Investigation Of Process,Tensile And Wear Properties Of Nanocrystalline H62 Brass Induced By Laser Shock Peening

As a novel and efficient surface treatment technology,laser shock peening?LSP?has the characteristics of ultra-high energy?more than 1GW/cm²?,ultra-high pressure?GPa-TPa?,ultra-high strain rate(10⁷ s^-1)and ultra-fast?ns?,and thereby has a great advantage in the field of surface modification of metal materials.The grain refinement and microstructure evolution caused by LSP are the key factors to improve the properties of the materials,but normally,the grain refinement can only reach the micron magnitude.Therefore,there are few reports about the phenomenon and forming mechanism of surface nanocrystallization,amorphization induced by laser shock peening and the relationship between surface nanocrystallination and material properties.H62 brass is a widely used copper alloy,but its wear resistance is relatively poor,which can not meet the higher-wear-resistance requirement in some industrial fields.Thus,The H62 brass is subjected to LSP treatment in this paper.The changes of microhardness,surface roughness,friction and wear properties before and after LSP at room temperature?RT?were investigated.And the effects of different working conditions on the friction and wear properties were discussed.The effects of laser shock layer number and temperature on the tensile properties and microstructure of H62 brass were investigated.The formation mechanism of nanocrystallization and amorphization of H62 brass were explored.The specific contents and conclusions of the study are as follows:?1?H62 brass was subjected to LSP with one and three coverage layers at RT,and the changes of microhardness,roughness and friction and wear properties before and after LSP were investigated.The results showed that the microhardness and surface roughness had been improved by LSP.The microhardness of H62 brass after LSP with one and three coverage layers are 159.5 HV and 167.6 HV,and increase by 15.9%and 21.8%,respectively.The surface roughness increased from 0.258?m?untreated?to 0.317?m?LSP with one coverage layer?and 0.366?m?LSP with three coverage layers?.The friction coefficient and specific wear rate of H62 brass decreased with the increase of the LSP coverage layer number.The specific wear rates of H62 brass after LSP with one and three coverage layers were 10.964×10^-4 mm³/Nm and 9.8875×10^-4 mm³/Nm,and decreased by 13.4%and 21.9%,respectively.With the increase of the LSP coverage layer,the wear mechanism changed from dominating with delamination wear to dominating with abrasive wear.?2?The friction and wear tests of the as-machined specimen and the LSPed specimen at RT were carried out.The effects of load and sliding velocity on the friction and friction properties of H62 brass before and after LSP were investigated.The friction coefficient of as-machined specimen and LSPed specimen decreased with the increase of load and decreased at first and then increased with the increase of sliding velocity.The specific wear rate increased with the increase of load and decreased with the increase of sliding velocity.At the sliding velocity of 10 mm/s,with the increase of load,the main wear mechanism of the as-machined specimen and LSPed specimens changed from abrasive wear to delamination wear,accompanied with oxidation wear.At the load of 20 N,with the increase of sliding velocity,the wear degree increased gradually.The main wear mechanism was delamination wear,accompanied with abrasive wear and oxidation wear.When the sliding velocity was low,adhesive wear also occurred in the as-machined specimen.With the increase of load and sliding velocity,the degree of oxidation of wear surface became higher.?3?H62 brass was subjected to LSP with one and three coverage layers at RT and warm laser shock peening?WLSP?with one coverage layer.The change of cross section microstructure of H62 brass under different LSP processes was investigated.The results indicated that surface nanocrystallization was realized on the surface of H62 brass treated by all three kinds of LSP processes.Even amorphization occurred on the surface of H62 brass subjected to LSP with three coverage layers at RT and WLSP with one coverage layer.The nanocrystalline size acquired on the specimen surface was smallest in the WLSPed specimen with one coverage layer?WLSP-1 specimen?,followed by the LSPed specimen with three coverage layers at RT?RTLSP-3 specimen?,and it was largest in the LSPed sample with one coverage layer at RT?RTLSP-1 specimen?.The formation of surface nanocrystallization at RT was mainly due to the dislocation movement.Multiple dislocation structures including dislocation lines,dislocation tangle and dislocation walls were generated by LSP.The original coarse grains were subdivided by formation of dislocation cells.During the LSP with three coverage layers,the grain refinement occurs in a smaller scale in the inner of the refined grain.And thus nano-grain with smaller size is formed.During the WLSP with one coverage layer,high density dislocation tangle and mechanical twins were formed in the inner of H62 brass.Dislocation slip and mechanical twins dominated the plastic deformation process.In addition,the dislocations were pinned by solute atoms due to the dynamic strain aging,resulting in higher density dislocations.Thus more dislocation and complex dislocation structure were formed in the inner of the refined grain.As a result,smaller nanocrystalline were generated.The accumulation of defects and formation of chemical disorder caused by plastic deformation led to the increase of free energy,resulting in the amorphization.?4?The tensile test of H62 brass subjected to LSP with one and three coverage layers at RT and WLSP with one coverage layer were carried out.And the changes of the tensile properties under different LSP processes were also analyzed.It was found that the strength and plasticity of H62 brass were improved by LSP.The ultimate tensile strength of RTLSP-1 specimen,RTLSP-3 specimen and WLSP-1specimen reached 368 MPa,372MPa and 389 MPa,and increased by 18.3%,19.6%and 25.1%,respectively.Compared with the as-machined specimen,the strain up to plastic instability point of the RTLSP-1 specimen,RTLSP-3 specimen and WLSP-1specimen increased by 2.3%,6.7%and 27.4%.All three kinds of specimens showed a good strength and plasticity.