum value reached 1225 N at laser power of 2800 W. The joint fractured at the fusion zone/steel interface when the laser power was less than 2800 W and it changed into fusion zone when the laser power reached 3000 W. The change of joint fracture load and fracture behavior was mainly attributed to the change of morphology of IMC and interfacial bonding length.
1. Introduction As countries pay more and more attention to energy conservation and emission reduction, the need of light-weight automotive structure is very prominent. The composite structure of aluminum, magnesium alloys and steels has the comprehensive advantages of light weight and high strength [1–4]. Therefore, the dissimilar combination of aluminum alloy and steel is necessary in the fabrication of hybrid structural parts. However, due to the huge differences of thermal physic properties and solid solubility between aluminum and steel, the joining of aluminum to steel is still a challenge. The formation of intermetallic compound (IMC), such as FeAl,
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FeAl2, FeAl3 and Fe2Al5 is a key factor affecting the quality of Al/steel welds. The welds may suffer from heavy cracking in service due to the inherent and nature brittleness of IMC [5,6]. To achieve a sound Al/ steel weld, the thickness of the Fe-Al IMC should be controlled below 10 μm [7]. Therefore, the development of adequate welding techniques to achieve high efficiency and high quality of Al/steel joining is urgent. Many welding technologies have been used to join aluminum alloy and steel. By virtue of high energy density, low distortion and narrow heat affected zone, laser welding/brazing was employed in the Al/steel joining. Alloy elements Si and Zn were commonly used to improve the joint quality [8–13]. Tan et al. [8] demonstrated that the laser Al/steel joint with the highest tensile strength was obtained by using Zn-22Al
Corresponding authors. E-mail addresses: [email protected] (J. Yang), [email protected] (H. Liu).
https://doi.org/10.1016/j.optlastec.2019.105882 Received 9 June 2019; Received in revised form 29 August 2019; Accepted 29 September 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
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filler metal among various Zn-based filler metals. Yang et al. [9] proved that Si was able to inhibit the growth of the interfacial reaction layer, resulting in an improvement of the fracture load of laser Al/steel joints, while Zn was able to reduce the brittleness of the reaction layer and consequently increased the fracture load. Qin et al. [10] confirmed that the tensile strength of Al/steel joints with ER4043(Al-5Si) filler metal firstly increased and then decreased with the increase of laser power, and the highest tensile strength of the joint reached 247.3 MPa, which was 85% of that of the base metal of 6013-T4 aluminum alloy. Dharmendra et al. [11] concluded that the thickness of IMC layer varied from 3 μm to 23 μm in laser Al/steel joint with Zn-Al filler metal, and the joints exhibited highest mechanical resistance of 220 MPa when the IMCs thickness ranged between 8 μm and 12 μm. Sun et al. [12] reported that all laser Al/steel joints failed at the brazed interface during tensile testing, and the maximum tensile strength ~120 MPa was achieved. The fracture surface was typical cleavage fracture mode, and the crack initiated and propagated in FeAl3 layer. Based on aforementioned literature, the effect of Si addition on the formation of Fe-Al IMCs has been established, that is, Si can reduce the IMCs layer thickness and thereby improving the joint mechanical properties. For the element Zn, it promotes the formation of tough and dispersed FeZn10 IMCs in layered Fe2Al5−xZnx, hence offering higher crack resistance during tensile loading and increases joint strength [14]. Nevertheless, the previous studies were mainly focused on the dissimilar Al/steel joints by laser welding/brazing technology; while, the influence of element Zn on the microstructure and mechanical properties of laser Al/steel joints using laser fusion welding technology was still unclear. Therefore, the purpose of this study is to investigate the influence element Zn on the microstructure and mechanical properties of dissimilar Al/steel joint using laser fusion welding. The results have shown that, in addition to FeZn10 and Fe2Al5−xZnx that commonly observed in laser welded/brazed joints [10,11], a new Fe-Al IMC, FeAl with much lower hardness and brittleness, is formed in laser fusion welded Al/steel joints. Thus, the joint fracture load was enhanced.
Table 2 Mechanical properties of AA5052 aluminum and Q235 steel. Materials
Tensile strength/MPa
Yield strength/MPa
AA5052 Al Q235 steel
260 460
190 235
Fig. 1. Schematic of laser fusion welding of Al to steel with Zn-22Al filler metal. Table 3 Process parameters in laser fusion welding of Al to steel. Laser power/ W
Defocusing amount/mm
Welding speed/m∙min−1
Wire feeding speed/m∙min−1
Flowing rate of shielding gas/L∙min−1
2400 2600 2800 3000
+30
0.5
2.0
15
After the welding, welding slag was removed by a stainless steel wire brush. Cross-sections of the laser joints were cut, and then mounted in phenolic resin. The samples were mechanically ground using 400, 600, 800, 1000 and 1200 grades SiC abrasive papers followed by polishing with a 1-μm diamond suspension. To analyze the microstructure and determine the chemical composition of the interfacial phases, the cross-sections were observed using Olympus 4XCJZ optical microscope and Hitachi S3400-N scanning electron microscope (SEM) equipped with EDAX Genesis energy-dispersive X-ray spectroscopy (EDS) analysis. The tensile-shear testing was evaluated at a cross-head speed of 1.0 mm/s. The shape and dimension of tensile specimens were shown in Fig. 2. The microhardness measurements were conducted on a computer automated machine at the load of 25 g and 15 s dwell time.
