Analysis of metallographic structure and hardness of magnesium alloy area using friction stir welding

Abstract

The present work envisages the friction stir welding of AZ40 M magnesium alloy to analyze the influence of different process parameters (rotation speeds: 600, 800, and 1000 r/min; feed speeds: 100, 120, and 150 mm/min) on the metallographic structure at different locations in the weld zone. The welded regularity, analysis of the distribution law of the weld surface, and section hardness value (HV) were obtained under different welding process parameters. Our results show that, when the current feed rate was constant, the grain size of the weld nugget increased with an increase in the rotation speed. When the rotation speed was constant, the grain size of the weld nugget area decreased initially, which subsequently increased with an increase in the advance speed. When the rotation speed was 600 r/min and the feed speed was 120 mm/min, the nugget region grain was uniform, fine, and exhibited a highest HV.

Introduction

Magnesium alloys have the advantages of low density, high specific strength and specific stiffness, good vibration damping effect, thermal conductivity, as well as easy cutting and easy casting. Currently, the magnesium alloys are widely used in the manufacturing of aircraft, high-speed trains, new energy vehicles, and other manufacturing areas. Magnesium alloy is light and suitable for the needs of modern manufacturing. ¹ However, its weld ability is poor. Various methods like the traditional fusion welding and the spontaneous combustion are difficult to ensure the welding of magnesium alloys. This undoubtedly led to a reduction in the application range of magnesium alloys. To promote the application of magnesium alloy in more fields, the welding experiment of magnesium alloy was studied by the friction stir welding (FSW) process. FSW is a solid-state welding method, in which the metal in the joint region is semiplasticized by agitating the two welded plates and then extruded under the constraints of the shoulder, the plate, and the table. FSW method is devoid of any harmful substances, gases, and so on, and is a green welding technology since it meets the environmental protection needs of China’s manufacturing industry. ^{2 –10} The biggest advantage of FSW is the ability to weld plates that cannot be welded by the fusion welding method. Currently, there is a rapid development in the research related to this technology.

Literature reveals that there has been an extensive research carried on the microstructure of welded joints of AZ31B magnesium alloy. ¹¹ For example, the metallographic structure of the friction stir welded joint of magnesium alloy was studied, in which the morphology of the “onion ring” was seen, whose morphology was affected by the base metal structure, process parameters, and the size of the pin structure; while other studies indicated that the appearance of the weld was well formed, but there was a through-hole (tunnel-shaped hole) defect inside. ^12,13 Liu et al. observed the grain refinement of the weld zone through metallographic structure after the FSW of dissimilar magnesium alloys ZK60 and AZ31. ¹⁴ In the present work, the AZ40 M magnesium alloy sheet was used for the FSW experiments. Different process parameters were used to study the influence on the grain size of AZ40 M magnesium alloy welded area and the grain change of weld area. At the same time, the observation of the metallographic structure for analyzing the hardness distribution law of the welding area can provide a certain technical support for the application of AZ40 M magnesium alloy in the FSW.

Experiments

Experimental materials

We used AZ40 M magnesium alloy in the present study, whose main chemical composition is shown in Table 1. The experiment used two plates of size 200 × 100 × 13 (mm) for welding.

Table 1.

Chemical compositions of AZ40 M magnesium alloy (mass fraction, %).

Al	Zn	Mn	Cu	Ni	Si	Fe	Mg
3.51	0.47	0.39	0.0042	0.00038	0.024	0.0016	Balance

Experimental equipment and method

The FSW-LS-A10 type friction stir connection device developed by AVIC Beijing Saifusite Technology Co., Ltd, Beijing, China, was used in the FSW experiment. The diameter of the tool shoulder was 24 mm, the length of the pin was 10 mm, the diameter of the big end of the pin was 10 mm, the diameter of the small end was 5 mm, and the pin had a right-hand thread, as shown in Figure 1.

Figure 1.

The FSW equipment and tool. FSW: friction stir welding.

The process parameters used in the FSW process are shown in Table 2.

