- Email: [email protected]
Effect of reactive alloy elements on friction stir welded butt joints of metallurgically immiscible magnesium alloys and steel
Journal of Manufacturing Processes 39 (2019) 138–145
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Effect of reactive alloy elements on friction stir welded butt joints of metallurgically immiscible magnesium alloys and steel
T
Tianhao Wanga, Shivakant Shuklaa, Bharat Gwalania,b, Mageshwari Komarasamya, ⁎ Luis Reza-Nietoa, Rajiv S. Mishraa,b, a b
Center for Friction Stir Processing and Advanced Materials Manufacturing Processes Institute, University of North Texas, TX, USA Advance Materials and Manufacturing Processing Institute (AMMPI), University of North Texas, TX, USA
ARTICLE INFO
ABSTRACT
Keywords: Dissimilar metals Immiscible Butt joining Intermetallic compound Thermodynamic and kinetic calculation
Magnesium alloys (AZ31 and WE43) and steel (316 stainless steel, SS316) were butt welded through friction stir welding. Even though Mg and Fe are metallurgically immiscible, intermetallic compounds (IMCs) formed at the welded interface due to the existence of alloying elements in both magnesium alloys and steel. The formation of IMCs among alloying elements and the matrix were verified by thermodynamic and kinetic calculations and microstructural characterization. The elastic modulus mismatch and resultant stress concentration during tensile testing lead to interfacial fracture and the very existence of IMCs impaired dissimilar joint strength due to the brittleness of the IMCs.
Introduction Requirements to reduce fuel consumption and carbon dioxide release in the automobile industries pushed the use of lightweight structural alloys such as aluminum and magnesium alloys. Since use of high-strength materials such as steel is essential for overall structural efficiency, joining light metals and steel becomes especially relevant to the automobile industries to maintain design flexibility. Note that magnesium alloys are one of the most likely candidates to obtain lightweight structures with lowest density among structural alloys. Friction stir welding (FSW) is a well-known solid-state joining technique for obtaining joints between dissimilar metals in various weld configurations. Several derivatives of FSW have been applied for lap welding of dissimilar materials, such as self-reverting friction stir welding [1], friction stir scribe technology [2], hybrid friction stir welding assisted by friction surfacing [3], friction-based filling stacking joining [4], etc. High-strength joints between aluminum alloys and steel via FSW have been achieved by many researchers [5–8]. Previous publications have confirmed that the joint strength increased with reduction in brittle Al-Fe intermetallic compound (IMC) thickness [8]. Dissimilar butt welding between pure magnesium and steel via FSW resulted in no IMCs since they are metallurgically immiscible elements [9]. In dissimilar joining of magnesium alloys such as AZ31 containing Al as alloying element, and steel, joint strength increased with increasing aluminum content which in turn resulted in thicker Al-Fe
⁎
related IMCs [9]. This observation was in contradiction with results reported on various Al alloys/steel welds. According to previous studies [10,11], stress concentration existed at the softer material next to the welded interface during tensile test. Magnesium alloy with lower aluminum content has lower intrinsic strength, which led to early crack initiation in the magnesium alloy next to the welded interface. This might be a reason to explain this difference in observation by Tanaka [8] and Kasai [9]. In this study, magnesium alloys (AZ31 and WE43) of similar strength were friction stir butt welded with 316 stainless steel (SS316). Welding parameters were optimized to obtain defect-free dissimilar joints between Mg alloys/SS316. AZ31 alloy contains ˜3 wt. % aluminum and ˜1 wt. % zinc while WE43 alloy contains ˜4 wt. % Y and ˜3 wt. % Nd. Although Mg and Fe are metallurgically immiscible, Al and Zn alloying elements in AZ31 and Y and Nd alloy elements in WE43 along with Fe, Cr, Ni and Mn in SS316 can form various IMCs. Effects of various intermetallic forming alloying elements on the strength and fracture behavior of butt-welded AZ31/SS316 and WE43/SS316 joints were evaluated for both mechanical properties and microstructural evolution. Experimental AZ31, WE43 sheet (thickness ˜6.0 mm) and 316 stainless steel sheet (thickness ˜5.75 mm) were friction stir butt welded using a W-Re tool.
Corresponding author at: Center for Friction Stir Processing and Advanced Materials Manufacturing Processes Institute, University of North Texas, TX, USA. E-mail address: [email protected] (R.S. Mishra).
https://doi.org/10.1016/j.jmapro.2019.02.009 Received 7 November 2018; Received in revised form 16 January 2019; Accepted 18 February 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
Journal of Manufacturing Processes 39 (2019) 138–145
T. Wang, et al.
Table 1 Chemical composition of base materials. Base materials
AZ31 WE43 SS316
Chemical composition (in wt. %) Al
Zn
Cu
Y
Nd
Ze
Mg
C
Mn
Si
Cr
Ni
Mo
Fe
2.83 – –
0.80 – –
0.002 – –
– 4.0 –
– 3.0 –
– 0.5 –
Bal. Bal. –
– – 0.08
– – 2.0
– – 0.03
– – 17.0
– – 12.0
– – 2.50
– – Bal.
Results and discussion
Table 2 Welding parameters for AZ31/SS316 and WE43/SS316 joints. Base materials
Rotation rate (rpm)
Traverse speed (ipm)
AZ31/SS316
350 400 500 600 650 350 450 550 600 750
0.5 1 1, 2 2 1 0.5, 2 1 2 1, 2 2
WE43/SS316
Difference in physio-thermal properties of Mg alloys and steel requires that FSW parameters should be selected cautiously to obtain defect-free dissimilar joints. Welding parameters, rotation rate and traverse speed applied in this study for dissimilar butt welding of AZ31/ SS316 and WE43/SS316 are plotted in Fig. 1(a) and (b) to show parameter window for defect-free joints. The two FSW characteristic defects during FSW [12], lack of fill (Fig. 1(c)) and galling surface (Fig. 1(d)) were observed. Lack of fill appeared in the magnesium next to the faying interface when tool rotation rate was low or the traverse speed was high. It was because steel was not plasticized sufficiently, which obstructed magnesium flow around the tool pin. A galling surface appeared on the weld surface when the rotation rate was high because high tool rotation rate led to excessive heat which ultimately led to sticky conditions. An AZ31/SS316 joint with tool rotation rate of 500 rpm and traverse speed of 1 ipm and a WE43/SS316 joint with tool rotation rate of 600 rpm and traverse speed of 1 ipm were defect-free and were used for further analysis. As shown in Fig. 2, an Al concentration region along the AZ31/SS316 welded interface (green circle in Fig. 2(a) and (c)) implied that Al in the AZ31 matrix diffused towards the AZ31/SS316 welded interface during FSW, forming possibly Al-X (Fe, Cr or Ni) IMCs. It was because Al is miscible with Fe, Cr and Ni and forms various IMCs due to negative enthalpy of mixing. In addition, the steel that was cut and stirred into the AZ31 nugget existed as steel fragments (red circle in Fig. 2(a) and (d–f)), which is common in dissimilar butt welding between Al and steel [13]. Interestingly, Al-Mn IMC particles are observed at the welded interface between AZ31 and SS316 (green circle in Fig. 3(a), (c) and (e)). The microstructure of the base SS316 clarifies the origin of Al-Mn IMC particles. Mn-Mo rich particles exist in the base SS316 matrix (Fig. 3(g)). During FSW, Mn-Mo rich particles reacted with Al in the
The chemical composition of all the base materials is listed in Table 1. During FSW, the steel was placed on the advancing side (AS). The tool offset from the Mg/steel interface favored towards the Mg side by 2.5 mm. The W-Re tool had a threaded conical pin of pin length, pin diameter at root and tip, and shoulder diameter of 3.8 mm, 7.6 mm, 5.0 mm, and 16.0 mm, respectively. Welding parameters for AZ31/ SS316 and WE43/SS316 including tool rotational rate and traverse speed are listed in Table 2. Plunge depth in steel, thickness difference, tool offset and tool tilt angle were kept constant and the values are 4.2 mm, 0.25 mm, 2.5 mm, 2.5˚, respectively. Standard tensile testing (ASTM E8-04) was used to evaluate the strength of the Mg alloy/steel joints and the base materials. The dimensions of the specimens are as follows: gage length, width and thickness are ˜30.0, 6.3 and 3.5 mm, respectively. Microhardness measurements were conducted on the cross section of welded joints with a load of 200 g and dwell time of 10 s. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) equipped with energy-dispersive X-ray spectroscopy (EDS) were used to investigate the microstructure of base materials and the welded interface between Mg alloys and steel.
