Research On Residual Stress Of GH4169 Super-alloy Fabricated By Selective Laser Melting

Selective laser melting(SLM)of GH1469 is promise to be applied widely.However,the stress caused by an non-uniform cooling during the SLM process,will result in warpage even cracking,which limits the engineering application of SLM of GH1469.The researches about residual stress of SLM processing are not systematical.The influence of the process parameters,scanning strategies,geometrical features of the fabricated parts and external conditions on the residual stress of GH4169 fabricated by SLM has been investigated in this thesis.Some valuable results are obtained.The main research contents and results are as follows:(1)Firstly,the residual stress measure method for GH4169 fabricated by SLM is observed.The X ray diffraction characteristics of the GH4169 superalloy were measured by the uniform intensity girder calibration,the results show that Kβ diffraction peak at148°with the Crystal plane(311)which can be used to obtain the actual stress.This result is used as basis to identify the Vickers micro-indentation method.The results show that the stress value measured by Vickers micro-indentation method and modified X ray diffraction method has a little difference and,within the tolerance,the overall variation trend of stress is same.(2)Secondly,the influence of process parameters on the residual stress of GH4169 fabricated by SLM was investigated.It is found that the laser power has the significantly effect on the residual stress among the laser powder,scanning velocity and scanning spacing.The residual stress increases with the laser power.As the scan velocity increases,the residual stress increases at first and then decreases,a maximum residual stress exists.As the scan velocity increase,the scan spacing corresponding to the maximum residual stress decreases.There is an optimal track length in the sequential strategy,too shorter track length each layer that was divided more number of part with the higher cooling rates and shorter heating time of laser on powder and leave the more effective energy transferring to powder bed,thereby led to a larger thermal stress owning to a high temperature caused by the higher remnant heat.Longer track length has the bigger constrain during the shrinking process,the inner layer shrinkage is mainly the longitudinal contraction of the molten pool,the track length is too long to have enough shrinkage compensation resulted in larger residual stress.(3)Thirdly,the residual stress distribution and evolution of the thin-wall and square part has also been detected.For the thin wall part,the maximum compressive stress is near the base plate,with the increase of the height,the compressive stress becomes to a tensile stress,and reaches to the maximum at the top.The higher the part,the larger the residual stress.For the square part,the stress of the top surface is tensile and the central stress is higher than that of the edge stress.(4)Lastly,the influence of external condition on the residual stress of GH4169 fabricated by SLM is also investigated.Owing to the little contact proportion,the residual stress of the surface increases at first and then remains constant as the thickness of the base plate increases.It has a little effect of the residual stress on the altitude direction with the change of the thickness of the base plate because the contact proportion is too small to constrain it.Under the condition of external support,the influence of the support structure on the residual stress is based on the constraint condition,the smaller the contact proportion,the smaller the residual stress.