Due to the layered nature of the manufacturing process with rapid cooling, the parts made by the additive manufacturing method of laser powder bed fusion have an anisotropic structure with significant residual stress and therefore require heat treatment after fabrication. This study intends to eva‎luate the influence of heat treatment on the phase and microstructural changes of Co-Cr-Mo-W alloy produced by laser powder bed fusion to optimize its mechanical characteristics for usage in dental implants with an appropriate average lifespan. For this purpose, a three-step heat treatment cycle, involving solution annealing at 1250°C for one hour, aging at 800°C for 12 hours, and then reversing at 1030°C for 20 minutes and Water cooling, was used. Optical and electron microscopy and X-ray diffraction were used to analyze the microscopic and phase structures of the samples before and after heat treatment at each step. The results showed that after the solution annealing heat treatment, the material's microstructure is homogenized, and the equiaxed grains having twin boundaries are formed. In the next step, the aging treatment caused grain refining, intermetallic precipitate formation, and a massive ε phase. In the following, the reverse treatment at the stable temperature of the γ phase caused the transformation of the brittle ε phase into the ductile γ phase, as well as the refinement of structure and homogenous distribution of the precipitates. The tensile test results showed that flexibility increased, yield strength and tensile strength decreased after solution annealing. Further, with the aging treatment, the yield strength and ultimate tensile strength increased, but the flexibility decreased significantly from 23 to 3%. In the last stage of the cycle, reverse heat treatment, an excellent combination of ultimate tensile strength of 1357 MPa, yield strength of 797 MPa, and flexibility of 17.8% with absorbed energy equal to 240 J/cm2 and toughness of 216 MPa has been obtained in comparison to other properties of cobalt-chromium alloys manufactured and heat treated in previous studies. In this stage, the strain hardening rate is steady, and deformation occurs uniformly and without local deformation. During the heat treatment cycle, the hardness of the samples decreased in the first stage of solution annealing due to the reduction of Stacking-Fault and the increase in grain size but lastly, it increased with the reverse treatment due to grain refining and the distribution of spherical precipitates inside the grains. The type of fracture in the as-built alloy is semi-cleavage, and its fracture surface has cleavages plates and fine dimples caused by cellular structure; However, after heat treatment, the type of fracture converted to ductile and intergranular fracture, with any cleavage plates on the fracture surface. In conclusion, the present work demonstrated that, in addition to improving the microstructure and mechanical properties of the Co-Cr-Mo-W alloy produced by laser powder bed fusion, the designed heat treatment partially compensated for the loss of properties caused by structural defects and increased confidence in the performance of the resulting component.

احمد رضائيان , احمد كرمانپور

محسن بدرسماي

بهزاد نيرومند , احسان فروزمهر