Microstructure and mechanical properties of fine-grained thin-walled AZ91 tubes processed by a novel combined SPD process

A combination of parallel tubular channel angular pressing (PTCAP) and tube backward extrusion (TBE) as a novel combined severe plastic deformation (SPD) was applied on AZ91 alloy to produce ultrafine-grained (UFG) thin-walled tubes. The effects of combined SPD process were investigated on the microstructure refinement and mechanical properties. Also, hydro-bulge test was carried out to reveal the mechanical properties. The results showed a notable increase in ultimate strength, yield strength and microhardness of the thin-walled UFG tube were achieved compared to that from PTCAP process. A remarkable grain refinement achieved. Applying three-passes-combined process refine the grain size to 8.8 μ m from an initial value of ∼ 150 μ m . For one pass- and two passes-processed thin-walled tubes, it was about 12.4 and 9.8 μ m , respectively. Yield and ultimate strengths were increased notably to 150 and 354 MPa for 2P+TBE tube, from the initial values of 86 and 166 MPa, respectively. The maximum microhardness was increased to about 105 Hv for the 2P+TBE tube from the initial value of 56 Hv. Hydro-bulge test showed that the bursting pressure increased to 246 bar for 2 passes-thin-walled tube from the initial value of 160 bar. It was 220 and 195 bar for 1 pass- and 3 passes-thin-walled tubes, respectively.