2. Experimental procedures Laser fusion welding was carried out by using the IPG YLS-5000 fiber laser processing system. Q235 galvanized steel was used in the dimension of 150 mm × 100 mm × 1.8 mm. AA5052 aluminum alloy was used in the dimension of 150 mm × 100 mm × 2.0 mm. Their chemical compositions and mechanical properties were shown in Tables 1 and 2. A flux-cored Zn-22Al alloy with a diameter of 1.6 mm was used as the filler metal. The corresponding chemical compositions of the Zn-22Al filler metal were listed in Table 1. Before welding, the work pieces were ground with abrasive paper and then cleaned with acetone to remove surface contaminant. The aluminum alloy was placed on top of the galvanized steel. The angle between shielding gas nozzle and filler wire was 90° and the laser head was perpendicular to the base metal, as shown in Fig. 1. To prevent oxidation during laser welding, pure argon (99.99 wt%) was used. Laser power was chosen as the only variable in the laser welding processes. Preliminary laser welding was conducted to obtain a visually acceptable weld appearance. The optimized parameters were listed in Table 3.
3. Results and discussion 3.1. Macrostructure Fig. 3 shows the weld appearances and cross-sections of joints with
Table 1 Chemical compositions of base materials and filler metal in weight percent (wt %). Materials
Cu
Mg
Mn
Fe
Si
Zn
Cr
Al
C
Ni
AA5052 Al Q235 steel Zn-22Al
0.1 – 0.8
2.2–2.8 – –
0.1 0.147 –
0.4 Bal. –
0.25 0.35 –
0.1 – Bal.
0.2 0.044 –
Bal. – 22
– 0.2 –
– – –
Fig. 2. Shape and dimension of tensile specimens. 2
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Fig. 3. Weld appearances and cross sections of laser joints produced with different laser powers: (a–b) 2400 W, (c–d) 2600 W, (e–f) 2800 W, (g–h) 3000 W.
interface were increased with the increase of laser power.
different laser powers. Fig. 3(a, c, e and g) illustrated the weld appearances of laser fusion welded Al/steel joint using Zn-22Al filler wire with 2400 W, 2600 W, 2800 W and 3000 W laser powers, respectively. Visually acceptable weld appearances were obtained in the laser power range. Fig. 3(b, d, f and h) showed the cross sections of joints with the laser powers of 2400 W, 2600 W, 2800 W and 3000 W, respectively. Two fusion zones were identified in the joints. Adjacent to the Al side, Zn-Al filler wire and parts of Al based metal were melted by laser irradiation and the melt solidified into fusion zone I during cooling. Adjacent to the steel side, certain amount of steel was melt under laser irradiation, and then it solidified into fusion zone II. In addition to fusion zones, brazed interface was formed at the interface, which was caused by the wetting and spreading of the melt in fusion zone I [15]. Fig. 3 also shows that the sizes of fusion zone I, fusion II and brazed
3.2. Microstructure According to the microstructure, three zones adjacent to the fusion zones and the brazed interface were chosen for further analysis, i.e., fusion zone I/fusion zone II interface, fusion zone II and brazed interface (Fig. 4). At 2400 W laser power, similar microstructure, i.e., a layered dark phase and a dispersed light phase, was observed at the fusion zone I/fusion zone II interface and the brazed interface (Fig. 4(a) and (c)). Based on EDS analysis, the layered dark phase was composed of 28.0 at% Fe, 70.7 at% Al and 1.3 at% Zn (Table 4), which was identified as Fe2Al5−xZnx IMC; while the dispersed light phase had the composition of 12.3 at% Fe, 50.4 at% Al and 37.3 at% Zn (Table 4), 3
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Fig. 4. The microstructure at selected regions of the joints with different laser powers: (a–c) 2400 W, (d–f) 2800 W, (g–i) 3000 W.
tortuous when comparing to that of 2800 W. Fig. 5 presents the corresponding EDS line scanning analysis at different laser powers. With the increase of laser power, similar element distribution was observed spanning from fusion zone I to steel. At IMC layer, a relatively stable element ratio Fe: (Al + Zn) of 7:3 was obtained, which corresponded to Fe2Al5−xZnx and FeZn10 identified in EDS point analysis. The shape change of element composition at the fusion zone I/IMC layer interface and the IMC layer/fusion zone II interface suggested the occurrence of intensive atomic diffusion. At fusion zone II, a relatively stable Fe and Al element distribution was obvious across the whole region with the ratio of 1:1. It corresponded to FeAl IMC identified in the EDS point analysis. Some occasional peaks of elements Al and Zn were found in Fig. 5(b–c) which was highlighted by dark arrows. The enrichment of Al and Zn corresponded to the white films formed at the grain boundaries of FeAl. This was mainly attributed to strong grain boundary penetration tendency of Zn-Al liquid [16,17].