Fig. 1. Relationship between welding parameters (tool rotation rate and traverse speed) and defect formations for dissimilar butt welding of (a) AZ31/SS316 and (b) WE43/SS316, and two characteristic FSW defects (c) lack of fill and (d) galling surface. 139
Journal of Manufacturing Processes 39 (2019) 138–145
T. Wang, et al.
Fig. 2. (a) Backscattered SEM and (b–f) EDS mapping on the cross section of AZ31/SS316 welded interface. Note that the steel fragments are marked with red circles and the Al-rich region is marked with a green circle. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Fig. 3. (a) Backscattered SEM and (b–f) EDS mapping on the Al-Mn IMC region at the AZ31/SS316 welded interface and (g) backscattered SEM and EDS linear scan on the Mn-rich particle in the base SS316. Note that the steel fragments are marked with red circles and the Al-Mn chemical reaction region is marked with a green circle. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
AZ31 and formed Al-Mn IMCs. To further explain the formation of IMCs in this study, thermodynamics and kinetics calculations on Al-Fe and Al-Mn systems were conducted. Effective heat of formation (ΔH') for both Al-Fe and Al-Mn systems can be calculated based on the effective heat of formation (EHF) model:
H
Al-Fe and Al-Mn systems are reported in [14,15]. At the lowest liquids composition of the Al-Fe and Al-Mn phase diagrams (˜0.90 atomic %), the calculated values of ΔH' for Al-Mn and Al-Fe based IMCs are labelled and listed in Fig. 4 and Table 3, respectively. The calculations indicated that formations of Al6Mn and Al4Mn were favored since their ΔH' values were more negative than those for other Al-Mn IMCs and AlFe IMCs. Based on kinetics data, diffusion length (IMC thickness) can be approximately predicted. The diffusion length (L) and diffusion coefficients (D) of Fe into Al, Mn into Al and Al into Fe can be calculated as:
H0
= ×
effective concentration of limiting element compound concentration of limiting element (1)
where H is the enthalpy change or heat of formation. H values for 140
L = (Dt)1/2
(2)
D = D0 exp(-Q/(RT))
(3)
Journal of Manufacturing Processes 39 (2019) 138–145
T. Wang, et al.
Fig. 4. Effective heat of formation at 298 K for various IMCs in (a) Al-Fe and (b) Al-Mn systems.
much larger than the expected diffusion length in dissimilar friction stir welding (< 5 μm). Plastic deformation of magnesium alloy during FSW can accelerate the diffusion process via increased vacancy concentration and dislocation density [21]. In addition, Mn-Mo-rich particles are broken during FSW with the formation of micro cracks, which might also enhance the diffusion of Al into Mn-Mo-rich particles. This might explain the high formation rate of the Al-Mn particles. Based on the calculated diffusion coefficients in Table 4, diffusion length of Fe into Al at 900 K can be obtained via L = (1.2 × 10−13 × t)1/2. When t ranged from 5 to 15 s [18], L equals to ˜0.8–1.3 μm. The calculated diffusion length is much larger than the measured value which is shown in the later section of the present paper. This is because the calculation was conducted based on the Al/Fe diffusion model, while Al in the AZ31 matrix is limited. As shown in Fig. 5, the Y and Nd concentration regions along the WE43/SS316 welded interface (green circle in Fig. 5(a), (c) and (d)) implied that Y and Nd in the WE31 matrix diffused towards the WE43/ SS316 welded interface during the FSW that formed IMCs. Besides, a Y and Nd depletion region (green arrow in Fig. 5(c) and (d)) existed next to the Y and Nd-rich region. In summary, all thermodynamically possibly-formed IMCs for AZ31/SS316 and WE43/SS316 (Table 5) indicated that multiple IMCs can form during FSW of AZ31/SS316 and WE43/SS316. IMC thickness for defect-free welded AZ31/SS316 and WE43/SS316 are ˜150 and 80 nm, respectively (Fig. 6(a) and (b)). To further study the alloying element diffusion at welded interface, TEM with EDS point analysis showed that Al diffused to welded interface region (points 3 and 4 in Fig. 6(a)) and Fe, Cr and Ni diffused into AZ31 matrix simultaneously (points 1 and 2 in Fig. 6(a)) for AZ31/SS316 welded joint. For WE43/SS316 welded interface, Y and Nd diffused to welded interface (point 3 in Fig. 6(b)) and SS316 matrix (point 2 in Fig. 6(b)) and Fe and Ni diffused into WE43 matrix (point 4 in Fig. 6(b)). Comparison of tensile test results for the AZ31/SS316 and WE43/ SS316 welded joints with the UTS of base materials (Fig. 7(a)) confirmed no significant difference in UTS of AZ31/SS316 joints and WE43/SS316 joints. The UTS of dissimilar joints is ˜160 MPa (˜73% of base Mg alloys). Microhardness profile of the cross section of welded
Table 3 Effective heat formation values for Al-Fe and Al-Mn IMCs. System
IMCs
ΔH° (kJmol−1at−1)
ΔH' (kJmol−1at−1)
Al-Fe
Al6Fe Al13Fe4 Al5Fe2 Al2Fe AlFe3 Al6Mn Al4Mn Al11Mn4 Al8Mn5
−11 −19 −22 −25 −22 −15 −21 −23 −26
−0.69 −0.68 −0.69 −0.68 −0.26 −0.93 −0.93 −0.76 −0.60
Al-Mn
where t is the time gap when FSW temperature is higher than diffusion requirement, D0 (m2/s) is the pre-exponential factor, Q (J/mol) is the activation energy, R (J/mol·K) is the gas constant and T (K) is the absolute temperature. The limited values of D0, Q and T were obtained from literature [16,17], and R is the gas constant (8.314 J/mol). The calculated D values are summarized in Table 4. Note that peak temperature is ˜600–750 K for friction stir welding of AZ31 magnesium alloy [19], and ˜900–1050 K for friction stir welding of 304 stainless steel [20]. Peak temperature at interface during friction stir welding of magnesium alloy and steel should be within these two ranges. Furthermore, the peak temperature needs to be close to the range of 900–1050 K to plasticize steel during friction stir butt welding between magnesium alloy and steel. The melting points of AZ31 and WE43 magnesium alloys are both ˜900 K. Therefore, it is reasonable to assume the peak temperature at magnesium alloy/steel interface during friction stir butt welding was similar and less than ˜ 900 K. The diffusion coefficient of Fe into Al at 900 K being much larger than the diffusion of Al in Fe at 1173 K implies that the diffusion of Fe into Al is favored and is dominant in the Al-Fe system when the temperature ranged from 900 to 1173 K. For the Al-Mn system, the D of Mn into Al at 900 K is negligible as compared with the diffusion coefficient in the Al-Fe system; and the data for diffusion of Al into Mn are not available. However, an abnormal diffusion behavior of Al into Mn was observed in this study. Al diffused into Mn-Mo-rich particles of around ˜10 μm in size, which is Table 4 Calculation of diffusion coefficients in Al-Fe and Al-Mn systems. Diffusion process
