Friction stir welding of AZ31 magnesium alloy rolled sheets: Influence of processing parameters

TLDR: In this article, the authors investigated the relationship between the processing parameters, the heat and plastic deformation produced and the resulting microstructure and mechanical properties in friction stir welding (FSW).

About: This article is published in Acta Materialia.The article was published on 2009-01-01 and is currently open access. It has received 415 citations till now. The article focuses on the topics: Welding & Friction stir welding.

Figures

Fig. 5. Radiography of AZ31 welds V = 200 mm min 1, W = 800–1400 rpm. The bright features correspond to lack of material in the weld region, and the dark features correspond to the flash.

Fig. 6. Grain size evolution in the TMAZ for ‘‘bead on plate” FSW (tool 4 mm pin/10 mm shoulder): (a) 600 rpm, 100 mm min 1; (b) 1800 rpm, 1000 mm min 1.

Fig. 7. Grain size evolution with shoulder diameter for 1000 rpm, 200 mm min 1 butt FSW: (a–c) 13 mm shoulder diameter; (d–f) 10 mm shoulder diameter.

Fig. 11. Evolution of microhardness (0.98 N, 15 s) in FSW.

Fig. 8. Intragranular precipitation evolution in the butt weld nugget (tool 5 mm pin/10 mm shoulder): (a) (600 rpm, 2000 mm min 1); (b) (1000 rpm, 200 mm min 1); (c) base metal.

Fig. 10. Comparison between the stress evolutions during welding (strain gauges) and XRD results for FSW (1000 rpm, 200 mm min 1, tool 5 mm pin/13 mm shoulder).

Frequently asked questions

Q1. What are the contributions in "Friction stir welding of az31 magnesium alloy rolled sheets: influence of processing parameters" ?

The temperature evolution during friction stir welding ( FSW ) and the resulting residual stresses of AZ31 Mg alloy were studied to get a better understanding of the mechanisms involved in this process. The relationship between the processing parameters, the heat and plastic deformation produced and the resulting microstructure and mechanical properties was investigated. 

Q2. What is the effect of the intense stirring on the weld?

At low welding speed, the intense stirring causes an increase in the trapped particles content, which can be responsible for the weld defects. 

Q3. What is the effect of the welding parameters on the microhardness of the butt wel?

Using high (W/V), induced a very low variation in microhardness val-ues compared with the base metal ones, whereas using low (W/V) induced an increase in the weld zone. 

Q4. Why is the residual stress in butt welds higher than in fusion welding?

Owing to the low temperature gradients observed in FSW compared with fusion welding processes, such as laser welding, lower residual stresses would be expected. 

Q5. What does increasing the shoulder diameter do?

increasing the shoulder diameter or the tool rotation speed (W) or decreasing the welding speed (V) produces an increase in the heat generated during the process and promotes grain growth. 

Q6. What is the effect of the friction between the tool and the workpiece?

During FSW, heat is generated by the friction between the tool and the workpiece and by the plastic deformation occurring around the tool. 

Q7. What is the effect of grain size evolution on welds?

Microstructure and microhardness experimental analysis enables the hardening effect due to grain size evolution across the weld to be studied. 

Q8. What is the reason for the need for weight reduction in the aircraft industry?

The need for weight reduction in the aircraft industry has raised interest in using magnesium alloys to replace aluminium alloys in some structural and mechanical parts. 

Q9. Did the nanoscale precipitates dissolve during the FSW process?

nanoscale precipitates identified as Al8Mn5 using energy dispersive spectroscopy, did not dissolve during FSW and were not modified by changing the welding parameters. 

Q10. What was the effect of the shoulder diameter on the welding of magnesium alloys?

They concluded that the heat generated was influenced mainly by the shoulder diameter and that an increase in applied pressure and tool rotation rate/ welding speed ratio (W/V) resulted in a higher temperature [4,7]. 

Q11. What is the main reason for the increasing use of magnesium alloys?

With their increasing application, having a reliable joining process is required, but welding magnesium alloys still faces many challenges. 

Q12. What was the pressure applied to the butt welds?

The applied pressure (F) for butt welds was in the range 6.5–9 kN except for the weld processed at 2000 mm min 1, where a 22 kN load was applied. 

Q13. What is the effect of increasing the shoulder diameter?

Increasing the shoulder diameter led to an increase in the peak temperature (Fig. 14) and a modification in the heat dissipation. 

Q14. What is the mechanical properties of the butt weld?

The larger grain refinement, and therefore larger microstructure heterogeneity, induced at low nugget temperature showed the best mechanical properties. 

Q15. What was the definition of the welds that did not present any porosity?

The welds obtained that did not present any porosity, any crack and that exhibited a flash size <2 mm were defined as ‘‘sound welds”. 

Q16. What is the relationship between the grain size and the hardness of the nanosized precipitates?

As the hardening effect of the nanosized precipitates is not modified during FSW, the hardness evolution is related mainly to the grain size evolution observed. 

Q17. What is the residual stress level of the butt welds?

The residual stress level obtained after FSW aluminium alloys is in the same range (<100 MPa [25–27]), but it corresponds to 30–60% of FSW yield stress and 20–50% of base metal yield stress [25], whereas in AZ31 it corresponds to 66–76% of FSW yield stress and 46% of base metal yield stress.