Table 4 The corresponding EDS results at selected points as shown in Fig. 4. Selected points
1 2 3 4 5 6 7 8 9 10 11
Element at%
Possible phase
Fe
Al
Zn
28.0 59.9 73.9 12.3 21.9 24.2 63.8 12.3 20.9 67.7 23.3
70.7 40.0 26.1 50.4 71.8 69.6 36.2 12.3 70.5 32.3 72.7
1.3 – – 37.3 6.2 5.9 – 37.3 8.6 – 4.0
Fe2Al5−xZnx FeAl FeAl Fe2Al5−xZnx Fe2Al5−xZnx Fe2Al5−xZnx FeAl FeZn10 Fe2Al5−xZnx FeAl Fe2Al5−xZnx
which was identified as FeZn10 IMC. It was noted that the IMC was thicker at the fusion I/fusion II interface than that of the brazed interface. It was probably caused by the change of the extent of temperature and time dependent diffusion reaction. A homogeneous reaction area was formed in fusion zone II (Fig. 4(b)). It had the composition of 73.9 at% Fe and 26.1 at% Al (Table 4), and it was identified as FeAl. It was worth to mention that bright film-like structures were found to occupy the grain boundaries of FeAl IMC. At 2800 W laser power, Fe2Al5−xZnx and FeZn10 were also generated at the fusion zone I/fusion zone II interface and the brazed interface. The morphology of Fe2Al5−xZnx was changed from flat to tortuous (Fig. 4(d) and f). Besides, some opening cracks were observed around the FeAl grains in fusion zone II, where films-like structures were also observed (Fig. 4(e) and Table 4). At 3000 W laser power, aside from similar phase constituent to that of 2600 W and 2800 W (Fig. 4(g-i) and Table 4), the morphology of Fe2Al5−xZnx was changed even more
3.3. Mechanical properties Fig. 6 shows relation between the fracture load and bonding length of laser Al/steel joints with different laser powers. The fracture load increased initially as the laser power increased and then decreased with the further increase of laser power, while the interfacial bonding length was monotonously increased from 3.0 mm to 4.2 mm with the increase of laser power. When the laser power was less than 2800 W, it was clearly that the increase of lengths of fusion I/fusion II interface and brazed interface had a positive effect on the fracture load of the sample. At 2800 W laser power, the joint reached the highest fracture load ~1225 N. With the further increase of laser power, the fracture load started to decrease which was closely associated with the distribution of IMC in fusion zone, which will be discussed later. The fracture load is 4
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significantly improved compared to the mechanical properties of the Al/steel joints in previous brazed welding studies. Fig. 7 shows microhardness profiles across the fusion zones and steel in the laser joints at different laser powers. Fig. 7(c) shows the microhardness profile at the fusion zone I/fusion zone II interface. The average microhardness of fusion zone I and fusion zone II were 80.1 HV and 322.1 HV, respective. It was obviously that IMC layer consisting of Fe2Al5−xZnx and FeZn10 had the highest microhardness ~406.7 HV due to the inherent and nature hardness and brittleness of Fe-Al IMC [18]. However, the microhardness of this two-phase layer (Fe2Al5−xZnx and FeZn10) is significantly lower than that of Fe2Al5 ~ 1100HV [19], which is due to the formation of soft FeZn10. It was noted that the microhardness of the FeAl phase is only around 400 HV [20]. Moreover, Rathod et al. reported that the Fe-Al IMCs were divided into two groups are grouped, i.e., soft and tough Fe-rich IMCs (FeAl and Fe3Al) and hard and brittle Al-rich IMCs (FeAl2, Fe2Al5, and FeAl3) [21]. Thus, FeAl was able to decrease the microhardness of IMC thus improving the joint fracture load [22,23], as shown in Fig. 6. Fig. 7(d) shows the microhardness profile at the brazed interface. Similarly, the microhardness of IMC layer increased sharply, which was attributed to the formation of hard and brittle IMC layer.
3.4. Fractography Fig. 8(a–c) presented fracture location of the joints at 2400 W, 2800 W and 3000 W laser powers, respectively. When the laser power ranged from 2400 W to 2800 W, similar fracture locations (along fusion zone I/fusion zone II interface and brazed interface) were observed. However, the fracture location changed to inside fusion zone I when the laser power increased to 3000 W. At 2400 W laser power, the crack propagated along the interface between IMC layer and fusion zone II near fusion zone side and the interface between IMC layer and steel near steel side (Fig. 8(d) and (g)). Similar crack propagation path was observed at 2800 W laser power (Fig. 8(e) and (h)). The fracture position of the joint with 3000 W laser power was changed to inside the fusion zone I as seen in Fig. 8(c). It was seen from Fig. 8(f) and (i) that some large Fe-Al-Zn IMC particles was formed adjacent to the fusion zone I/Al interface, which had the chemical composition of 74.2 at% Fe, 22.1 at% Al and 3.8 at% Zn. These brittle IMCs particles were able to deflect the crack into fusion zone I, which led to the reduction in joint fracture load.
3.5. Joining mechanism
Fig. 5. EDS line scanning in fusion zone I and fusion zone II for Al/steel joints at different laser powers: (a) 2400 W, (b) 280000 W and (c) 3000 W.