Temperature range (K)
D0 (m2/s)
Q (kJ/mol)
D (m2/s)
Reference
Fe into Al Mn into Al Al into Fe Al into Mn
793–922 773–918 1173–1358
5.3 × 10−3 3.8 × 10−2 1.0 × 10−2
183.4 221.8 267.1
1.2 × 10−13 (900 K) 5.1 × 10−15 (900 K) 1.3 × 10−14 (1173 K)
[16] [16] [17]
a
a
a
a
Not available.
141
a
Journal of Manufacturing Processes 39 (2019) 138–145
T. Wang, et al.
Fig. 5. (a) Backscattered SEM and (b–h) EDS mapping on the cross section of WE43/SS316 welded interface. Note that the steel fragments are marked with red circles, the Y and Nd-rich region is marked with a green circle, and the Y and Nd depletion regions are marked with green arrows. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
between AZ31 and WE43. Hardness values in the TMAZ of the steel side is higher than that of the base steel. The hardness increase in the SZ is based on recrystallization, and the hardness increase in the TMAZ of the steel side is linked to work-hardening and the resultant recrystallization. Both AZ31/SS316 and WE43/SS316 joints fractured through welded interface (Fig. 7(a)). IMC with thickness of ˜80-150 nm should not be the main factor for fracture at welded interface because IMC thickness needs to be greater than a few μm to be deleterious for tensile properties of the weld [7]. Previous studies on butt welding of copper/ steel [10] and aluminum alloy/steel [11] showed that stress concentration occurred at weld interface at the early stage of tensile testing due to the mismatch of elastic modulus. Therefore, stress concentration at the welded interface during tensile testing could be the main reason for interfacial fracture for AZ31/SS316 and WE43/SS316 joints. Fractography of AZ31/SS316 dissimilar joint (Figs. 8 and 9) revealed certain features on the fractured surface of the SS316 part (Fig. 8(a)) that called for further investigation (Fig. 8(b)) and EDS mapping (Fig. 8(c–g)) on the region selected in Fig. 8 (a). The surface of the
Table 5 Possible IMCs formed during FSW of AZ31/SS316 and WE43/SS316. Intermetallics
SS316
Fe
WE43
Mg
Al
Zn
Mg
Y
Nd
̶
Al13Fe4 Al5Fe2… Al7Cr Al11Cr2 Al3Ni Al3Ni2… Al12Mn A6Mn…
ZnFe Zn4Fe Zn13Cr Zn17Cr ZnNi
̶
Fe17Y2 Fe23Y6… ̶
Fe8Nd Fe7Nd2
Ni17Y2 Ni5Y… Mn12Y Mn23Y6…
a
Cr
̶
Ni
MgNi2 Mg2Ni ̶
Mn a
AZ31
Zn9Mn
̶ MgNi2 Mg2Ni
a
a
Not available ― Immiscible.
AZ31/SS316 and WE43/SS316 joints (Fig. 7(b)) revealed that stir zone (SZ) has higher hardness than thermomechanically affected zone (TMAZ), heat affected zone (HAZ) and base material (BM) of magnesium alloys; and that there is no obvious difference in hardness values
Fig. 6. TEM images and EDS point analysis on welded interface of (a) AZ31/SS316 and (b) WE43/SS316.
142
Journal of Manufacturing Processes 39 (2019) 138–145
T. Wang, et al.
Fig. 7. (a) UTS of AZ31/SS316 joint, WE43/SS316 joint, base AZ31, base WE43, and base SS316 and (b) microhardness profile across the weld cross section of both AZ31/SS316 and WE43/SS316 joints. Fig. 8. (a) Backscattered SEM of fractured surface of SS316 part of AZ31/SS316 joint, (b) back scattered SEM of selected region in red rectangle in (a) and (c–g) EDS mapping of region (b). Note that broken Al-Mn particles are marked with red circles and sticking Mg on the fractured SS316 part are marked with green circles. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
fractured SS316 part consisted of steel and Mg sticking to it (green circles in Fig. 8(b) and (f)). In addition, broken Al-Mn particles formed at the welded interface can be observed (denoted by red circles in Fig. 8(b), (e) and (g)). Next, the surfaces of the fractured AZ31 part (Fig. 9(a)) revealed steel fragments along with Al-Fe IMCs (green circles in (Fig. 9(b), (d) and (e)) remaining on the fractured surface of the AZ31 part. Also, broken Al-Mn particles were observed on the AZ31 side as well (red circles in Fig. 9 (f), (h) and (i)). Existence of Al-Mn particles and Al-Fe IMCs at the fractography of welded AZ31/SS316 interface displayed that IMCs might also impaire the joint strength. Similarly, fractography of dissimilar joining of WE43/SS316 (Figs. 10 and 11), backscattered SEM (Fig. 10(b)) and EDS mapping (Fig. 10(c–h)) on the fractured surface of the SS316 part of WE43/ SS316 joints were conducted on the region marked in Fig. 10(a). The surface of the fractured SS316 part consisted of steel with Y and Nd
diffusing into it (red arrow in Fig. 10(b) and (d–h)) and Mg sticking to it (green arrows in Fig. 10(b) and (c)). On the other hand, the surface of the fractured WE43 part (Fig. 11(a)) showed some steel fragments (red arrows in (Fig. 11(b) and (f–h)) remaining on the fractured surface of the WE31 part. Conclusions (1) Interdiffusion occurred between alloying elements in magnesium alloys and 316 stainless steel, and alloying elements in the magnesium alloys (eg. Al in AZ31 and Y and Nd in the WE43) diffused towards to the welded interface to form an Al-rich region for welded AZ31/SS316 and a Y-Nd-rich region for welded WE43/ SS316. The IMC thickness are ˜150 and 80 nm for welded AZ31/ SS316 and WE43/SS316, respectively.
