Microstructure and Mechanical Properties of a Fine-Grained AZ91 Magnesium Alloy Prepared by Multi-Pass Friction Stir Processing

Abstract

Friction stir processing (FSP) is a novel severe plastic deformation technique developed in recent years to produce fine-grained structural materials. Through increasing the processing pass, further grain refinement can be achieved. In this paper, the microstructure and mechanical properties of AZ91 magnesium alloy prepared by the single-pass and two-pass FSP were studied. The results showed that the coarse, network-like eutectic β-Mg17Al12 phase was broken into particles and some of them dissolved into the magnesium matrix, and the α-Mg grains were remarkably refined after FSP. The average grain sizes of the single-pass and two-pass FSP alloys were 8.3 μm and 5.8 μm respectively. The ultimate tensile strengths of the specimens were 284.5 MPa and 319.7 MPa, and elongations were 13% and 14.5%, respectively. The improved mechanical properties of the two-pass FSP specimen were mainly attributed to the finer grain size and more homogenized microstructure.

Evaluation of corrosion resistance of multipass friction stir processed AZ31 magnesium alloy

“…The average grain size in the 1‐PASS sample is ~10 μm, while that of the 3‐PASS sample is ~6 μm. The results indicate that subsequent passes can further reduce the grain size in AZ31 plates, which is consistent with the previous studies …”

“…In fact, many previous studies have reported that further grain refinement is widely observed in AZ31, AZ61, and AZ91 alloys produced by multipass FSP. [1,23,28,40] The reason may have a close relationship with the mechanisms of dynamic recrystallization (DRX) and grain structure evolution during FSW/FSP. Suhuddin et al [41] deeply studied the DRX mechanisms and grain structure evolution of AZ31 alloy during FSW by a "stop-action technique."…”

“…The results indicate that subsequent passes can further reduce the grain size in AZ31 plates, which is consistent with the previous studies. [1,23,28] Measured hardness values of the 1-PASS and 3-PASS samples are presented in Figure 3. Hardness is scattered between 50 and 65 HV.…”

The Effect of Rapid Deformation Process to Improve Creep and Tensile Resistance of AZ91 Magnesium Alloy Plates

“…Lua and Zhangb. [13] reported that after FSP, the β-Mg17Al12 networks broke into smaller particles, but the microstructure of AZ91 alloys was not finely equiaxed.…”

“…Their results show that some coarse β-Mg17Al12 phases that existed after the first pass of FSP break and dissolve into the matrix under the action of the second pass of FSP. Lu et al [31] conducted twopass FSP (with water cooling) on cast AZ91 plates. Their results show that the microhardness, tensile strength, and elongation of the processed specimens were 94.7 HV, 155.5 MPa, and 31.5%, respectively, which were more than that of the base plates.…”