基于梯度降温的叠层制备热残余应力

针对叠层制备工艺的热残余问题,为消除传统的基于同步降温假设的理论解与实际热残余现象的差异,本文在充分考虑成形过程中沿长度和厚度方向形成的温度梯度的基础上,分别建立在层平面和厚度方向引起的热残余变形和应力的解析解,并根据不同叠层制备工艺,将降温梯度概括为同步降温、均等梯度降温、非均等梯度降温、瞬态降温的4种模式.算例表明,梯度降温会造成在层平面和厚度方向均产生热残余现象.讨论了4种梯度降温模式对热残余程度的影响,梯度越大影响越大;合理解释了同一种材料制备的工件也会因降温梯度而产生明显的弯曲变形;对于梯度材料,叠层制备顺序会显著影响热残余的程度.研究表明,梯度降温假设符合实际制备、工艺,更准确地揭示了叠层制备热残余现象产生的机理,优化制备工艺缩小降温梯度是解决热残余问题的有效途径.

Thermal residual stresses in laminated object manufacturing due to gradient cooling

Thermal residual problems in laminated object manufacturing are considered. To remove the errors between the theoretical solutions by the synchronous cooling assumption and the real thermal residual phenomenon, a gradient cooling assumption more suitable for real manufacturing processes is presented. Considering the temperature gradients along length and thickness directions in manufacturing processes, the analytical solutions of thermal residual stresses and deformations in layer plane and through thickness are built. Four cooling gradient models, namely synchronous cooling, average gradient cooling, non-average gradient cooling and quick cooling, are summarized according to different manufacturing techniques. It is shown that in the numerical examples cooling gradients result in thermal residual phenomena in layer plane and through thickness. The effects of four cooling gradient models on thermal residual stresses are discussed. Thermal residual stresses increase with the increase of cooling gradient. The models can reasonably explain the bending deformation due to the cooling gradients in a workpiece made of one material. Processing sequence in functional gradient materials has a strong impact on the thermal residual level. The results show that the gradient cooling assumption accords with real manufacturing techniques and reveals the mechanism of thermal residual phenomenon in laminated object manufacturing more accurately. Optimizing manufacturing techniques and reducing cooling gradients are the effective methods to solve thermal residual problems.