143
Journal of Manufacturing Processes 39 (2019) 138–145
T. Wang, et al.
Fig. 9. (a) Backscattered SEM of fractured surface of AZ31 part of AZ31/ SS316 joint, (b) backscattered SEM of selected region in red rectangle in (a), (c–e) EDS mapping of region (b), (f) backscattered SEM of selected region in green rectangle in (a) and (g–i) EDS mapping of region (f). Note that broken Al-Mn particles are marked with red circles and Al-Fe IMCs on the fractured SS316 part are marked with green circles. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Fig. 10. (a) Backscattered SEM of fractured surface of SS316 part of WE43/SS316 joint, (b) backscattered SEM of selected region in red rectangle in (a) and (c–h) EDS mapping on the selected region of (b). Note that sticking Mg on the fractured SS316 surface is marked with green arrows and steel with Y and Nd diffusing into it are marked with red arrows. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
(2) Ultimate tensile strength of dissimilar butt joints of AZ31/SS316 and WE43/SS316 reached ˜160 MPa (˜73% of base Mg alloys)). Welded AZ31/SS316 and WE43/SS316 broke through welded interface during tensile testing mainly because elastic modulus mismatch between magnesium alloy and steel and resultant stress
concentration. Existence of IMCs at weld interface only impaired the joint strength to some extent. (3) An abnormal diffusion of Al into Mn-Mo rich particles in SS316 was observed and verified by thermodynamics and kinetics calculations in the butt welding of AZ31 and 316 stainless steel. 144
Journal of Manufacturing Processes 39 (2019) 138–145
T. Wang, et al.
Fig. 11. (a) Backscattered SEM of fractured surface of WE43 part of WE43/SS316 joint, (b) backscattered SEM of selected region in red rectangle in (a) and (c–h) EDS mapping on the selected region of (b). Note that sticking steel fragments on the fractured WE43 surface are marked with red arrows and the remaining Mg is marked with green arrows. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Acknowledgments [9]
This work was supported under the NSF-IUCRC grant for Friction Stir Processing (NSF-IIP 1157754). The additional support of Boeing, General Motors, Pacific Northwest National Laboratory, Army Research Laboratory and Korea Aerospace Research Institute for the UNT site is acknowledged. This report was prepared as an account of work sponsored by an agency of the US Government. The views and opinions of the authors expressed herein do not necessarily state or reflect those of the US Government or any agency thereof. We also acknowledge the UNT Materials Research Faculty (MRF).
[10] [11] [12] [13] [14]
References
[15]
[1] Huang Y, Wang J, Wan L, Meng X, Liu H, Li H. Self-riveting friction stir lap welding of aluminum alloy to steel. Mater Lett 2016;185:181–4. [2] Y. Hovanski, G.J. Grant, S. Jana, Mattlin, K.F., Battelle Memorial Institute Inc, 2013. Friction stir welding tool and process for welding dissimilar materials. U.S. Patent 8, 434,661. [3] Huang Y, Lv Z, Wan L, Shen J, dos Santos JF. A new method of hybrid friction stir welding assisted by friction surfacing for joining dissimilar Ti/Al alloy. Mater Lett 2017;207:172–5. [4] Huang Y, Meng X, Xie Y, Li J, Wan L. New technique of friction-based filling stacking joining for metal and polymer. Compos Part B: Eng 2019;163:217–23. [5] Lee WB, Schmuecker M, Mercardo UA, Biallas G, Jung SB. Interfacial reaction in steel–aluminum joints made by friction stir welding. Scr Mater 2006;55(4):355–8. [6] Kimapong K, Watanabe T. Friction stir welding of aluminum alloy to steel. Weld J 2004;83(10):277. [7] Jiang WH, Kovacevic R. Feasibility study of friction stir welding of 6061-T6 aluminium alloy with AISI 1018 steel. Proc Inst Mech Eng Part B J Eng Manuf 2004;218(10):1323–31. [8] Tanaka T, Morishige T, Hirata T. Comprehensive analysis of joint strength for
[16] [17] [18] [19] [20] [21]
145
dissimilar friction stir welds of mild steel to aluminum alloys. Scr Mater 2009;61(7):756–9. Kasai H, Morisada Y, Fujii H. Dissimilar FSW of immiscible materials: steel/magnesium. Mater Sci Eng A 2015;624:250–5. Wang T, Shukla S, Nene SS, Frank M, Wheeler RW, Mishra RS. Towards obtaining sound butt joint between metallurgically immiscible pure Cu and stainless steel through friction stir welding. Metall Mater Trans A 2018;49(7):2578–82. Wang T, Komarasamy M, Liu K, Mishra RS. Friction stir butt welding of strainhardened aluminum alloy with high strength steel. Mater Sci Eng A 2018;737:85–9. Arbegast WJ. A flow-partitioned deformation zone model for defect formation during friction stir welding. Scr Mater 2008;58(5):372–6. Uzun H, Dalle Donne C, Argagnotto A, Ghidini T, Gambaro C. Friction stir welding of dissimilar Al 6013-T4 to X5CrNi18-10 stainless steel. Mater Des 2005;26(1):41–6. Theron CC, Ndwandwe OM, Lombaard JC, Pretorius R. First phase formation at interfaces: comparison between Walser-Bené and effective heat of formation model. Mater Chem Phys 1996;46(2–3):238–47. Liu XJ, Ohnuma I, Kainuma R, Ishida K. Thermodynamic assessment of the aluminum-manganese (Al-Mn) binary phase diagram. J Phase Equilibria Diffus 1999;20(1):45. Du Y, Chang YA, Huang B, Gong W, Jin Z, Xu H, Yuan Z, Liu Y, He Y, Xie FY. Diffusion coefficients of some solutes in fcc and liquid Al: critical evaluation and correlation. Mater Sci Eng: A 2003;363(1–2):140–51. MADELUNG O. Numerical data and functional relationships in science and technology. Landolt Bornstein, New Series, Group III; 1982. p. 22. Liu X, Lan S, Ni J. Analysis of process parameters effects on friction stir welding of dissimilar aluminum alloy to advanced high strength steel. Mater Des 2014;59:50–62. Chang CI, Lee CJ, Huang JC. Relationship between grain size and Zener–Holloman parameter during friction stir processing in AZ31 Mg alloys. Scr Mater 2004;51(6):509–14. Cho JH, Boyce DE, Dawson PR. Modeling strain hardening and texture evolution in friction stir welding of stainless steel. Mater Sci Eng A 2005;398(1–2):146–63. Nagahama K, Miki I. Precipitation during Recrystallization in Al–Mn and Al–Cr alloys. Trans Jpn Inst Met 1974;15(3):185–92.
