Abstract:  Large amounts of heat generate during the preparation of ultrafine grained materials，which may decrease the strength of materials． In order to solve this problem，a new method named cryogenictemperature extrusion machining ( CT-EM) was put forward． In this method，Al 7075 alloy was chosen as the workpiece material and roomtemperature extrusion machining ( RT-EM) was also conducted for a comparison in terms of chip morphology，microstructure and mechanical properties． The results indicate that ( 1) the chips prepared by CT-EM and RT-EM are continuous and segmented，respectively; ( 2) the surface of the chip obtained by CT-EM possesses fewer defects with better integrity; ( 3) the maximum hardness of CT-EM and the RT-EM samples are 190 ( HV，the same as follows) and 175，respectively，which are far greater than that of the original material ( 105) ; ( 4) after the CT-EM process，the grains are highly refined to less than 200 nm in diameter; ( 5) the grain refinement and dislocation strengthening are two main strengthening mechanisms for Al 7075 alloy; and ( 6) as CT-EM Al 7075 alloy possesses higher dislocation density，the contribution of dislocation strengthening to hardness is greater． 

Key words: Al 7075 alloy, cryogenic temperature, extrusion machining, microstructure, hardness