Transformation of Laves phases and its effect on the mechanical properties of TIG welded Mg-Al-Ca-Mn alloys

doi:10.1016/j.jmst.2022.01.005

Abstract

Abstract:

The effects of Al content and Ca/Al mass ratio on the microstructure and mechanical properties of tungsten inert gas (TIG) welded Mg-2Ca-xAl-0.5Mn (x=0, 1, 5) alloy joints were studied in present work. Results showed that increasing Al content was effective in reducing the dendrite spacing at the fusion zone (FZ) edge. The Laves phases in the FZ and the heat-affected zone (HAZ) can be changed from Mg₂Ca to (Mg, Al)₂Ca with the decrease of Ca/Al ratio, and the (Mg, Al)₂Ca could be further transformed to Al₂Ca under welding thermal cycle. Furthermore, dynamic dissolution and precipitation of Laves phases and Al₈Mn₅ phases occurred in the HAZ, resulting in a gradient microstructure and hardness peak in this area. The tensile properties of the joints were significantly improved with the increase of Al content, which was mainly due to the modification of Laves phases.

Key words: Mg-Al-Ca-Mn alloy, WeldLaves phaseMicrostructure, Mechanical properties

Sensen Chai, Shiyu Zhong, Qingshan Yang, Daliang Yu, Qingwei Dai, Hehe Zhang, Limeng Yin, Gang Wang, Zongxiang Yao. Transformation of Laves phases and its effect on the mechanical properties of TIG welded Mg-Al-Ca-Mn alloys[J]. J. Mater. Sci. Technol., 2022, 120: 108-117.

Figures/Tables 16

Table 1. Chemical compositions of the alloy sheets and welding fillers (wt.%).

Sample No.	Nomenclatures	Ca	Al	Mn	Mg	Ca/Al
X2	Mg-2Ca-0.5Mn	2.11	-	0.62	Bal.	-
XA21	Mg-1Al-2Ca-0.5Mn	2.23	1.35	0.54	Bal.	1.65
XA25	Mg-5Al-2Ca-0.5Mn	1.95	4.85	0.53	Bal.	0.40

Fig. 1. Microstructure of the (a) X2, (b) XA21 and (c) XA25 alloy sheets.

Fig. 2. Microstructure of the XA25 alloy joints illustrating the zoning strategy.

Fig. 3. Microstructure of the FZ, FZ edge and HAZ in the (a-c) X2, (d-f) XA21 and (g-i) XA25 alloy joints. The annotation in each image indicates where it was taken.

Fig. 4. XRD results examined from the BM and FZ of the X2, XA21 and XA25 alloy joints.

Fig. 5. FE-SEM images of the initial (a) X2, (b) XA21 and (c) XA25 alloy sheets.

Fig. 6. FE-SEM images taken from the FZ of (a, d) X2, (b, e) XA21 and (c, f) XA25 alloy joints.

Fig. 7. TEM BFIs taken from the FZ of (a) XA21 and (b, c) XA25 alloy joints. Corresponding SADPs were taken with: (a) B

Table 2. EDS results tested from Fig. 7.

Point	Composition/wt.%			Ca/Al	Composition/at.%			Ca/Al
Point	Ca	Al	Mg	Ca/Al	Ca	Al	Mg	Ca/Al
1	31.79	9.7	58.51	3.28	22.28	10.1	67.62	2.21
2	41.71	43.08	15.21	0.97	31.89	48.93	19.18	0.65
3	32.14	40.55	27.31	0.79	23.39	43.84	32.77	0.53

Fig. 8. FE-SEM images taken from the HAZ of (a, d) X2, (b, e) XA21 and (c, f) XA25 alloy joints. The annotation in each image indicates where it was taken.

Fig. 9. FE-SEM images taken from the HAZ and PZ of (a-d) XA21 and (e-h) XA25 alloy joints. The annotation in each image indicates where it was taken.

Fig. 10. TEM BFIs taken from the HAZ of XA25 alloy joints: (a) Al2Ca phase and its SADP of B

Table 3. EDS results tested from Fig. 10.

Point	Composition/wt.%				Ca/Al	Composition/at.%				Ca/Al
Point	Ca	Al	Mn	Mg	Ca/Al	Ca	Al	Mn	Mg	Ca/Al
1	36.92	38.52	0.34	24.2	0.96	27.48	42.60	0.18	29.71	0.65
2	30.53	44.43	0.32	24.7	0.69	22.20	47.99	0.17	29.62	0.46
3	1.77	26.8	36.92	34.49	-	1.41	31.74	21.47	45.35	-
4	2.64	21.09	18.27	57.97	-	1.85	21.92	9.32	66.89	-
5	2.41	22.01	21.46	54.09	-	1.72	23.35	11.18	63.72	-

Fig. 11. Microscopic Vickers hardness distribution of the X2, XA21 and XA25 alloy joints.

Fig. 12. Tensile properties of the X2, XA21, XA25 alloy joints and corresponding BM sheets.

Fig. 13. Tensile fracture morphologies of the (a, d) X2, (b, e) XA21 and (c, f) XA25 alloy joints.

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