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Effect of reactive alloy elements on friction stir welded butt joints of metallurgically immiscible magnesium alloys and steel
T
Tianhao Wanga, Shivakant Shuklaa, Bharat Gwalania,b, Mageshwari Komarasamya, ⁎ Luis Reza-Nietoa, Rajiv S. Mishraa,b, a b
Center for Friction Stir Processing and Advanced Materials Manufacturing Processes Institute, University of North Texas, TX, USA Advance Materials and Manufacturing Processing Institute (AMMPI), University of North Texas, TX, USA
ARTICLE INFO
ABSTRACT
Keywords: Dissimilar metals Immiscible Butt joining Intermetallic compound Thermodynamic and kinetic calculation
Magnesium alloys (AZ31 and WE43) and steel (316 stainless steel, SS316) were butt welded through friction stir welding. Even though Mg and Fe are metallurgically immiscible, intermetallic compounds (IMCs) formed at the welded interface due to the existence of alloying elements in both magnesium alloys and steel. The formation of IMCs among alloying elements and the matrix were verified by thermodynamic and kinetic calculations and microstructural characterization. The elastic modulus mismatch and resultant stress concentration during tensile testing lead to interfacial fracture and the very existence of IMCs impaired dissimilar joint strength due to the brittleness of the IMCs.
Introduction Requirements to reduce fuel consumption and carbon dioxide release in the automobile industries pushed the use of lightweight structural alloys such as aluminum and magnesium alloys. Since use of high-strength materials such as steel is essential for overall structural efficiency, joining light metals and steel becomes especially relevant to the automobile industries to maintain design flexibility. Note that magnesium alloys are one of the most likely candidates to obtain lightweight structures with lowest density among structural alloys. Friction stir welding (FSW) is a well-known solid-state joining technique for obtaining joints between dissimilar metals in various weld configurations. Several derivatives of FSW have been applied for lap welding of dissimilar materials, such as self-reverting friction stir welding [1], friction stir scribe technology [2], hybrid friction stir welding assisted by friction surfacing [3], friction-based filling stacking joining [4], etc. High-strength joints between aluminum alloys and steel via FSW have been achieved by many researchers [5–8]. Previous publications have confirmed that the joint strength increased with reduction in brittle Al-Fe intermetallic compound (IMC) thickness [8]. Dissimilar butt welding between pure magnesium and steel via FSW resulted in no IMCs since they are metallurgically immiscible elements [9]. In dissimilar joining of magnesium alloys such as AZ31 containing Al as alloying element, and steel, joint strength increased with increasing aluminum content which in turn resulted in thicker Al-Fe
⁎
related IMCs [9]. This observation was in contradiction with results reported on various Al alloys/steel welds. According to previous studies [10,11], stress concentration existed at the softer material next to the welded interface during tensile test. Magnesium alloy with lower aluminum content has lower intrinsic strength, which led to early crack initiation in the magnesium alloy next to the welded interface. This might be a reason to explain this difference in observation by Tanaka [8] and Kasai [9]. In this study, magnesium alloys (AZ31 and WE43) of similar strength were friction stir butt welded with 316 stainless steel (SS316). Welding parameters were optimized to obtain defect-free dissimilar joints between Mg alloys/SS316. AZ31 alloy contains ˜3 wt. % aluminum and ˜1 wt. % zinc while WE43 alloy contains ˜4 wt. % Y and ˜3 wt. % Nd. Although Mg and Fe are metallurgically immiscible, Al and Zn alloying elements in AZ31 and Y and Nd alloy elements in WE43 along with Fe, Cr, Ni and Mn in SS316 can form various IMCs. Effects of various intermetallic forming alloying elements on the strength and fracture behavior of butt-welded AZ31/SS316 and WE43/SS316 joints were evaluated for both mechanical properties and microstructural evolution. Experimental AZ31, WE43 sheet (thickness ˜6.0 mm) and 316 stainless steel sheet (thickness ˜5.75 mm) were friction stir butt welded using a W-Re tool.
Corresponding author at: Center for Friction Stir Processing and Advanced Materials Manufacturing Processes Institute, University of North Texas, TX, USA. E-mail address: [email protected] (R.S. Mishra).
https://doi.org/10.1016/j.jmapro.2019.02.009 Received 7 November 2018; Received in revised form 16 January 2019; Accepted 18 February 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
Journal of Manufacturing Processes 39 (2019) 138–145
T. Wang, et al.
Table 1 Chemical composition of base materials. Base materials
AZ31 WE43 SS316
Chemical composition (in wt. %) Al
Zn
Cu
Y
Nd
Ze
Mg
C
Mn
Si
Cr
Ni
Mo
Fe
2.83 – –
0.80 – –
0.002 – –
– 4.0 –
– 3.0 –
– 0.5 –
Bal. Bal. –
– – 0.08
– – 2.0
– – 0.03
– – 17.0
– – 12.0
– – 2.50
– – Bal.
Results and discussion
Table 2 Welding parameters for AZ31/SS316 and WE43/SS316 joints. Base materials
Rotation rate (rpm)
Traverse speed (ipm)
AZ31/SS316
350 400 500 600 650 350 450 550 600 750
0.5 1 1, 2 2 1 0.5, 2 1 2 1, 2 2
WE43/SS316
Difference in physio-thermal properties of Mg alloys and steel requires that FSW parameters should be selected cautiously to obtain defect-free dissimilar joints. Welding parameters, rotation rate and traverse speed applied in this study for dissimilar butt welding of AZ31/ SS316 and WE43/SS316 are plotted in Fig. 1(a) and (b) to show parameter window for defect-free joints. The two FSW characteristic defects during FSW [12], lack of fill (Fig. 1(c)) and galling surface (Fig. 1(d)) were observed. Lack of fill appeared in the magnesium next to the faying interface when tool rotation rate was low or the traverse speed was high. It was because steel was not plasticized sufficiently, which obstructed magnesium flow around the tool pin. A galling surface appeared on the weld surface when the rotation rate was high because high tool rotation rate led to excessive heat which ultimately led to sticky conditions. An AZ31/SS316 joint with tool rotation rate of 500 rpm and traverse speed of 1 ipm and a WE43/SS316 joint with tool rotation rate of 600 rpm and traverse speed of 1 ipm were defect-free and were used for further analysis. As shown in Fig. 2, an Al concentration region along the AZ31/SS316 welded interface (green circle in Fig. 2(a) and (c)) implied that Al in the AZ31 matrix diffused towards the AZ31/SS316 welded interface during FSW, forming possibly Al-X (Fe, Cr or Ni) IMCs. It was because Al is miscible with Fe, Cr and Ni and forms various IMCs due to negative enthalpy of mixing. In addition, the steel that was cut and stirred into the AZ31 nugget existed as steel fragments (red circle in Fig. 2(a) and (d–f)), which is common in dissimilar butt welding between Al and steel [13]. Interestingly, Al-Mn IMC particles are observed at the welded interface between AZ31 and SS316 (green circle in Fig. 3(a), (c) and (e)). The microstructure of the base SS316 clarifies the origin of Al-Mn IMC particles. Mn-Mo rich particles exist in the base SS316 matrix (Fig. 3(g)). During FSW, Mn-Mo rich particles reacted with Al in the
The chemical composition of all the base materials is listed in Table 1. During FSW, the steel was placed on the advancing side (AS). The tool offset from the Mg/steel interface favored towards the Mg side by 2.5 mm. The W-Re tool had a threaded conical pin of pin length, pin diameter at root and tip, and shoulder diameter of 3.8 mm, 7.6 mm, 5.0 mm, and 16.0 mm, respectively. Welding parameters for AZ31/ SS316 and WE43/SS316 including tool rotational rate and traverse speed are listed in Table 2. Plunge depth in steel, thickness difference, tool offset and tool tilt angle were kept constant and the values are 4.2 mm, 0.25 mm, 2.5 mm, 2.5˚, respectively. Standard tensile testing (ASTM E8-04) was used to evaluate the strength of the Mg alloy/steel joints and the base materials. The dimensions of the specimens are as follows: gage length, width and thickness are ˜30.0, 6.3 and 3.5 mm, respectively. Microhardness measurements were conducted on the cross section of welded joints with a load of 200 g and dwell time of 10 s. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) equipped with energy-dispersive X-ray spectroscopy (EDS) were used to investigate the microstructure of base materials and the welded interface between Mg alloys and steel.