Based on the analyses above, the joining mechanism of steel to aluminum by laser fusion welding was clarified with aid of the schematic illustrations in Fig. 9. Firstly, Zn-22Al filler metal melted and covered the steel base metal under the laser irradiation (Fig. 9(a)). As the continuous deposition of molten filler metal droplets, part of base metal Al alloy was melted and mixed with molten filler metal (Fig. 9(b)). At the direct laser irradiation zone, the steel substrate started to melt forming fusion zone II when the temperature reached 1583 °C. At the weld root (laser unirradiated zone), the brazed interface was produced by the spreading of the melt from fusion zone I. Meanwhile, Zn and Al elements from Zn-22Al filler metal were extensively diffused into the liquid steel and solid steel sides under elevated temperature (Fig. 9(c)). After cooling, FeAl phase first precipitated at the laser direct irradiation zone as the temperature decreased to 1310 °C forming fusion zone II. When the temperature decreased to 1169 °C, Fe2Al5−xZnx started to solidify at FeAl surface by heterogeneous nucleation. Finally, FeZn10 was formed at grain boundaries of Fe2Al5−xZnx owing to the grain boundaries of liquid Zn-Al and the following metallurgical reactions (Fig. 9(e)) [24]. When the joint cooled to room temperature, the final joint was obtained, as shown in Fig. 9(f).
Fig. 6. The average fracture loads and bonding length of joints at different laser powers [22,23].
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Fig. 7. (a) and (b) Schematic illustration of indentation distribution along longitudinal direction, and longitudinal direction, respectively, (c) and (d) microhardness profiles of across fusion zone I/fusion zone II interface and brazed interface, respectively.
4. Conclusions
(2) Newly formed FeAl IMC had much lower hardness than Fe2Al5−xZnx, which decreased the hardness and brittleness of interfacial region and improved the joint fracture load. (3) The phase constituents kept unchanged with the increase of laser power increased, while the morphology of interfacial phases and bonding length changed significantly. The maximum joint fracture load reached 1225 N at laser power of 2800 W. (4) Two fracture modes were observed. When laser power was less than 2800 W, the joint fracture at the interfaces between fusion zone I and fusion zone II as well as fusion zone I and steel. When laser power increased to 3000 W, the fracture occurred inside fusion zone I.
In this study, the influence of laser power on microstructure and mechanical properties of laser fusion welded Al/steel joints using a Znbased filler wire was investigated. The main conclusions were summarized as follows: (1) A fusion welded region consisting two fusion zones, i.e., fusion zone I and fusion zone II, was formed which was composed of Fe2Al5−xZnx, FeZn10 and FeAl IMCs at the interface, while Fe2Al5−xZnx and FeZn10 phase were formed at the brazed region between fusion zone I and steel.
Fig. 8. The fracture locations of the joints at laser power of 2400 W, 2800 W and 3000 W: (a–c) fractured paths for the joints at laser power of 2400 W, 2800 W and 3000 W, respectively, (d and g) red-square-highlighted region in Fig. 8 (a), (e and h) red-squarehighlighted region in Fig. 8(b), (f) redsquare-highlighted region in fusion zone I in Fig. 8(c), and (i) enlarged view in (f). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 9. Schematic of joining mechanism: (a) melting of liquid filler under laser irradiation, (b) melting of Al base metal, (c) melting of steel and elemental diffusion, (d) formation of FeAl in the fusion zone II, (e) formation of Fe2Al5–xZnx phase and FeZn10 phase at the fusion zone I/fusion zone II and brazed interface; (f) formation of laser Al/steel joint.
Acknowledgement
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The authors would like to acknowledge the support provided by National Natural Science Foundation of China (51805315, 51665038 and 51775091), Zhejiang Key Project of Research and Development Plan (2019C01114), the Jiangxi Science Fund for Distinguished Young Scholars (2018ACB21015) & the Talent Program of Shanghai University of Engineering Science. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optlastec.2019.105882. References [1] W. Guo, Z. Wan, P. Peng, Q. Jia, G. Zou, Y. Peng, Microstructure and mechanical properties of fiber laser welded QP980 steel, J. Mater. Process. Technol. 256 (2018) 229–238. [2] F. Khodabakhshi, L.H. Shah, A.P. Gerlich, Dissimilar laser welding of an AA6022AZ31 lap-joint by using Ni-interlayer: Novel beam-wobbling technique, processing parameters, and metallurgical characterization, Opt. Laser Technol. 112 (2019) 349–362. [3] H. Li, J. Zhang, X. Wang, The study of magnetic field assisted system for tungsten inert gas welding of magnesium alloy and steel, Mater. Res. Express 6 (8) (2019) 0865d3. [4] J. Yang, Z. Yu, Y. Li, H. Zhang, N. Zhou, Laser welding/brazing of 5182 aluminium alloy to ZEK100 magnesium alloy using a nickel interlayer, Sci. Technol. Weld.
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Full length article
Microstructure and mechanical properties of laser fusion welded Al/steel joints using a Zn-based filler wire
T
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Jiahao Sua, Jin Yanga,b, , Yulong Lic, Zhishui Yua,b, Jieshi Chena,b, Wanqin Zhaoa,b, ⁎ Hongbing Liua,b, , Caiwang Tand a
School of Materials Engineering, Shanghai University of Engineering Science, Shanghai 201620, China Shanghai Collaborative Innovation Center of Laser Advanced Manufacturing Technology, Shanghai 201620, China c Key Lab for Robot and Welding Automation of Jiangxi Province, Mechanical and Electrical Engineering School, Nanchang University, Nanchang 330031, China d State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China b
H I GH L IG H T S
fusion welding of Al alloy to steel using Zn-based filler wire was performed. • Laser FeAl IMC was formed at the interface, which enhanced the fracture load. • New fracture load first increased and then decreased with the rising laser power. • The • The fracture modes of the joints changed with the laser power.
