Research On Surface Burnishing Strengthening Of ZK60 Magnesium Alloy

The effect of burnishing process parameters on the surface integrity and microstructure of ZK60 magnesium alloy was studied,and the reasonable process parameters of burnishing treatment of ZK60 magnesium alloy were put forward.The strengthening mechanism and the effect law of burnishing process parameters on strengthening results have been revealed by means of investigation of mechanical properties,tribological behaviours and fatigue characteristics for burnished magnesium alloy ZK60.The results of burnishing process parameters on the surface integrity and microstructure of ZK60 magnesium alloy show that within the range of test parameters,with the increase of burnishing pressure,the number of burnishing passes and the decrease of burnishing speed and feed rate,the surface roughness decreases,and the surface microhardness,residual compressive stress and the depth of strengthening layer increase.Among them,the burnishing pressure has the greatest impact,followed by the number of burnishing passes,and the impact of burnishing speed and feed rate is relatively small.With the increase of surface depth,The absolute value of surface residual compressive stress can reach 102.6MPa.Along the depth direction,the absolute value of residual compressive stress increases first,then decreases and then increases.Along the depth direction,the microhardness increases first and then decreases to the hardness of the matrix material,the maximum hardness was 105.81 HV_0.25.The surface roughness Ra decreased from 1.14?m to 0.05?m.After burnishing treatment,the phase transformation of ZK60 magnesium alloy did not take place.The coarse grains in the surface layer were broken and refined,the grains were distributed in irregular shape,the grain boundaries were distorted,the crystals slip,the dislocation density was increased,and some deformation twins were produced.The results of the study on the wear resistance of ZK60 magnesium alloy show that the wear and the average friction coefficient of the burnishing treated sample were reduced compared with the untreated sample,and the wear scar morphology was more smooth and shallow.In the process of friction and wear,the burnishing treated sample can pass through the running in stage faster and enter the stable wear stage.The friction and wear mechanism of the untreated sample and the burnishing treated sample are the composite wear formed by adhesion wear and abrasive wear.The reduction of surface roughness and the improvement of surface microhardness make the wear resistance of the samples more excellent.The strength of ZK60 magnesium alloy increases and the plasticity decreases after burnishing treatment.The yield strength increased from 237.8 MPa to 264.5 MPa,the tensile strength increased from 307.3 MPa to 322.7 MPa,and the elongation decreased from 22.7%to18.3%.Compared with the edge morphology of the tensile fracture,the dimple and torn edges of the fracture edge of the burnishing treated sample were reduced,and a large number of quasi-cleavage planes appeared,showing the morphology characteristics of the mixed fracture of toughness and brittleness.The results of the fatigue property and fatigue fracture behavior of ZK60 magnesium alloy show that the fatigue life of the burnishing treated sample was larger than that of the untreated sample,and the greater the burnishing pressure is,the greater the fatigue life of the sample is.The fatigue crack of the untreated sample originated on the most surface of the sample,and then spreads in a fan-shaped way towards the interior of the sample;the position of the fatigue crack initiation of the burnishing treated sample transferred from the most surface to the secondary surface,and then spreads in a fan-shaped way towards the interior of the sample.Surface burnishing treatment can increase the residual compressive stress and decrease the surface roughness of ZK60 magnesium alloy,which is beneficial to improve fatigue performance and prolong fatigue life.The burnishing strengthening mechanism of ZK60 magnesium alloy is finishing strengthening,stress strengthening and strain strengthening.The elimination of surface defects and stress concentration reduces the notch sensitivity of the sample surface.The maximum"effective tensile stress"generated by the superposition of the residual compressive stress and the applied tensile stress is transferred to the tensile stress area of the sub surface,resulting in the transfer of the fatigue crack initiation position from the most surface to the sub surface.It is difficult to generate and start dislocation slip in the material,which delays the nucleation and proliferation rate of cracks on the sub surface,thus improving the fatigue performance and prolonging the fatigue life of magnesium alloy.