Fig. 1. Relationship between welding parameters (tool rotation rate and traverse speed) and defect formations for dissimilar butt welding of (a) AZ31/SS316 and (b) WE43/SS316, and two characteristic FSW defects (c) lack of fill and (d) galling surface. 139
Journal of Manufacturing Processes 39 (2019) 138–145
T. Wang, et al.
Fig. 2. (a) Backscattered SEM and (b–f) EDS mapping on the cross section of AZ31/SS316 welded interface. Note that the steel fragments are marked with red circles and the Al-rich region is marked with a green circle. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Fig. 3. (a) Backscattered SEM and (b–f) EDS mapping on the Al-Mn IMC region at the AZ31/SS316 welded interface and (g) backscattered SEM and EDS linear scan on the Mn-rich particle in the base SS316. Note that the steel fragments are marked with red circles and the Al-Mn chemical reaction region is marked with a green circle. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
AZ31 and formed Al-Mn IMCs. To further explain the formation of IMCs in this study, thermodynamics and kinetics calculations on Al-Fe and Al-Mn systems were conducted. Effective heat of formation (ΔH') for both Al-Fe and Al-Mn systems can be calculated based on the effective heat of formation (EHF) model:
H
Al-Fe and Al-Mn systems are reported in [14,15]. At the lowest liquids composition of the Al-Fe and Al-Mn phase diagrams (˜0.90 atomic %), the calculated values of ΔH' for Al-Mn and Al-Fe based IMCs are labelled and listed in Fig. 4 and Table 3, respectively. The calculations indicated that formations of Al6Mn and Al4Mn were favored since their ΔH' values were more negative than those for other Al-Mn IMCs and AlFe IMCs. Based on kinetics data, diffusion length (IMC thickness) can be approximately predicted. The diffusion length (L) and diffusion coefficients (D) of Fe into Al, Mn into Al and Al into Fe can be calculated as:
H0
= ×
effective concentration of limiting element compound concentration of limiting element (1)
where H is the enthalpy change or heat of formation. H values for 140
L = (Dt)1/2
(2)
D = D0 exp(-Q/(RT))
(3)
Journal of Manufacturing Processes 39 (2019) 138–145
T. Wang, et al.
Fig. 4. Effective heat of formation at 298 K for various IMCs in (a) Al-Fe and (b) Al-Mn systems.
much larger than the expected diffusion length in dissimilar friction stir welding (< 5 μm). Plastic deformation of magnesium alloy during FSW can accelerate the diffusion process via increased vacancy concentration and dislocation density [21]. In addition, Mn-Mo-rich particles are broken during FSW with the formation of micro cracks, which might also enhance the diffusion of Al into Mn-Mo-rich particles. This might explain the high formation rate of the Al-Mn particles. Based on the calculated diffusion coefficients in Table 4, diffusion length of Fe into Al at 900 K can be obtained via L = (1.2 × 10−13 × t)1/2. When t ranged from 5 to 15 s [18], L equals to ˜0.8–1.3 μm. The calculated diffusion length is much larger than the measured value which is shown in the later section of the present paper. This is because the calculation was conducted based on the Al/Fe diffusion model, while Al in the AZ31 matrix is limited. As shown in Fig. 5, the Y and Nd concentration regions along the WE43/SS316 welded interface (green circle in Fig. 5(a), (c) and (d)) implied that Y and Nd in the WE31 matrix diffused towards the WE43/ SS316 welded interface during the FSW that formed IMCs. Besides, a Y and Nd depletion region (green arrow in Fig. 5(c) and (d)) existed next to the Y and Nd-rich region. In summary, all thermodynamically possibly-formed IMCs for AZ31/SS316 and WE43/SS316 (Table 5) indicated that multiple IMCs can form during FSW of AZ31/SS316 and WE43/SS316. IMC thickness for defect-free welded AZ31/SS316 and WE43/SS316 are ˜150 and 80 nm, respectively (Fig. 6(a) and (b)). To further study the alloying element diffusion at welded interface, TEM with EDS point analysis showed that Al diffused to welded interface region (points 3 and 4 in Fig. 6(a)) and Fe, Cr and Ni diffused into AZ31 matrix simultaneously (points 1 and 2 in Fig. 6(a)) for AZ31/SS316 welded joint. For WE43/SS316 welded interface, Y and Nd diffused to welded interface (point 3 in Fig. 6(b)) and SS316 matrix (point 2 in Fig. 6(b)) and Fe and Ni diffused into WE43 matrix (point 4 in Fig. 6(b)). Comparison of tensile test results for the AZ31/SS316 and WE43/ SS316 welded joints with the UTS of base materials (Fig. 7(a)) confirmed no significant difference in UTS of AZ31/SS316 joints and WE43/SS316 joints. The UTS of dissimilar joints is ˜160 MPa (˜73% of base Mg alloys). Microhardness profile of the cross section of welded
Table 3 Effective heat formation values for Al-Fe and Al-Mn IMCs. System
IMCs
ΔH° (kJmol−1at−1)
ΔH' (kJmol−1at−1)
Al-Fe
Al6Fe Al13Fe4 Al5Fe2 Al2Fe AlFe3 Al6Mn Al4Mn Al11Mn4 Al8Mn5
−11 −19 −22 −25 −22 −15 −21 −23 −26
−0.69 −0.68 −0.69 −0.68 −0.26 −0.93 −0.93 −0.76 −0.60
Al-Mn
where t is the time gap when FSW temperature is higher than diffusion requirement, D0 (m2/s) is the pre-exponential factor, Q (J/mol) is the activation energy, R (J/mol·K) is the gas constant and T (K) is the absolute temperature. The limited values of D0, Q and T were obtained from literature [16,17], and R is the gas constant (8.314 J/mol). The calculated D values are summarized in Table 4. Note that peak temperature is ˜600–750 K for friction stir welding of AZ31 magnesium alloy [19], and ˜900–1050 K for friction stir welding of 304 stainless steel [20]. Peak temperature at interface during friction stir welding of magnesium alloy and steel should be within these two ranges. Furthermore, the peak temperature needs to be close to the range of 900–1050 K to plasticize steel during friction stir butt welding between magnesium alloy and steel. The melting points of AZ31 and WE43 magnesium alloys are both ˜900 K. Therefore, it is reasonable to assume the peak temperature at magnesium alloy/steel interface during friction stir butt welding was similar and less than ˜ 900 K. The diffusion coefficient of Fe into Al at 900 K being much larger than the diffusion of Al in Fe at 1173 K implies that the diffusion of Fe into Al is favored and is dominant in the Al-Fe system when the temperature ranged from 900 to 1173 K. For the Al-Mn system, the D of Mn into Al at 900 K is negligible as compared with the diffusion coefficient in the Al-Fe system; and the data for diffusion of Al into Mn are not available. However, an abnormal diffusion behavior of Al into Mn was observed in this study. Al diffused into Mn-Mo-rich particles of around ˜10 μm in size, which is Table 4 Calculation of diffusion coefficients in Al-Fe and Al-Mn systems. Diffusion process
Temperature range (K)
D0 (m2/s)
Q (kJ/mol)
D (m2/s)
Reference
Fe into Al Mn into Al Al into Fe Al into Mn
793–922 773–918 1173–1358
5.3 × 10−3 3.8 × 10−2 1.0 × 10−2
183.4 221.8 267.1
1.2 × 10−13 (900 K) 5.1 × 10−15 (900 K) 1.3 × 10−14 (1173 K)
[16] [16] [17]
a
a
a
a
Not available.