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser fusion welding Dissimilar metals Intermetallic compounds Microhardness Fracture load
Laser fusion welding technique was used for lap joining of Q235 steel to AA5052 aluminum alloy using a fluxcored Zn-22Al filler wire. The influence of laser power on the microstructure and mechanical properties of laser Al/steel joints was investigated. A fusion welded region and a brazed region were formed at the fusion zone/steel interface: Fe2Al5−xZnx and FeZn10 were formed at the both regions, while a new FeAl IMCs with much lower hardness and brittleness was observed in the welded region. The phase constitutions at the both regions were unvaried with the change of laser power, while the phase morphologies were slightly changed. The tensile-shear testing results showed that the joint fracture load first increased then decreased with the rising laser power and the maximum value reached 1225 N at laser power of 2800 W. The joint fractured at the fusion zone/steel interface when the laser power was less than 2800 W and it changed into fusion zone when the laser power reached 3000 W. The change of joint fracture load and fracture behavior was mainly attributed to the change of morphology of IMC and interfacial bonding length.
1. Introduction As countries pay more and more attention to energy conservation and emission reduction, the need of light-weight automotive structure is very prominent. The composite structure of aluminum, magnesium alloys and steels has the comprehensive advantages of light weight and high strength [1–4]. Therefore, the dissimilar combination of aluminum alloy and steel is necessary in the fabrication of hybrid structural parts. However, due to the huge differences of thermal physic properties and solid solubility between aluminum and steel, the joining of aluminum to steel is still a challenge. The formation of intermetallic compound (IMC), such as FeAl,
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FeAl2, FeAl3 and Fe2Al5 is a key factor affecting the quality of Al/steel welds. The welds may suffer from heavy cracking in service due to the inherent and nature brittleness of IMC [5,6]. To achieve a sound Al/ steel weld, the thickness of the Fe-Al IMC should be controlled below 10 μm [7]. Therefore, the development of adequate welding techniques to achieve high efficiency and high quality of Al/steel joining is urgent. Many welding technologies have been used to join aluminum alloy and steel. By virtue of high energy density, low distortion and narrow heat affected zone, laser welding/brazing was employed in the Al/steel joining. Alloy elements Si and Zn were commonly used to improve the joint quality [8–13]. Tan et al. [8] demonstrated that the laser Al/steel joint with the highest tensile strength was obtained by using Zn-22Al
Corresponding authors. E-mail addresses: [email protected] (J. Yang), [email protected] (H. Liu).
https://doi.org/10.1016/j.optlastec.2019.105882 Received 9 June 2019; Received in revised form 29 August 2019; Accepted 29 September 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
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filler metal among various Zn-based filler metals. Yang et al. [9] proved that Si was able to inhibit the growth of the interfacial reaction layer, resulting in an improvement of the fracture load of laser Al/steel joints, while Zn was able to reduce the brittleness of the reaction layer and consequently increased the fracture load. Qin et al. [10] confirmed that the tensile strength of Al/steel joints with ER4043(Al-5Si) filler metal firstly increased and then decreased with the increase of laser power, and the highest tensile strength of the joint reached 247.3 MPa, which was 85% of that of the base metal of 6013-T4 aluminum alloy. Dharmendra et al. [11] concluded that the thickness of IMC layer varied from 3 μm to 23 μm in laser Al/steel joint with Zn-Al filler metal, and the joints exhibited highest mechanical resistance of 220 MPa when the IMCs thickness ranged between 8 μm and 12 μm. Sun et al. [12] reported that all laser Al/steel joints failed at the brazed interface during tensile testing, and the maximum tensile strength ~120 MPa was achieved. The fracture surface was typical cleavage fracture mode, and the crack initiated and propagated in FeAl3 layer. Based on aforementioned literature, the effect of Si addition on the formation of Fe-Al IMCs has been established, that is, Si can reduce the IMCs layer thickness and thereby improving the joint mechanical properties. For the element Zn, it promotes the formation of tough and dispersed FeZn10 IMCs in layered Fe2Al5−xZnx, hence offering higher crack resistance during tensile loading and increases joint strength [14]. Nevertheless, the previous studies were mainly focused on the dissimilar Al/steel joints by laser welding/brazing technology; while, the influence of element Zn on the microstructure and mechanical properties of laser Al/steel joints using laser fusion welding technology was still unclear. Therefore, the purpose of this study is to investigate the influence element Zn on the microstructure and mechanical properties of dissimilar Al/steel joint using laser fusion welding. The results have shown that, in addition to FeZn10 and Fe2Al5−xZnx that commonly observed in laser welded/brazed joints [10,11], a new Fe-Al IMC, FeAl with much lower hardness and brittleness, is formed in laser fusion welded Al/steel joints. Thus, the joint fracture load was enhanced.
