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FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | F E B R U A R Y / M A R C H 2 0 1 9 6 0 lated to the HIP process: first surface oxidation and second partial recrystallization. Improvement in the mechanical properties from the HIP process has also been attributed to the dissolution of undesirable Laves and delta phases aswell as pore closure. To evaluate the effect of HIP parameters on porosity closure, special porous bars were produced using nonoptimal DMLM parameters at 20 μ m layer thickness. The porosity level produced was around 7-8%. It is believed that this level of porosity is near the threshold where significant amounts of surface connectivity for the embedded pores may begin to be observed. Those samples were HIP heat treated using various temperatures and pressures as shown in Fig. 3 to understand trends in pore closure capability. Metallography was then performed to evaluate the porosity level change and porosity was quantified using image anal- ysis. Figure 3 summarizes the results of this ex- periment. Low HIP temperatures with low pres- sure or low temperature with high HIP pressure conditions were found to be more susceptible to retained porosity while those at higher tem- peratures resulted in significant pore closure. The HIP process may also lead to surface oxidation on the DMLM parts. It was observed that higher temperatures were more suscepti- ble compared to lower temperatures. Commer- cially, for high HIP pressures, there are limited numbers of furnaces available for high tempera- ture cycles. HIP optimization was carried out based on the porosity, oxidation control, and cost/capacity. Further HIP optimization con- cluded that the best commercial solution is in- termediate temperature at low pressure for the DMLM 20 μ mparameter set. Solution heat treatment plays a key role in setting the fraction and morphology of the delta phase as it relates to the delta solvus temperature. Ideally, it is desirable to have a discrete delta phase along the grain boundaries. If the delta phase is continuous it may impact notch sensitivity and, in the absence of delta phase on the grain boundaries, it may impact the strength of the material. To acieve the right bal- ance, the typical solution temperature is 14-28°C (25-50°F) below delta solvus temperature, so a cycle of 982°C (1800°F) for 1 hour is used. Finally, standard age temperatures were used to complete the heat treat cycles (718°C/1325°F and 621°C/1150°F for 8 hours each). In summary, heat treatment is an important aspect of the additive manufacturing process. It is important for users of this technology to understand the impact of heat treat- ment processes on the quality and final metallurgical struc- ture of additively manufactured components. ~HTPro For more information: Andrew Wessman, staff engineer, GE Additive, 9701 Windisch Rd., West Chester Township, OH 45069, 859.485.2971, andrew.wessman@ge.com, www. ge.com/additive . 8 Fig. 2 — Residual strain measurements by x-ray diffraction for the as-printed DMLM alloy 718 coupons. Fig. 3 — Effect of HIP parameters on porosity.