| Al-Mg-Si-(Cu) aluminum alloys of 6000 series have widespread applications for their good mechanical properties, excellent weld-ability, resistance to corrosion and good formability in the fields of automobile manufacturing and aviation. However, compared with the aluminum alloys of 7000 series, the strength of the aluminum alloys of 6000 series is still need to improve. Severe plastic deformation(SPD) technique is an effective method to refine grain and can improve the mechanical properties of metal materials obviously. Equal channel angular pressing(ECAP) has broad application prospects for its production capacity of ultrafine–grained structural materials by using shear deformation. The research of the microstructure and ductility of SPD aluminum alloys is relatively systematic at home and abroad, but the research of its wear behavior is very few. Therefore, it has a vital significance to study the wear properties of 6061 ultrafine-grained aluminum alloy and analyze the influences of microstructure and mechanical properties on friction and wear behavior.In the present work, the effects of the different temperature of dynamic aging ECAP on microstructure and mechanical properties in 6061 Al-Mg-Si aluminum alloys are investigated by means of X–ray diffractometer(XRD), differential scanning calorimetry(DSC) analysis, tensile test, and transmission electron microscopy(TEM) etc. Particularly, the wear behaviors of the ECAPed aluminum alloy with highest strength and good ductility were compared to the wear tests of solid solution state and peaking aging state(T6) under different loads. The wear surface, the wear subsurface and wear debris were analysed by scanning electron microscope(SEM), noncontact optical profilometer and energy dispersive spectrum(EDS) to explore the wear mechanism of ultrafine-grained aluminum alloy and the effect of microstructure on the friction and wear behavior of ultrafine-grained materials. The main results of this work are listed as follows:(1) The range of average grain size, lattice strain and dislocation density can be obtained from the XRD results of dynamic aging ECAPed 6061 aluminum alloy. The range of average dislocation density of the ECAPed samples are 0.25×1014m–2 to 1.75×1014 m–2. The average grain size of the ECAPed 6061 samples are about 100 nm to 173 nm. The grain size is in the ultrafine-grain range.(2) Both DSC and TEM analyses show that various amount of β " and β’ precipitates are detected in ECAPed 6061 aluminum alloys under different temperature. There are a large amount of β" precipitates and dislocations in 6061 alloy ECAPed at 170 °C. The very fine precipitates may play a role of pinning on the surrounding dislocation and increase the strength of alloy.(3) The mechanical properties of 6061 aluminum alloys are significantly increased by combination of ECAP and appropriate aging process. The ultimate tensile strength increased 2~3 times larger than that of solid-solution treatment samples(from 151 MPa up to 310~450 MPa) and the yield strength increased 4~6 times larger than that of solid-solution treatment samples(from 67 MPa up to 272~425 MPa). The alloy ECAPed at 110 °C reach the level of high strength aluminum alloy and the ultimate tensile strength, yield strength and uniform elongation are 450 MPa, 425 MPa and 15%, respectively. The high strength of ECAPed samples are attributed to the cooperation effects of precipitation strengthening, dislocation strengthening and grain boundary strengthening.(4) The average friction coefficients of samples ECAPed at 110°C range from 0.4567 to 0.528 at the load from 5 to 25 N. The average friction coefficients and wear rate of samples ECAPed at 110°C are lower than these of solid solution state and T6 state. The process of aging and ECAP played a key effect on the antifriction performance with more excellent wear resistance.(5) The varieties and transformation sequence of the wear mechanism of the ultrafine–grained and coarse-grained 6061 aluminum alloy at room temperature are abrasive wearâ†'oxidation wear + delamination wearâ†' adhesive wearâ†'surface fatigue wear. The dominant wear mechanisms of the alloy ECAPed at 110 °C are abrasive wear, oxidation wear + delamination wear and adhesive wear under 5 N, 10 N and 25 N respectively.(6) After dynamic aging ECAP process at 110°C, the wear mechanisms of ultrafine–grained 6061 aluminum alloy under 5 N and 10 N are the same as those of coarse-grained 6061 aluminum alloy under 10 N and 25 N respectively. In other words, 6061 aluminum alloy ECAPed at 110°C can delayed the process of the wear mechanisms.(7) It was reported that wear resistance is usually decreased during the wear process in most ultrafine–grained materials with high strength and low ductility. However, the creating and diffusing of cracks between friction layer and plastic deformation zone is susppressed by high ductility of ultrafine–grained 6061 alloy ECAPed at 110° C. As a consequence, the friction coefficient and wear rate are decreased effectively.(8) The wear resistance of ultrafine grained materials is affected by both the strength and ductility. In the 6061 aluminum alloy of dynamic aging ECAP in this paper, the high strength and good ductility are achieved for the cooperative interaction of high density of nanoscal β" precipitates and dislocations. The improvement of wear resistance in the ultrafine–grained 6061 aluminum alloy is attributed to the combination of higer strength and work hardening ability. |
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