Table 2.

FSW process parameters.

Sample number	Rotation speed of the tool ω (r/min)	Feed speed of the tool ν (mm/min)
1	600	100
2	600	120
3	600	150
4	800	100
5	800	120
6	800	150
7	1000	100
8	1000	120
9	1000	150

FSW: friction stir welding.

Before the FSW test of the magnesium alloy, the parts to be welded were sanded using sandpaper or grinding wheel to remove the oxides on the surface of the magnesium alloy. Subsequently, the polished impurities were washed away with water, followed by washing with absolute ethanol, and blown with a hairdryer. After that, it was subjected to fixed welding.

The metallographic analysis of the weld area for each parameter was carried out by using the MDS400 inverted metallographic microscope (Chongqing Aote Optical Instrument Co., Ltd, Chongqing, China). The hardness test of the weld zone was carried out using the HV-1000 Vickers hardness tester (Shanghai Caikang Optical Instrument Co., Ltd, Shanghai, China).

Analysis of results

Metallographic analysis

Figure 2 shows the metallographic structure of the welded section of the magnesium alloy sheet at a rotation speed of 600 r/min and a feed speed of 100 mm/min. It can be seen in Figure 3 that the zones 1 and 4 were thermomechanically affected zones (TMAZ), zones 2, 3, 5, and 8 were nugget zones, zones 6 and 9 were heat-affected zones (HAZ), and zones 7 and 10 were base metal (BM) zones. In Figure 2, the regions 2, 3, 5, and 8 exhibited a grain refinement and were equiaxed crystals. Although the grains in the regions 1 and 4 were refined, there was a tendency to flatten, which indicates that the plastic extrusion process occurred during the stirring process. The grains in the regions 6 and 9 were in a stratified state, wherein some grains were in a refinement state, and some grains remained similar to the BM (BM is a typical HAZ state, and the grains are elongated near the BM region). The flattened state indicated that there was an intense agitation and extrusion in this part. The grains in the regions 7 and 10 parent metal were maintained in a flat strip shape in its original rolled state.

Figure 2.

Microstructure of FSW section of magnesium alloy (the rotational speed of tool ω = 600 r/min, the feed speed of tool ν = 100 mm/min). FSW: friction stir welding.

Figure 3.

Metallographic structure of FSW welding nugget region of magnesium alloy sheet under different parameters. (a) ω = 600 r/min, ν = 100 mm/min; (b) ω = 600 r/min, ν = 120 mm/min; (c) ω = 600 r/min, ν = 150 mm/min; (d) ω = 800 r/min, ν = 100 mm/min; (e) ω = 800 r/min, ν = 120 mm/min; (f) ω = 800 r/min, ν = 150 mm/min; (g) ω = 1000 r/min, ν = 100 mm/min; (h) ω = 1000 r/min, ν = 120 mm/min; and (i) ω = 1000 r/min, ν = 150 mm/min. FSW: friction stir welding.

Figure 3 shows the metallographic structure and parent metal structure of the weld nugget under different parameters. It can be seen from Figure 3 that the grains in the nugget obtained under each parameter were fine and uniform. Comparing Figure 3(a), (d), and (g), Figure 3(b), (e), and (h), and Figure 3(c), (f), and (i), it was found that when the feed speed of the tool was constant, the grain size of the weld nugget increased with an increase of the rotation speed of the tool. This is because, in the unit volume, the feed speed of the tool was constant, and an increased heat was generated by the increase in the rotation speed of the tool. However, the speed of heat dissipation was constant, so that the large rotation speed of the tool did not dissipate due to the increased heat, and the grain in the nugget region was again refined and grown. Comparing Figure 3(a) to (c), Figure 3(d) to (f), and Figure 3(g) to (i), it was found that when the rotation speed of the tool was constant, the nugget grains first decreased and later increased in size. This is because when the rotation speed of the tool was constant and the feed speed of the tool was 100 mm/min, the heat generated by the friction stirring promoted the grain refinement in the weld zone to form a weld. When the feed speed of the tool was 120 mm/min, the increase of friction stirring heat promoted the grain refinement in the weld zone. When the feed speed of the tool was 150 mm/min, the friction stirring heat was too large, and the partially refined grains continued to grow, so that the phenomenon occurred, as shown in Figure 3. From the size and uniformity of the grain in the nugget region as shown in Figure 3, the optimum process parameters for the AZ40 M magnesium alloy FSW were the rotation speed of the tool: 600 r/min and the feed speed of the tool: 120 mm/min.