Table 2 Mechanical properties of AA5052 aluminum and Q235 steel. Materials
Tensile strength/MPa
Yield strength/MPa
AA5052 Al Q235 steel
260 460
190 235
Fig. 1. Schematic of laser fusion welding of Al to steel with Zn-22Al filler metal. Table 3 Process parameters in laser fusion welding of Al to steel. Laser power/ W
Defocusing amount/mm
Welding speed/m∙min−1
Wire feeding speed/m∙min−1
Flowing rate of shielding gas/L∙min−1
2400 2600 2800 3000
+30
0.5
2.0
15
After the welding, welding slag was removed by a stainless steel wire brush. Cross-sections of the laser joints were cut, and then mounted in phenolic resin. The samples were mechanically ground using 400, 600, 800, 1000 and 1200 grades SiC abrasive papers followed by polishing with a 1-μm diamond suspension. To analyze the microstructure and determine the chemical composition of the interfacial phases, the cross-sections were observed using Olympus 4XCJZ optical microscope and Hitachi S3400-N scanning electron microscope (SEM) equipped with EDAX Genesis energy-dispersive X-ray spectroscopy (EDS) analysis. The tensile-shear testing was evaluated at a cross-head speed of 1.0 mm/s. The shape and dimension of tensile specimens were shown in Fig. 2. The microhardness measurements were conducted on a computer automated machine at the load of 25 g and 15 s dwell time.
2. Experimental procedures Laser fusion welding was carried out by using the IPG YLS-5000 fiber laser processing system. Q235 galvanized steel was used in the dimension of 150 mm × 100 mm × 1.8 mm. AA5052 aluminum alloy was used in the dimension of 150 mm × 100 mm × 2.0 mm. Their chemical compositions and mechanical properties were shown in Tables 1 and 2. A flux-cored Zn-22Al alloy with a diameter of 1.6 mm was used as the filler metal. The corresponding chemical compositions of the Zn-22Al filler metal were listed in Table 1. Before welding, the work pieces were ground with abrasive paper and then cleaned with acetone to remove surface contaminant. The aluminum alloy was placed on top of the galvanized steel. The angle between shielding gas nozzle and filler wire was 90° and the laser head was perpendicular to the base metal, as shown in Fig. 1. To prevent oxidation during laser welding, pure argon (99.99 wt%) was used. Laser power was chosen as the only variable in the laser welding processes. Preliminary laser welding was conducted to obtain a visually acceptable weld appearance. The optimized parameters were listed in Table 3.
3. Results and discussion 3.1. Macrostructure Fig. 3 shows the weld appearances and cross-sections of joints with
Table 1 Chemical compositions of base materials and filler metal in weight percent (wt %). Materials
Cu
Mg
Mn
Fe
Si
Zn
Cr
Al
C
Ni
AA5052 Al Q235 steel Zn-22Al
0.1 – 0.8
2.2–2.8 – –
0.1 0.147 –
0.4 Bal. –
0.25 0.35 –
0.1 – Bal.
0.2 0.044 –
Bal. – 22
– 0.2 –
– – –
Fig. 2. Shape and dimension of tensile specimens. 2
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Fig. 3. Weld appearances and cross sections of laser joints produced with different laser powers: (a–b) 2400 W, (c–d) 2600 W, (e–f) 2800 W, (g–h) 3000 W.
interface were increased with the increase of laser power.
different laser powers. Fig. 3(a, c, e and g) illustrated the weld appearances of laser fusion welded Al/steel joint using Zn-22Al filler wire with 2400 W, 2600 W, 2800 W and 3000 W laser powers, respectively. Visually acceptable weld appearances were obtained in the laser power range. Fig. 3(b, d, f and h) showed the cross sections of joints with the laser powers of 2400 W, 2600 W, 2800 W and 3000 W, respectively. Two fusion zones were identified in the joints. Adjacent to the Al side, Zn-Al filler wire and parts of Al based metal were melted by laser irradiation and the melt solidified into fusion zone I during cooling. Adjacent to the steel side, certain amount of steel was melt under laser irradiation, and then it solidified into fusion zone II. In addition to fusion zones, brazed interface was formed at the interface, which was caused by the wetting and spreading of the melt in fusion zone I [15]. Fig. 3 also shows that the sizes of fusion zone I, fusion II and brazed
3.2. Microstructure According to the microstructure, three zones adjacent to the fusion zones and the brazed interface were chosen for further analysis, i.e., fusion zone I/fusion zone II interface, fusion zone II and brazed interface (Fig. 4). At 2400 W laser power, similar microstructure, i.e., a layered dark phase and a dispersed light phase, was observed at the fusion zone I/fusion zone II interface and the brazed interface (Fig. 4(a) and (c)). Based on EDS analysis, the layered dark phase was composed of 28.0 at% Fe, 70.7 at% Al and 1.3 at% Zn (Table 4), which was identified as Fe2Al5−xZnx IMC; while the dispersed light phase had the composition of 12.3 at% Fe, 50.4 at% Al and 37.3 at% Zn (Table 4), 3
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Fig. 4. The microstructure at selected regions of the joints with different laser powers: (a–c) 2400 W, (d–f) 2800 W, (g–i) 3000 W.
tortuous when comparing to that of 2800 W. Fig. 5 presents the corresponding EDS line scanning analysis at different laser powers. With the increase of laser power, similar element distribution was observed spanning from fusion zone I to steel. At IMC layer, a relatively stable element ratio Fe: (Al + Zn) of 7:3 was obtained, which corresponded to Fe2Al5−xZnx and FeZn10 identified in EDS point analysis. The shape change of element composition at the fusion zone I/IMC layer interface and the IMC layer/fusion zone II interface suggested the occurrence of intensive atomic diffusion. At fusion zone II, a relatively stable Fe and Al element distribution was obvious across the whole region with the ratio of 1:1. It corresponded to FeAl IMC identified in the EDS point analysis. Some occasional peaks of elements Al and Zn were found in Fig. 5(b–c) which was highlighted by dark arrows. The enrichment of Al and Zn corresponded to the white films formed at the grain boundaries of FeAl. This was mainly attributed to strong grain boundary penetration tendency of Zn-Al liquid [16,17].
