Study On Mechanical Properties And Milling Performance Of Stainless Steel Manufactured By Laser Additive Manufacturing

Aimed at the lack of intensive study on the theory of additive manufacturing technology and the lack of systematic study on the machinability of additive manufactured alloy parts,the thesis is focused on study on the mechanical properties and milling performance of laser additive manufactured stainless steel.The heat transfer process and the temperature filed in laser additive manufacturing metal process were studied.The effect of additive manufacturing processing parameters on the microstructure,mechanical properties and milling performance of stainless steel parts was experimentally analyzed,and the additive manufacturing parameters and milling parameters were optimized.The difference in microstructure,mechanical properties and milling performance between laser additive manufactured stainless steel and forged stainless steel was investigated.The heat transfer process and the temperature filed in laser additive manufacturing metal process were studied.The temperature field models and cooling rate models for laser additive manufacturing metal were established,and the effect of additive manufacturing parameters on temperature filed and its cooling rate was analyzed.The results showed that the temperature around the laser spot was increased with an increase in the laser power and decreased with an increase in the laser scanning speed.In addition,the cooling rate was decreased with an increase in the laser power and the initial temperature of the parts.The effect of building direction on the microstructure,mechanical properties and milling performance of laser additive manufactured stainless steel parts was investigated.It was found that the microstructure was homogeneous on the horizontal section,at the building direction of 0°,while a number of longer grains were present in the microstructure on the vertical section,at the building direction of 90°.The ultimate tensile strength,yield strength,elongation after fracture and hardness at the building direction of 0° were higher than those at the building direction of 90°.For both building directions,the decrement in surface roughness and the increment in cutting force and tool wear,were observed with an increase in the cutting speed.The cutting force,tool wear and surface roughness of the additive manufactured stainless steel at the building direction of 0° were relatively higher than those at the building direction of 90°.The effect of laser scanning speed on the microstructure,mechanical properties and milling performance of laser additive manufactured stainless steel parts was investigated.It was found that with an increase in laser scanning speed,the grain size,ultimate tensile strength and yield strength were decreased while the hardness was increased,the elongation was firstly increased and then decreased.In the milling experiments,the tool wear was affected significantly by the laser scanning speed.When the laser scanning speed was 300 mm/min,the tool wear was increased with an increase in the cutting speed.When the laser scanning speeds were 500,700 and 900 mm/min,the tool wear was firstly increased and then decreased with an increase in the cutting speed,and it was increased with an increase in the laser scanning speed.For each laser scanning speed,the cutting force was increased and the surface roughness of the machined surface was decreased,with an increase in the cutting speed.The effect of laser power on the microstructure,mechanical properties and milling performance of laser additive manufactured stainless steel parts was investigated.The results showed that with an increase in the laser power,the ultimate tensile strength,yield strength and hardness were decreased while the grain size was increased,the elongation after fracture was firstly decreased and then increased.In the milling experiments,the tool wear was affected significantly by the laser power.When the laser power was 1500W,the tool wear was increased with an increase in the cutting speed.When the laser powers were 2000,2500 and 3000 W,the tool wear was firstly increased and then decreased with an increase in the cutting speed,and it was increased with an increase in the laser power.For each laser power,the cutting force was increased and the surface roughness of the machined surface was decreased,with an increase in the cutting speed.The effect of layer thickness on the microstructure,mechanical properties and milling performance of laser additive manufactured stainless steel parts was investigated.The decrement in grain size and the increment in hardness were observed,with an increase in the layer thickness.However,there was little effect on the tensile properties.In the milling experiments,the tool wear was affected significantly by the layer thicknesses.When the layer thicknesses were 0.5 and 1.0 mm,the tool wear was firstly increased and then decreased with an increase in the cutting speed.When the layer thicknesses were 1.5 and 2.0 mm,with an increase in the cutting speed,the failure mode of cutting tool was transferred from wear to fracture,and the tool fracture was getting more heavily.For each layer thickness,the cutting force was increased and the surface roughness of the machined surface was decreased,with an increase in the cutting speed.The additive manufacturing parameters and the milling parameters were optimized.When the laser scanning speed,laser power,layer thickness,building direction and environment temperature were 700 mm/min,1500 W,1.0 mm,0° and +25?respectively,the mechanical properties of the additive manufactured stainless steel parts were higher.When the cutting speed,feed rate and depth of cut were 150 m/min,0.05 mm/z and 0.1 mm respectively,the tool wear and the surface roughness of machined surface were lower.The difference in microstructure,mechanical properties and milling performance between laser additive manufactured stainless steel parts and forged stainless steel parts was analyzed.It was found that the microstructure of the forged parts was isotropic and the grain size was coarse,while the microstructure of the additive manufactured parts was anisotropic and the grain size was thin.Compared with the forged parts,the ultimate tensile strength,yield strength and hardness of additive manufactured parts,which was built by optimum additive manufacturing parameters,were higher,but the elongation after fracture was lower.When the cutting speed was 150 m/min,the tool wear and the surface roughness of the machined surface of the additive manufactured parts,which was built by optimum additive manufacturing parameters,were lower.