141
a
Journal of Manufacturing Processes 39 (2019) 138–145
T. Wang, et al.
Fig. 5. (a) Backscattered SEM and (b–h) EDS mapping on the cross section of WE43/SS316 welded interface. Note that the steel fragments are marked with red circles, the Y and Nd-rich region is marked with a green circle, and the Y and Nd depletion regions are marked with green arrows. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
between AZ31 and WE43. Hardness values in the TMAZ of the steel side is higher than that of the base steel. The hardness increase in the SZ is based on recrystallization, and the hardness increase in the TMAZ of the steel side is linked to work-hardening and the resultant recrystallization. Both AZ31/SS316 and WE43/SS316 joints fractured through welded interface (Fig. 7(a)). IMC with thickness of ˜80-150 nm should not be the main factor for fracture at welded interface because IMC thickness needs to be greater than a few μm to be deleterious for tensile properties of the weld [7]. Previous studies on butt welding of copper/ steel [10] and aluminum alloy/steel [11] showed that stress concentration occurred at weld interface at the early stage of tensile testing due to the mismatch of elastic modulus. Therefore, stress concentration at the welded interface during tensile testing could be the main reason for interfacial fracture for AZ31/SS316 and WE43/SS316 joints. Fractography of AZ31/SS316 dissimilar joint (Figs. 8 and 9) revealed certain features on the fractured surface of the SS316 part (Fig. 8(a)) that called for further investigation (Fig. 8(b)) and EDS mapping (Fig. 8(c–g)) on the region selected in Fig. 8 (a). The surface of the
Table 5 Possible IMCs formed during FSW of AZ31/SS316 and WE43/SS316. Intermetallics
SS316
Fe
WE43
Mg
Al
Zn
Mg
Y
Nd
̶
Al13Fe4 Al5Fe2… Al7Cr Al11Cr2 Al3Ni Al3Ni2… Al12Mn A6Mn…
ZnFe Zn4Fe Zn13Cr Zn17Cr ZnNi
̶
Fe17Y2 Fe23Y6… ̶
Fe8Nd Fe7Nd2
Ni17Y2 Ni5Y… Mn12Y Mn23Y6…
a
Cr
̶
Ni
MgNi2 Mg2Ni ̶
Mn a
AZ31
Zn9Mn
̶ MgNi2 Mg2Ni
a
a
Not available ― Immiscible.
AZ31/SS316 and WE43/SS316 joints (Fig. 7(b)) revealed that stir zone (SZ) has higher hardness than thermomechanically affected zone (TMAZ), heat affected zone (HAZ) and base material (BM) of magnesium alloys; and that there is no obvious difference in hardness values
Fig. 6. TEM images and EDS point analysis on welded interface of (a) AZ31/SS316 and (b) WE43/SS316.
142
Journal of Manufacturing Processes 39 (2019) 138–145
T. Wang, et al.
Fig. 7. (a) UTS of AZ31/SS316 joint, WE43/SS316 joint, base AZ31, base WE43, and base SS316 and (b) microhardness profile across the weld cross section of both AZ31/SS316 and WE43/SS316 joints. Fig. 8. (a) Backscattered SEM of fractured surface of SS316 part of AZ31/SS316 joint, (b) back scattered SEM of selected region in red rectangle in (a) and (c–g) EDS mapping of region (b). Note that broken Al-Mn particles are marked with red circles and sticking Mg on the fractured SS316 part are marked with green circles. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
fractured SS316 part consisted of steel and Mg sticking to it (green circles in Fig. 8(b) and (f)). In addition, broken Al-Mn particles formed at the welded interface can be observed (denoted by red circles in Fig. 8(b), (e) and (g)). Next, the surfaces of the fractured AZ31 part (Fig. 9(a)) revealed steel fragments along with Al-Fe IMCs (green circles in (Fig. 9(b), (d) and (e)) remaining on the fractured surface of the AZ31 part. Also, broken Al-Mn particles were observed on the AZ31 side as well (red circles in Fig. 9 (f), (h) and (i)). Existence of Al-Mn particles and Al-Fe IMCs at the fractography of welded AZ31/SS316 interface displayed that IMCs might also impaire the joint strength. Similarly, fractography of dissimilar joining of WE43/SS316 (Figs. 10 and 11), backscattered SEM (Fig. 10(b)) and EDS mapping (Fig. 10(c–h)) on the fractured surface of the SS316 part of WE43/ SS316 joints were conducted on the region marked in Fig. 10(a). The surface of the fractured SS316 part consisted of steel with Y and Nd
diffusing into it (red arrow in Fig. 10(b) and (d–h)) and Mg sticking to it (green arrows in Fig. 10(b) and (c)). On the other hand, the surface of the fractured WE43 part (Fig. 11(a)) showed some steel fragments (red arrows in (Fig. 11(b) and (f–h)) remaining on the fractured surface of the WE31 part. Conclusions (1) Interdiffusion occurred between alloying elements in magnesium alloys and 316 stainless steel, and alloying elements in the magnesium alloys (eg. Al in AZ31 and Y and Nd in the WE43) diffused towards to the welded interface to form an Al-rich region for welded AZ31/SS316 and a Y-Nd-rich region for welded WE43/ SS316. The IMC thickness are ˜150 and 80 nm for welded AZ31/ SS316 and WE43/SS316, respectively.