Table 4 The corresponding EDS results at selected points as shown in Fig. 4. Selected points
1 2 3 4 5 6 7 8 9 10 11
Element at%
Possible phase
Fe
Al
Zn
28.0 59.9 73.9 12.3 21.9 24.2 63.8 12.3 20.9 67.7 23.3
70.7 40.0 26.1 50.4 71.8 69.6 36.2 12.3 70.5 32.3 72.7
1.3 – – 37.3 6.2 5.9 – 37.3 8.6 – 4.0
Fe2Al5−xZnx FeAl FeAl Fe2Al5−xZnx Fe2Al5−xZnx Fe2Al5−xZnx FeAl FeZn10 Fe2Al5−xZnx FeAl Fe2Al5−xZnx
which was identified as FeZn10 IMC. It was noted that the IMC was thicker at the fusion I/fusion II interface than that of the brazed interface. It was probably caused by the change of the extent of temperature and time dependent diffusion reaction. A homogeneous reaction area was formed in fusion zone II (Fig. 4(b)). It had the composition of 73.9 at% Fe and 26.1 at% Al (Table 4), and it was identified as FeAl. It was worth to mention that bright film-like structures were found to occupy the grain boundaries of FeAl IMC. At 2800 W laser power, Fe2Al5−xZnx and FeZn10 were also generated at the fusion zone I/fusion zone II interface and the brazed interface. The morphology of Fe2Al5−xZnx was changed from flat to tortuous (Fig. 4(d) and f). Besides, some opening cracks were observed around the FeAl grains in fusion zone II, where films-like structures were also observed (Fig. 4(e) and Table 4). At 3000 W laser power, aside from similar phase constituent to that of 2600 W and 2800 W (Fig. 4(g-i) and Table 4), the morphology of Fe2Al5−xZnx was changed even more
3.3. Mechanical properties Fig. 6 shows relation between the fracture load and bonding length of laser Al/steel joints with different laser powers. The fracture load increased initially as the laser power increased and then decreased with the further increase of laser power, while the interfacial bonding length was monotonously increased from 3.0 mm to 4.2 mm with the increase of laser power. When the laser power was less than 2800 W, it was clearly that the increase of lengths of fusion I/fusion II interface and brazed interface had a positive effect on the fracture load of the sample. At 2800 W laser power, the joint reached the highest fracture load ~1225 N. With the further increase of laser power, the fracture load started to decrease which was closely associated with the distribution of IMC in fusion zone, which will be discussed later. The fracture load is 4
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significantly improved compared to the mechanical properties of the Al/steel joints in previous brazed welding studies. Fig. 7 shows microhardness profiles across the fusion zones and steel in the laser joints at different laser powers. Fig. 7(c) shows the microhardness profile at the fusion zone I/fusion zone II interface. The average microhardness of fusion zone I and fusion zone II were 80.1 HV and 322.1 HV, respective. It was obviously that IMC layer consisting of Fe2Al5−xZnx and FeZn10 had the highest microhardness ~406.7 HV due to the inherent and nature hardness and brittleness of Fe-Al IMC [18]. However, the microhardness of this two-phase layer (Fe2Al5−xZnx and FeZn10) is significantly lower than that of Fe2Al5 ~ 1100HV [19], which is due to the formation of soft FeZn10. It was noted that the microhardness of the FeAl phase is only around 400 HV [20]. Moreover, Rathod et al. reported that the Fe-Al IMCs were divided into two groups are grouped, i.e., soft and tough Fe-rich IMCs (FeAl and Fe3Al) and hard and brittle Al-rich IMCs (FeAl2, Fe2Al5, and FeAl3) [21]. Thus, FeAl was able to decrease the microhardness of IMC thus improving the joint fracture load [22,23], as shown in Fig. 6. Fig. 7(d) shows the microhardness profile at the brazed interface. Similarly, the microhardness of IMC layer increased sharply, which was attributed to the formation of hard and brittle IMC layer.
3.4. Fractography Fig. 8(a–c) presented fracture location of the joints at 2400 W, 2800 W and 3000 W laser powers, respectively. When the laser power ranged from 2400 W to 2800 W, similar fracture locations (along fusion zone I/fusion zone II interface and brazed interface) were observed. However, the fracture location changed to inside fusion zone I when the laser power increased to 3000 W. At 2400 W laser power, the crack propagated along the interface between IMC layer and fusion zone II near fusion zone side and the interface between IMC layer and steel near steel side (Fig. 8(d) and (g)). Similar crack propagation path was observed at 2800 W laser power (Fig. 8(e) and (h)). The fracture position of the joint with 3000 W laser power was changed to inside the fusion zone I as seen in Fig. 8(c). It was seen from Fig. 8(f) and (i) that some large Fe-Al-Zn IMC particles was formed adjacent to the fusion zone I/Al interface, which had the chemical composition of 74.2 at% Fe, 22.1 at% Al and 3.8 at% Zn. These brittle IMCs particles were able to deflect the crack into fusion zone I, which led to the reduction in joint fracture load.