Figure 4 shows the metallographic structure of the upper TMAZ of the magnesium alloy FSW welding zone under different parameters. As can be seen from Figure 4, the grains of Figure 4(a), (c), (d), (h), and (g) were not uniform and were coarse. The grains in Figure 4(b), (e), (f), and (i) were relatively uniform, while the grains in Figure 4(b) were larger, but they had a significant extrusion process relative to the metallographic structure of the parent metal, and it was flat and uniform size relative to the parent metal grain. The crystal grains were relatively fine in Figure 4(i) and uniform. Figure 4(e) and (f) showed the presence of larger grains.

Figure 4.

Metallographic structure of thermal–mechanical affected zone at the upper end of magnesium alloy sheet FSW under different parameters. (a) ω = 600 r/min, ν = 100 mm/min; (b) ω = 600 r/min, ν = 120 mm/min; (c) ω = 600 r/min, ν = 150 mm/min; (d) ω = 800 r/min, ν = 100 mm/min; (e) ω = 800 r/min, ν = 120 mm/min; (f) ω = 800 r/min, ν = 150 mm/min; (g) ω = 1000 r/min, ν = 100 mm/min; (h) ω = 1000 r/min, ν = 120 mm/min; and (i) ω = 1000 r/min, ν = 150 mm/min. FSW: friction stir welding.

Hardness analysis

Figure 5 shows the hardness distribution of the vertical weld direction of the FSW weld surface obtained under each parameter. It can be clearly seen from Figure 5 that the hardness curve was substantially symmetrical with the centerline of the weld, which indicates that the grain sizes on the front side and the return side were substantially same. The hardness value (HV) of the sample 2 in Figure 5 was the largest, which was related to the minimum grain size in Figure 3(b). The HV results in Figure 5 were in good agreement with the metallographic structure in Figure 3.

Figure 5.

Distribution law of the welding area surface hardness of each sample.

Figure 6 shows the HVs at different points on the centerline of the weld cross section from the surface under each parameter. In Figure 6, the average hardness of the HAZ of the samples 1–9 was 76HV_0.3, 80HV_0.3, 66HV_0.3, 66HV_0.3, 64HV_0.3, 69 HV_0.3, 65HV_0.3, 75HV_0.3, and 76HV_0.3, respectively. The BM HV was 77HV_0.3. It can be seen from Figure 4 that the grain of the TMAZ of the sample 2 was finer and more uniform, and therefore, the hardness of the sample 2 exceeded the hardness of the BM.

Figure 6.

Hardness values on the weld centerline of welding cross under different parameters of each sample.

Conclusions

Our study offered the following conclusions:

1. With a constant advancing speed, the grain size of the weld nugget increased with an increase in the rotating speed. When the rotating speed was constant, the grain size of the weld nugget initially decreased and subsequently increased with an increase in the advancing speed.

2. The grain flattening state occurred in the TMAZ obtained under any parameter, which is a result of the plastic extrusion of the tool shoulder.

3. The surface hardness of the weld obtained under each process parameter was symmetrical with respect to the centerline of the weld, and the HVs of the forward side and the return side were substantially similar. Sample 2 exhibited a highest HV.

4. The maximum HV of the weld zone cross-section TMAZ obtained under each process parameter was in the order: sample 2 > sample 1.

5. The optimal FSD process parameter for AZ40 M magnesium alloy included the rotation speed of the tool to be 600 r/min and the feed speed of the tool to be 120 mm/min.