February AMP_Digital

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 5 9 an acceptablemetallurgical result. Tomaintain good quality, stress relief is most often performed in a vacuum heat treat- ment process for as-built surface conditions that will not be subsequently machined. Hot isostatic pressing: Hot isostatic pressing (HIP) refers to a process by which high temperatures and pressures are applied simultaneously. HIP is employed in heat treating AM parts to close non-surface connected porosity that may lead to fatigue debit or poor tensile performance. While not necessarily used for all AM parts, many users with high per- formance or safety critical applications have included HIP in the standard process for a wide variety of materials. The goal is to control and minimize non-interconnected porosity anduseHIPas a backstop against defects that coulddevelop fromnonoptimal process conditions in the DMLM process. Solution, quenching, and temper heat treatments: Solu- tion heat treatment and quenching is often required where prior processing of thematerial does not result in an optimal metallurgical structure for meeting the requirements for the part in use. This may be because prior heat treatment pro- cesses such as HIP cannot provide an adequate cooling rate to generate ideal precipitate morphologies or generating such a precipitate structure requires an aging or tempering process to develop slow-to-form distributions. These pro- cesses tend to be alloy-specific. However, given the differ- ent starting condition of AM materials relative to cast and wrought materials of the same alloy, developing an appro- priate heat treatment specific to the AM material may be required—even for alloys where a standard cast or wrought material heat treatment already exists. HEAT TREATMENT AND MICROSTRUCTURE DEVELOPMENT FOR DMLM ALLOY 718 Alloy 718 has been utilized successfully in both stat- ic and rotating turbo machinery applications for four de- cades. The combination of high strength, fatigue capabili- ty, rupture strength, and corrosion and creep resistance at temperatures through 650°C are key attributes of this alloy. Conventional manufacturing routes include cast, wrought, sheet, joining and fabrication by welding and brazing, pow- der metallurgical processing, and metal injection molding. Recent investigation of aerospace materials like Alloy 718 produced by AM technology has provided an opportunity for disruptive component manufacturing methods, geometries, and component capabilities that expand the design space for complex applications. At GE Aviation (GEA), development of DMLM Alloy 718 was a natural choice following the successful commercial application of DMLM CoCrMo in GEA and Safran’s LEAP plat- form fuel tip and the GE90 T25 sensor. GEA’s DMLM Alloy 718 development started with demonstrator military applica- tions and has expanded to include multiple commercial en- gine applications across the size range of the GEA product line. The DMLM process development for Alloy 718 involved a combination of laser processing parameter investigation and heat treatment development to produce an acceptable build geometry and metallurgical microstructures. The heat treat for DMLM Alloy 718 includes stress relief, HIP, and solu- tion and aging steps. The purpose of the stress relief is to minimize thermal strains incurred during the DMLM process, which in turn reduces distortion during post-processing. Stress relief may also be utilized to start recrystal- lization of the as-printed grain structure, which typically consists of small weld pools of varying orientation. HIP is subsequently used to close non-surface connected poros- ity. Finally, solution and age treatments generate generally equiaxed grains anddevelop an effective precipitate size and morphology in the microstructure for strengthening. Com- mon precipitates include delta, gamma prime, and gamma double prime. The goal of the heat treat development on Alloy 718 was to obtain strain free, pore free, equiaxed, fin- er grains and balanced (in terms of phases) microstructure without detrimental metallurgical anomalies in thematerial. Each step of heat treat optimization is described with more details in the following paragraphs. To understand the residual stress in the part, as-print- ed coupons were stress relieved at various temperatures be- tween800° and1100°C(1472° and2012°F)withthesamehold time. Subsequently, x-ray diffraction (XRD) was performed to quantify the residual strains. Figure 2 summarizes the re- sults. These results show that there is an order of magnitude difference in residual matrix strains detected by XRD at most of the tested temperatures compared with the as-printed coupon. Based on these results, the lowest temperature was selected as an adequate stress relief temperature. During heat treatment development for DMLM Alloy 718, two additional metallurgical effects were observed re- 7 Fig. 1 — Optical micrograph of the as-printedmicrostructure in the etched condition with an arrow indicating the build direction.