【作者】 郑翔；

【导师】 郑志军； 李浩；

【作者基本信息】 华南理工大学 ， 安全工程（专业学位）， 2023， 硕士

【摘要】 激光选区熔化(Selective Laser Melting,SLM)技术是目前较为成熟的金属增材制造技术。SLM技术广泛应用于各行各业,但由于其自身的成形原理及复杂冶金过程,难以避免产生组织缺陷。而激光重熔是目前解决这一问题的有效方法,但目前关于激光重熔对温度场、应力场分布及性能的影响却少见报道。本文以316L不锈钢为研究对象,分析SLM激光重熔成形对温度场、应力场的影响及制备样品分析其对冶金质量和组织性能的影响,为SLM激光重熔工艺优化及应用提供理论基础。(1)深入对比未重熔与激光重熔温度场特征,结果表明,激光第二次扫描前仅需移动激光束,其时间极短使得部分热量残留在成形区域,从而提高快速最低温度,起到强烈的预热作用。单层成形时温度快速提升69%,多层成形时因热量扩散程度增大使得温度提升10%。而且分析不同重熔激光能量密度下的温度结果发现,激光第二次扫描温度和整体最低温度会随着重熔能量密度的增大而增大,冷却速率也随着能量密度的增大而增大,而形貌因子随着能量密度的增大先增大后减小。(2)深入对比未重熔与激光重熔应力场特征,结果表明,激光重熔每层两次扫描使得局部重熔区域增大,释放更多热应力,从而降低残余应力,至少比未重熔情况降低12.5%,但会因第二次扫描输入能量不足,容易造成焊道拐角处应力增大,导致原本处于基板衔接处的应力集中区域转移到表面,形成更加不均匀的应力分布,导致变形位移预测范围增大。其次,分析不同重熔激光能量密度对残余应力、变形位移的影响发现,随着激光重熔能量密度的增大,残余应力不断增大,变形位移范围因应力分布逐渐均匀而逐渐缩小。(3)结合模拟结果,对相同工艺参数实验样品的表面粗糙度、致密度、成分及微观组织和性能等进行分析。结果表明,随着重熔激光能量密度的增大,重熔区域逐步增大并消除部分孔隙,但增大至较高的能量密度时,飞溅和粘粉增强,蒸发增强导致新缺陷产生,使得表面粗糙度和孔隙尺寸随着重熔能量密度的增大呈现先减小后增大的趋势。当能量密度为50J/m时,表面粗糙度和孔隙尺寸均达到最小值,比未重熔情况降低35%以上,致密度最高,为99.95%。另外,由于受到碳原子分布均匀强化、残余应力增大和孔隙变化的影响,硬度和耐磨性也随着激光重熔能量密度的增大出现先增强后减弱的趋势。当能量密度为70J/m时,硬度和耐磨性最好,比未重熔增强15%以上。而且激光重熔使晶粒取向差异性更加明显,XOY面偏向(220)衍射峰,ZOX面偏向(111)衍射峰,但微观组织无明显变化。更多还原

【Abstract】 Selective Laser Melting(SLM)is a mature metal additive manufacturing technology that is widely used in various industries,but due to its own forming principle and complex metallurgical process,it is difficult to avoid tissue defects.Laser remelting is an effective method to solve this problem,but there are few reports on the effects of laser remelting on the temperature field,stress field distribution and properties.This paper takes 316 L stainless steel as the research object,analyzes the influence of SLM laser remelting on the temperature field and stress field and prepares samples to analyze its influence on metallurgical quality and tissue properties,so as to provide a theoretical basis for the optimization and application of SLM laser remelting process.(1)An in-depth comparison of the temperature field characteristics of unremelted and laser remelting shows that the laser only needs to move the laser beam before the second scan,which is extremely short and allows some of the heat to remain in the forming area,thus increasing the rapid minimum temperature and providing a strong preheating effect.The rapid temperature increase is 69% for single-layer forming and 10% for multi-layer forming due to increased heat diffusion.Furthermore,analysis of the temperature results at different remelting laser energy densities shows that the second laser scan temperature and the overall minimum temperature increase with increasing remelting energy density,the cooling rate also increases with increasing energy density,and the shape factor increases and then decreases with increasing energy density.(2)In-depth comparison of the stress field characteristics of unremelted and laser remelted,the results show that laser remelting two scans per layer makes the local remelting area larger,releasing more thermal stresses,thus reducing residual stresses,at least 12.5%lower than the unremelted case,but will be due to the second scan input energy is not enough,easily resulting in increased stresses at the corners of the weld channel,resulting in the stress concentration area originally at the substrate articulation transferred to the this results in a more uneven stress distribution and an increase in the predicted range of deformation and displacement.Secondly,analysis of the effect of different remelting laser energy densities on residual stresses and deformation displacements revealed that as the laser remelting energy density increased,the residual stresses continued to increase and the deformation displacement range was gradually reduced due to the gradual uniformity of the stress distribution.(3)Combined with the simulation results,the surface roughness,density,composition and microstructure and properties of the experimental samples with the same process parameters were analysed.The results show that as the remelting laser energy density increases,the remelting area gradually increases to eliminate some of the porosity,but at higher energy densities,sputtering and sticky powder are enhanced,and evaporation is enhanced leading to new defects,making the surface roughness and porosity size show a trend of first decreasing and then increasing with the increase of remelting energy density.At an energy density of 50 J/m,both surface roughness and pore size reach a minimum value,which is more than 35% lower than the unremelted case,and the highest density is 99.95%.In addition,the hardness and wear resistance also tend to increase and then decrease with increasing energy density of laser remelting due to the reinforcement of the uniform distribution of carbon atoms,the increase of residual stress and the change of pores.At an energy density of 70 J/m,the hardness and wear resistance are best,with an increase of more than 15% compared to unremelted.Furthermore,the laser remelting makes the grain orientation variability more pronounced,with the XOY plane biased towards the(220)diffraction peak and the ZOX plane biased towards the(111)diffraction peak,but with no significant change in microstructure.更多还原