Microstructures and mechanical properties of resistance spot welded magnesium alloy joints

Microstructures and mechanical properties of resistance spot welded magnesium alloy joints

Materials Science and Engineering A 460–461 (2007) 494–498 Microstructures and mechanical properties of resistance spot welded magnesium alloy joints...

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Materials Science and Engineering A 460–461 (2007) 494–498

Microstructures and mechanical properties of resistance spot welded magnesium alloy joints D.Q. Sun ∗ , B. Lang, D.X. Sun, J.B. Li Key Laboratory of Automobile Materials, School of Materials Science and Engineering, Jilin University, Changchun 130025, China Received 14 November 2006; received in revised form 18 January 2007; accepted 19 January 2007

Abstract The resistance spot welded magnesium alloy joints consist mainly of weld nugget and heat-affected zone (HAZ). The nugget contains two different structures, the cellular-dendritic structure at the edge of the nugget and the equiaxed dendritic structure in the center of the nugget. The structure transition is attributed to the changes of solidification conditions. In HAZ, the grain boundary melting occurred and grain boundaries became coarse. The magnesium alloy nugget has a high hot cracking susceptibility. The cracks appeared in the nuggets when the welding current was higher than 15 kA. With an increase of welding currents from 15 kA to 23 kA, the joint tensile shear load rises from 1460 N to 3000 N and the nugget diameter increases from 4.2 mm to 6.5 mm. The joint strength rises are mainly related to increasing the nugget diameter. It is favorable to select relatively high welding current for improving mechanical properties of spot welded magnesium alloy joint. © 2007 Elsevier B.V. All rights reserved. Keywords: Resistance spot welding; Magnesium alloy; Microstructure; Mechanical properties

1. Introduction Magnesium and magnesium alloys have recently attracted great attention in academic research and industrial application owing to some unique properties such as their low density, high specific strength, elastic modulus and damping capacity, and recyclable characteristics [1–3]. Especially within transportation industries, the weight saving effect of replacing steel and aluminium parts is an important factor in reducing fuel consumption and increasing speed. Besides the development of new alloy types, manufacturing techniques such as welding and joining play an important role in exploiting the new fields of applications. There has been some research activity on welding of magnesium alloys in recent years. Various welding processes, principally laser welding, electron beam welding, tungsten inert gas welding, friction stir welding and transient liquid phase bonding, were employed in the investigations and many useful insights and data have been obtained [4–12]. Resistance spot welding (RSW) is a popular process for assembly lines



Corresponding author. Tel.: +86 431 85094687; fax: +86 431 85094687. E-mail address: [email protected] (D.Q. Sun).

0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.01.073

in the automotive and other industries manufacturing a variety of products made of thin gauge metals, and has great potential for magnesium alloy sheet joining in the automotive industry. Therefore, the resistance spot weldability of magnesium alloys is of significant interest for automotive manufacturing process. However, reports in the literature dealing with the resistance spot welding of magnesium alloys are limited, and further research and development efforts are required to utilize the full potential of these materials. The present work investigates microstructures, crack features and mechanical properties of resistance spot welded magnesium alloy joints. Its purpose was to obtain better understanding of the resistance spot weldability of magnesium alloys and provide some foundation for improving mechanical properties of the magnesium alloy joints. 2. Experimental The base metal employed in this investigation is 1.2 mm thick magnesium alloy sheet and its chemical composition is presented in Table 1. Spot welding specimens (100 mm × 25 mm × 1.2 mm) were cut from the sheet and installed as the lapping joint, as shown in Fig. 1. The spot welding test was carried out by a TDZ-3X100 resistance spot weld-

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Table 1 Chemical composition of magnesium alloy (wt%) Al Zn Si Fe Cu Mn Ni Mg

2.90 0.837 0.067 0.0045 0.005 0.431 0.0013 Balance

Fig. 2. Optical micrograph of spot welded joint.

3. Results and discussion 3.1. Microstructure of joint

Fig. 1. Shape and size of specimens.

ing machine. Copper electrodes with a crown-shaped tip with crown radius of 100 mm and tip diameter of 20 mm were used. The welding current, welding time and electrode force were 15–23 kA, 8.0 cycles and 2.5 kN, respectively. After welding, the spot welded joints were sectioned through the nugget center normal to the plane of the specimens and then ground, polished and etched for metallographic examination and nugget diameter measurement. The microstructure of spot welded joints was examined using optical microscopy and scanning electron microscopy (SEM, JSM-5600). The nugget diameter measurement was made from the resulting optical micrographs. Joint tensile shear test was carried out using a MT810 material testing machine. The nugget diameter and joint tensile shear load were determined, based on the average value over three measurements per condition.

Fig. 2 shows optical micrograph of resistance spot welded magnesium alloy joint. It can be seen that the joint consists mainly of weld nugget and heat-affected zone (HAZ) encircling the nugget. The experimental results indicated that the weld nugget size is related to the welding current and a higher current level produces a larger nugget, as shown in Fig. 3. In the RSW process, the welding current flowed through the faying surface of two metal sheets compressed by a pair of water-cooled copper electrodes and the faying surface was locally melted to form the nugget by the heat resulting from contact electrical resistance. The higher current leads to the larger nugget, since the supply of heat is larger, the time to reach the melting temperature is shorter and the time spent at melting temperature or above the melting point is longer. Metallographic examination of the weld nugget revealed that the nugget contains two different structures under the conditions of this investigation. A cellular-dendritic structure develops at the edge of the nugget and the crystal grows epitaxially from the unmelt base metal (Fig. 4(a)), while the equiaxed dendritic structure appears in the center of the nugget, as shown in Fig. 4(b). The transition from cellular-dendritic to equiaxed dendritic structures in the nugget is attributed to the changes of solidification conditions. Owing to the good heat conductivity

Fig. 3. Effect of welding current on the nugget size: (a) 15 kA and (b) 23 kA.