143
Journal of Manufacturing Processes 39 (2019) 138–145
T. Wang, et al.
Fig. 9. (a) Backscattered SEM of fractured surface of AZ31 part of AZ31/ SS316 joint, (b) backscattered SEM of selected region in red rectangle in (a), (c–e) EDS mapping of region (b), (f) backscattered SEM of selected region in green rectangle in (a) and (g–i) EDS mapping of region (f). Note that broken Al-Mn particles are marked with red circles and Al-Fe IMCs on the fractured SS316 part are marked with green circles. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Fig. 10. (a) Backscattered SEM of fractured surface of SS316 part of WE43/SS316 joint, (b) backscattered SEM of selected region in red rectangle in (a) and (c–h) EDS mapping on the selected region of (b). Note that sticking Mg on the fractured SS316 surface is marked with green arrows and steel with Y and Nd diffusing into it are marked with red arrows. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
(2) Ultimate tensile strength of dissimilar butt joints of AZ31/SS316 and WE43/SS316 reached ˜160 MPa (˜73% of base Mg alloys)). Welded AZ31/SS316 and WE43/SS316 broke through welded interface during tensile testing mainly because elastic modulus mismatch between magnesium alloy and steel and resultant stress
concentration. Existence of IMCs at weld interface only impaired the joint strength to some extent. (3) An abnormal diffusion of Al into Mn-Mo rich particles in SS316 was observed and verified by thermodynamics and kinetics calculations in the butt welding of AZ31 and 316 stainless steel. 144
Journal of Manufacturing Processes 39 (2019) 138–145
T. Wang, et al.
Fig. 11. (a) Backscattered SEM of fractured surface of WE43 part of WE43/SS316 joint, (b) backscattered SEM of selected region in red rectangle in (a) and (c–h) EDS mapping on the selected region of (b). Note that sticking steel fragments on the fractured WE43 surface are marked with red arrows and the remaining Mg is marked with green arrows. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Acknowledgments [9]
This work was supported under the NSF-IUCRC grant for Friction Stir Processing (NSF-IIP 1157754). The additional support of Boeing, General Motors, Pacific Northwest National Laboratory, Army Research Laboratory and Korea Aerospace Research Institute for the UNT site is acknowledged. This report was prepared as an account of work sponsored by an agency of the US Government. The views and opinions of the authors expressed herein do not necessarily state or reflect those of the US Government or any agency thereof. We also acknowledge the UNT Materials Research Faculty (MRF).
[10] [11] [12] [13] [14]
References
[15]
[1] Huang Y, Wang J, Wan L, Meng X, Liu H, Li H. Self-riveting friction stir lap welding of aluminum alloy to steel. Mater Lett 2016;185:181–4. [2] Y. Hovanski, G.J. Grant, S. Jana, Mattlin, K.F., Battelle Memorial Institute Inc, 2013. Friction stir welding tool and process for welding dissimilar materials. U.S. Patent 8, 434,661. [3] Huang Y, Lv Z, Wan L, Shen J, dos Santos JF. A new method of hybrid friction stir welding assisted by friction surfacing for joining dissimilar Ti/Al alloy. Mater Lett 2017;207:172–5. [4] Huang Y, Meng X, Xie Y, Li J, Wan L. New technique of friction-based filling stacking joining for metal and polymer. Compos Part B: Eng 2019;163:217–23. [5] Lee WB, Schmuecker M, Mercardo UA, Biallas G, Jung SB. Interfacial reaction in steel–aluminum joints made by friction stir welding. Scr Mater 2006;55(4):355–8. [6] Kimapong K, Watanabe T. Friction stir welding of aluminum alloy to steel. Weld J 2004;83(10):277. [7] Jiang WH, Kovacevic R. Feasibility study of friction stir welding of 6061-T6 aluminium alloy with AISI 1018 steel. Proc Inst Mech Eng Part B J Eng Manuf 2004;218(10):1323–31. [8] Tanaka T, Morishige T, Hirata T. Comprehensive analysis of joint strength for
[16] [17] [18] [19] [20] [21]
145
dissimilar friction stir welds of mild steel to aluminum alloys. Scr Mater 2009;61(7):756–9. Kasai H, Morisada Y, Fujii H. Dissimilar FSW of immiscible materials: steel/magnesium. Mater Sci Eng A 2015;624:250–5. Wang T, Shukla S, Nene SS, Frank M, Wheeler RW, Mishra RS. Towards obtaining sound butt joint between metallurgically immiscible pure Cu and stainless steel through friction stir welding. Metall Mater Trans A 2018;49(7):2578–82. Wang T, Komarasamy M, Liu K, Mishra RS. Friction stir butt welding of strainhardened aluminum alloy with high strength steel. Mater Sci Eng A 2018;737:85–9. Arbegast WJ. A flow-partitioned deformation zone model for defect formation during friction stir welding. Scr Mater 2008;58(5):372–6. Uzun H, Dalle Donne C, Argagnotto A, Ghidini T, Gambaro C. Friction stir welding of dissimilar Al 6013-T4 to X5CrNi18-10 stainless steel. Mater Des 2005;26(1):41–6. Theron CC, Ndwandwe OM, Lombaard JC, Pretorius R. First phase formation at interfaces: comparison between Walser-Bené and effective heat of formation model. Mater Chem Phys 1996;46(2–3):238–47. Liu XJ, Ohnuma I, Kainuma R, Ishida K. Thermodynamic assessment of the aluminum-manganese (Al-Mn) binary phase diagram. J Phase Equilibria Diffus 1999;20(1):45. Du Y, Chang YA, Huang B, Gong W, Jin Z, Xu H, Yuan Z, Liu Y, He Y, Xie FY. Diffusion coefficients of some solutes in fcc and liquid Al: critical evaluation and correlation. Mater Sci Eng: A 2003;363(1–2):140–51. MADELUNG O. Numerical data and functional relationships in science and technology. Landolt Bornstein, New Series, Group III; 1982. p. 22. Liu X, Lan S, Ni J. Analysis of process parameters effects on friction stir welding of dissimilar aluminum alloy to advanced high strength steel. Mater Des 2014;59:50–62. Chang CI, Lee CJ, Huang JC. Relationship between grain size and Zener–Holloman parameter during friction stir processing in AZ31 Mg alloys. Scr Mater 2004;51(6):509–14. Cho JH, Boyce DE, Dawson PR. Modeling strain hardening and texture evolution in friction stir welding of stainless steel. Mater Sci Eng A 2005;398(1–2):146–63. Nagahama K, Miki I. Precipitation during Recrystallization in Al–Mn and Al–Cr alloys. Trans Jpn Inst Met 1974;15(3):185–92.