Effects of constrained groove pressing and temperature-assisted ultrasonic shot peening on microstructure and mechanical properties of a two-phase Mg–Li alloy

Abstract

Two passes of constrained groove pressing (CGP) were conducted on a two-phase Mg–Li alloy followed by temperature-assisted ultrasonic shot peening (TA-USP) with different peening durations and temperatures. The effects of TA-USP and CGP on microstructural evolution and mechanical properties were investigated in details. Surface gradient microstructure including nanocrystalline and partial amorphization is introduced by TA-USP. Grain refinement is more significant with longer durations and lower temperatures. Pretreatment by CGP before TA-USP introduces fine and homogeneous microstructure in the alloy due to severe plastic deformation. The average grain sizes of α-Mg and β-Li are refined from the as-annealed 31.6 μm and 12.9 μm to 2.5 μm and 2.2 μm, respectively. The depth of USP-affected region increases with the increase of peening duration and temperature, and a depth of about 300 μm is achieved by direct TA-USP at room temperature with 400 s. The microstructure gradient is reduced at elevated temperatures. The hardness and strength increase while the ductility decreases with the increase of peening duration. The decrease in hardness with the increase of peening temperature is more significant than peening duration. Compared with direct TA-USP, pretreatment by CGP before TA-USP can obviously enhance the surface strengthening. When processed by TA-USP at 200 °C with 100 s, the tensile strength and yield strength increase from 141.9 MPa and 106.2 MPa to 158.3 MPa and 134.7 MPa, respectively. The elongation to failure decreases from 29.1% to an acceptable value of about 16.1%.

Keywords

• Constrained groove pressing
• Temperature-assisted ultrasonic shot peening
• Microstructure
• Mechanical properties
• LA81 Mg–Li alloy sheets