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Fig. 4. Microstructure of weld nugget: (a) cellular-dendritic structure and (b) equiaxed dendritic structure.

of magnesium alloy, the liquid temperature at the edge of the nugget drops so quickly that the liquid is in a high supercooling state after power stops, which promotes epitaxial growth of cellular-dendritic crystal. As solidification progresses, the solute concentrations of Al and Zn atoms in front of solid/liquid interface increase and the temperature gradient tends to decrease in liquid near the center of the nugget due to the relatively poor heat extraction and the released latent heat. Under such conditions, the sufficient constitutional supercooling results in the formation of equiaxed dendritic crystal by radial dendritic growth of the nuclei. In HAZ immediately adjacent to the nugget, the grain boundary melting occurred and grain boundaries became coarse compared with unaffected base metal (Figs. 2 and 4(a)). In this investigation, it was found that the magnesium alloy weld nugget has a high cracking susceptibility. The cracks appeared in the nuggets when the welding current was higher than 15 kA in the experiment. In cross-section of the nugget, the direction of cracks was approximately perpendicular to the faying surface and cracks initiated at grain boundaries and propagated along the grain boundaries, as shown in Figs. 3(b) and 5. Fig. 6 shows SEM opened crack surface morphology. The cellular-dendritic and equiaxed dendritic appearances could be seen clearly and the dendrite tips were rounded, smooth and undamaged. Intergranular characteristics of the cracks and dendritic morphology of opened crack surface are typical features of hot cracking and evidence of crack formation during the later stages of solidification. On the other hand, it also proves that an almost continuous low melting point liquid film was present between the dendrites at the moment of crack formation. Therefore, the cracks in the weld nuggets belong to the solidification

Fig. 5. Cracks in magnesium alloy nugget.

cracking according to the classification by Hemsworth et al. [13]. The low melting point liquid film and tensile stress are responsible for initiation and propagation of the cracks. During the later stages of solidification, the low melting point liquid films between the dendrites form due to segregation of Al and Zn atoms. The liquid films weaken the nugget microstructure to the extent that cracks form at grain boundaries under the influence of the tensile stress developed during cooling. The high welding current results in the high hot cracking susceptibility because of increasing the tensile stress. It is favorable to select relatively low welding current for preventing the appearance of cracks.

Fig. 6. SEM opened crack surface morphology: (a) cellular-dendrite appearance and (b) equiaxed dendrite appearance.

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Fig. 7. Effects of welding current on tensile shear load and nugget diameter. Fig. 10. Overall fracture morphology of spot welded joint.

Fig. 8. The relationship between tensile shear load and nugget diameter.

3.2. Mechanical properties of joint The experimental results indicated that the welding current has an obvious effect on spot welded magnesium alloy joint strength. Figs. 7 and 8 show effects of welding current on joint tensile shear load and nugget diameter, and the relationship

between joint tensile shear load and nugget diameter, respectively. It can be seen that with an increase of welding currents from 15 kA to 23 kA, the joint tensile shear load rises from 1460 N to 3000 N in spite of fact that the nuggets contain cracks at high current level, the nugget diameter increases from 4.2 mm to 6.5 mm, and the tensile shear load changes as a function of the nugget diameter. These results suggest that the nugget diameter is the main controlling factor of joint strength and the joint strength rises are mainly associated with increasing the nugget diameter. In addition, the welding current has also an effect on the fracture appearance of the spot welded joint. In tensile shear test, the crack developed around the edge of the nugget. At relatively low currents (15 kA and 17 kA), nuggets failed across the interface for tensile shear specimens. At relatively high currents (19 kA, 21 kA and 23 kA), nuggets failed by forming a buttonhole, as shown in Fig. 9. Fig. 10 shows overall tensile shear fracture morphology of spot welded magnesium alloy joint. Different fracture surface morphologies can be seen at higher magnification. In the region A of Fig. 10, the plastic deformation and tear dimple features were observed (Fig. 11(a)), while the region B displayed relatively flat fracture surfaces, as shown in Fig. 11(b). Based on above results, it is favorable to select relatively high welding current for improving mechanical properties of spot welded magnesium alloy joint.

Fig. 9. The fracture appearance of the joint.

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Fig. 11. Fracture surface morphology of the joint: (a) fracture surface with plastic deformation and dimples; (b) relatively flat fracture surface.

4. Conclusions

References

(1) The magnesium alloy weld nugget contains the cellulardendritic structure at the edge of the nugget and the equiaxed dendritic structure in the center of the nugget. Features of HAZ are the boundary melting and coarsening. (2) The weld nugget has a high hot cracking susceptibility. The cracks appear in the nuggets when the welding current is higher than 15 kA. The high welding current increases the cracking tendency because of increasing the tensile stress developed during cooling. (3) With an increase of welding currents from 15 kA to 23 kA, the joint strength rises are mainly associated with increasing the nugget diameter. It is favorable to select relatively high welding current for improving mechanical properties of spot welded magnesium joint.

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