3.5. Joining mechanism
Fig. 5. EDS line scanning in fusion zone I and fusion zone II for Al/steel joints at different laser powers: (a) 2400 W, (b) 280000 W and (c) 3000 W.
Based on the analyses above, the joining mechanism of steel to aluminum by laser fusion welding was clarified with aid of the schematic illustrations in Fig. 9. Firstly, Zn-22Al filler metal melted and covered the steel base metal under the laser irradiation (Fig. 9(a)). As the continuous deposition of molten filler metal droplets, part of base metal Al alloy was melted and mixed with molten filler metal (Fig. 9(b)). At the direct laser irradiation zone, the steel substrate started to melt forming fusion zone II when the temperature reached 1583 °C. At the weld root (laser unirradiated zone), the brazed interface was produced by the spreading of the melt from fusion zone I. Meanwhile, Zn and Al elements from Zn-22Al filler metal were extensively diffused into the liquid steel and solid steel sides under elevated temperature (Fig. 9(c)). After cooling, FeAl phase first precipitated at the laser direct irradiation zone as the temperature decreased to 1310 °C forming fusion zone II. When the temperature decreased to 1169 °C, Fe2Al5−xZnx started to solidify at FeAl surface by heterogeneous nucleation. Finally, FeZn10 was formed at grain boundaries of Fe2Al5−xZnx owing to the grain boundaries of liquid Zn-Al and the following metallurgical reactions (Fig. 9(e)) [24]. When the joint cooled to room temperature, the final joint was obtained, as shown in Fig. 9(f).
Fig. 6. The average fracture loads and bonding length of joints at different laser powers [22,23].
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Fig. 7. (a) and (b) Schematic illustration of indentation distribution along longitudinal direction, and longitudinal direction, respectively, (c) and (d) microhardness profiles of across fusion zone I/fusion zone II interface and brazed interface, respectively.
4. Conclusions
(2) Newly formed FeAl IMC had much lower hardness than Fe2Al5−xZnx, which decreased the hardness and brittleness of interfacial region and improved the joint fracture load. (3) The phase constituents kept unchanged with the increase of laser power increased, while the morphology of interfacial phases and bonding length changed significantly. The maximum joint fracture load reached 1225 N at laser power of 2800 W. (4) Two fracture modes were observed. When laser power was less than 2800 W, the joint fracture at the interfaces between fusion zone I and fusion zone II as well as fusion zone I and steel. When laser power increased to 3000 W, the fracture occurred inside fusion zone I.
In this study, the influence of laser power on microstructure and mechanical properties of laser fusion welded Al/steel joints using a Znbased filler wire was investigated. The main conclusions were summarized as follows: (1) A fusion welded region consisting two fusion zones, i.e., fusion zone I and fusion zone II, was formed which was composed of Fe2Al5−xZnx, FeZn10 and FeAl IMCs at the interface, while Fe2Al5−xZnx and FeZn10 phase were formed at the brazed region between fusion zone I and steel.
Fig. 8. The fracture locations of the joints at laser power of 2400 W, 2800 W and 3000 W: (a–c) fractured paths for the joints at laser power of 2400 W, 2800 W and 3000 W, respectively, (d and g) red-square-highlighted region in Fig. 8 (a), (e and h) red-squarehighlighted region in Fig. 8(b), (f) redsquare-highlighted region in fusion zone I in Fig. 8(c), and (i) enlarged view in (f). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 9. Schematic of joining mechanism: (a) melting of liquid filler under laser irradiation, (b) melting of Al base metal, (c) melting of steel and elemental diffusion, (d) formation of FeAl in the fusion zone II, (e) formation of Fe2Al5–xZnx phase and FeZn10 phase at the fusion zone I/fusion zone II and brazed interface; (f) formation of laser Al/steel joint.
Acknowledgement
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The authors would like to acknowledge the support provided by National Natural Science Foundation of China (51805315, 51665038 and 51775091), Zhejiang Key Project of Research and Development Plan (2019C01114), the Jiangxi Science Fund for Distinguished Young Scholars (2018ACB21015) & the Talent Program of Shanghai University of Engineering Science. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optlastec.2019.105882. References [1] W. Guo, Z. Wan, P. Peng, Q. Jia, G. Zou, Y. Peng, Microstructure and mechanical properties of fiber laser welded QP980 steel, J. Mater. Process. Technol. 256 (2018) 229–238. [2] F. Khodabakhshi, L.H. Shah, A.P. Gerlich, Dissimilar laser welding of an AA6022AZ31 lap-joint by using Ni-interlayer: Novel beam-wobbling technique, processing parameters, and metallurgical characterization, Opt. Laser Technol. 112 (2019) 349–362. [3] H. Li, J. Zhang, X. Wang, The study of magnetic field assisted system for tungsten inert gas welding of magnesium alloy and steel, Mater. Res. Express 6 (8) (2019) 0865d3. [4] J. Yang, Z. Yu, Y. Li, H. Zhang, N. Zhou, Laser welding/brazing of 5182 aluminium alloy to ZEK100 magnesium alloy using a nickel interlayer, Sci. Technol. Weld.
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