Study On Thermal-oxidative Aging Mechanism,Microstructure And Properties Of Polyamide-12 During Selective Laser Sintering

Selective Laser Sintering(SLS)is one of the most important and widely used technologies in Additive Manufacturing(AM).SLS is superimposed by high-energy laser beam to sinter powder materials,which can directly form plastic functional parts or combine with casting to make structurally complex parts.There are many types of materials used in SLS,among which the Polyamide-12(PA12)in the polymer material has become the most widely used material for SLS due to its excellent performance and powder characteristics.However,PA12 is prone to thermal aging degradation during SLS process,resulting in reduced part performance and low powder recovery utilization.Material has become a bottleneck restricting the development of SLS.In view of the problems above,PA12 powders was prepared by solvent precipitation method,and the thermal aging of PA12 in air was analyzed.SLS process was used to print the PA12 for four times in succession.Different batches of aging powders and corresponding test pieces were obtained and characterized.The thermo-oxidative aging mechanism,microstructure evolution process and the performance of the parts were studied.The main research results are as follows:(1)PA12 powders can produce parts with high sintering quality and good dimensional accuracy.However,after aging,the color of the powder turns yellow and the quality of the surface is seriously reduced,which will no longer be suitable for SLS process.Adding appropriate antioxidants can greatly improve the thermal stability of the material.When adding a total content of 0.5% of antioxidants,the type is hindered phenol 1098 and phosphite 168,and the ratio of thses two antioxidants is 2:1,the antioxidation of the composite powder is significantly improved.(2)During the powder aging process of the SLS precessed PA12 material,it was found for the first time that the peak of the Brill transition resulted in a higher overall melting temperature of the aged powder,and the sintering window expanded beyond after three times of powder recovery.X-ray photoelectron spectroscopy demonstrate the presence of solid and melt state polycondensation,resulting in a continuously rise in molecular weight and viscosity of the aged powder and secondary sintering of the surrounding powder as well as higher nucleation temperatures for aged powders and laser sintering detected in differential scanning calorimetry.The aging component has a crystallization delay.After three recovery cycles,the effect of solid state polycondensation reduced the crystallinity.(3)The optimized process parameters are: preheating temperature 158℃,processing temperature 167℃,laser power 18 W,scanning rate 4000mm/s,layer spacing 0.3mm,and layer height 0.1mm,which can reduce the tendency of warpage deformation,dimensional accuracy decrease and indistinguishable border of the parts.The SLS parts of PA12 will have a small amount of pores,and the mechanical properties will be lower than those of the molded parts,which will be transformed into brittle fracture.The higher the degree of powder aging,the lower the tensile and bending strength of the parts,which can not meet the basic requirements of ordinary plastic functional parts.