Effect Of Laser Surface Melting On The Surface Microstructure And Fatigue Crack Growth Behavior Of Al-Si Cast Alloys

Laser surface melting (LSM) is a new technology for the surface modification and this technology has attracted widespread interest. Al-Si cast alloys are widely used in the automobile industry as popular alloys owing to their excellent castability, low thermal expansion coefficient and high thermal stability. ZL108 eutectic alloys are widely used as the piston material; Hyper-eutectic Al-Si alloys are respected because of superior wear-resistance and high hardness. It is becoming a new ideal piston material. These two alloys were treated by LSM. And the effect of LSM on the surface microstructure and fatigue crack growth behavior was investigated in this experiment. The main works are described as follows.The output power,scanning velocity and defocusing distance were selected as the processing parameters during LSM. The surface of eutectic ZL108 alloys and Al-20%Si hyper-eutectic alloys were treated by using different parameters, the three factors' effect on microstructure of samples were studied. When the output power and defocusing distance are constant, the melted depth is slowly increase and then it begin to decrease for ZL108 alloy when the scanning velocity continuously increases; The melted depth reduces as scanning velocity increases for Al-20%Si hyper-eutectic alloy. Maintaining the velocity and defocusing distance invariable, the pool depth increases with the laser power also increases. In condition of the same output power and scanning velocity, the depth of melting layer gradually decreases with the defocusing distance increases.The microstructure of the melting layer was analyzed for the alloys after LSM, which usually consisted of three zones. They are melting zone,transition zone and substrate zone from surface. Whatever the alloy, the coarse block primary Si particles are eliminated in the melting zone, LSM also causes the primary a(Al) and Al-Si eutectic to be get refiner obviously. The dendrite growth which has a clear direction is basically perpendicular to the melt-substrate interface in the transition zone. Furthermore, The primary a(Al) dendrite arm is very long and the secondary arm is very short. As for Al-20%Si hyper-eutectic alloy, the growth of a(Al) dendrite was hindered by the partially-melted primary Si during solidification. Some porosities were observed in the sub-surface after LSM, which was caused by the super-saturated gas in the liquid metal.The results show that processing parameters also affects the microhardness of melted-layer. The specimen's microhardness gradually decreases with the scanning velocity increases. When the velocity and defocusing distance are constant the microhardness decreases with power raises. In condition of the same laser power and scanning velocity, the improvement of the hardness appears to reduce as the defocusing distance increases. The refinement of microstructure and the increased saturation in the melted-layer cause the improvement of microhardness.Fatigue crack growth behavior was comparatively studied for the two alloys before and after LSM. The experimental results show that the fatigue crack growth rate (FCGR) of the laser-treated specimens is evidently lower than the untreated in the condition of same stress intensity factor range whatever stress ratio R is 0.1 or 0.5. Moreover, the difference becomes less with the increase of FCGR and converges finally. The reasons for improvement of fatigue crack growth properties are probably that the refinement of the brittle Si,the stress field formed during the rapid solidification and the distortion of lattice in the supersaturated solid solution.The facture toughness was also tested for the alloys before and after LSM. The results show that the fracture toughness remains essentially unchanged for the alloys after LSM. So, LSM has less effect on the fracture toughness.