Research On Friction And Wear Properties Of AISI304 Stainless Steel Strengthened By Laser Shock Peening And Ultrasonic Impact Peening

As advanced surface strengthening technologies,ultrasonic impact peening(UIP)and laser shock peening(LSP)have unique advantages in the surface strengthening of metal materials.The advantage of UIP is that it can produce higher surface hardening while LSP can induce a large depth of compressive residual stress layer.By combining UIP and LSP,improved surface performance of metal materials can be achieved.AISI304 stainless steel is extensively used in engineering areas.However,poor hardness and strength limits its application in actual production with strict requirement on wear resistance.In this dissertation,UIP,LSP and UIP+LSP are used to strengthen the surface of AISI304 stainless steel.The effects of UIP,LSP and UIP+LSP on the surface integrity,friction and wear performance of AISI304 stainless steel are investigated,and the mechanism of improved wear resistance is discussed.The main results of this dissertation are as follows:(1)The strengthening mechanism of AISI304 stainless steel by surface severe plastic deformation was theoretically analyzed.Based on the surface integrity test of the samples,the change of the surface integrity of AISI304 stainless steel after UIP,LSP and UIP+LSP was studied.The results show that the surface roughness of the sample increases after surface treatments.The gradient hardened layer and a compressive residual stress layer are generated after UIP and LSP.After surface treatments,the maximum hardness of the sample is on the surface of the sample,while the maximum residual compressive stress is on the subsurface layer.Compared with UIP and LSP,the magnitude and depth of the microhardness and compressive residual stress are significantly improved after UIP+LSP.(2)X-ray diffraction and advanced electron microscopy were employed to analyze the plastic deformation behavior of AISI304 stainless steel after UIP,LSP and UIP+LSP.The results show that no martensite transformation occurred after LSP while considerable martensite phases were produced by UIP.After UIP+LSP,the martensite content in the surface of the sample slightly increased compared with the sample after UIP and the martensite content increases with the increase of plastic deformation.A large number of dislocations and deformation twins generated in AISI304 stainless steel after surface strengthening treatments.(3)The effects of LSP,UIP and UIP+LSP on the friction and wear property of AISI304 stainless steel under different loads were investigated.Moreover,the strengthening mechanism of UIP+LSP on the friction and wear performance of AISI304 stainless steel is obtained.The results show that the friction coefficient of the sample gradually decreases with the increase of the load after the surface strengthening treatments,and the decrease rate gradually becomes slow.The friction coefficient,specific wear rate of the sample under different loads significantly decreased after UIP+LSP,and the friction and wear property is better than that of the specimens after LSP and UIP.The wear mechanism of the original sample and the LSPed sample is abrasive wear and adhesive wear.Moreover,the original sample shows slight fatigue wear.As the load increases,the oxidation on the surface of the UIP+LSP processed specimen becomes obvious,and the wear mechanism is transformed into a combined effect of abrasive wear,oxidative wear,and adhesive wear.(4)The main strengthening mechanism of UIP+LSP on the friction and wear performance of AISI304 stainless steel can be summarized as follows.UIP significantly improves the hardness in the surface layer of AISI304 stainless steel,and the high surface hardness strengthens its resistance to further plastic deformation and wear.Due to the increased surface strength after UIP,the surface layer of the sample after LSP can accommodate compressive residual stress with higher magnitude.The compressive residual stress delays the generation and propagation of cracks,and prevents fatigue wear.Therefore,the friction and wear performance of AISI304 stainless steel is effectively improved by combining UIP and LSP.