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 8 M etal additive manufacturing (AM) is a rap- idly growing and advancing technology for making engineered metallic components across several industries. The ability to produce complex components in either limited production quantities or customized configurations hasmeant AM has so far seen themost rapid interest and adop- tion in the aerospace and medical industries. However, the improving economics and technicalmaturity of these processes are now driving increased adoption across other sectors including defense and transportation. In the broadest terms, additive can include any process by which a component is made by depositing material in a manner specified by 3D model data. In the case of metal AM, this is primarily accomplished by provid- ing material to the location required for building the part, either by spreading thin layers of powder in a powder bed or through a feed nozzle for wire or powder. Energy is then added, typically tomelt thematerial, after which solidmate- rial is built in the location desired. Themost commonmeans of adding energy are via laser or electronbeam, butmethods such as electric arcs, gas velocity, or chemical energy can also be used. DIRECT METAL LASER MELTING One of the most common additive manufacturing methods for metal parts is the laser powder bed fusion pro- cess, known at GE Additive as direct metal laser melting (DMLM). In this process, a 3D part model is broken into a se- ries of 2D layers from 20-100 microns in thickness, which are melted using a scanning laser. After each completed layer, the part is lowered in the build chamber by the height of the layer and a powder spreading systemapplies a layer of pow- der on the top surface of the part, where the scanning layer melts the next layer of the part. The DMLM process can pro- duce parts with a high degree of dimensional accuracy and good surface finish, minimizing the amount of post process machining that is required to obtain a finished part. In the as-built condition, the microstructure consists of layers of overlapping melt pools with very fine precipitates and sec- ondary phases owing to the rapid cooling of the structures during each pass of the laser. An example of this structure is shown in Fig. 1. For DMLM parts, heat treatment is important to set the final microstructure, which impacts mechanical properties. General heat treatment processes for DMLM parts include stress relief, hot isostatic pressing (HIP), and alloy specific solution, quench, age and tempering processes, if required. Stress relief: DMLM produces high thermal gradients layer by layer that generate a high degree of localized resid- ual stresses throughout the part. Generally, in the as-printed DMLM condition, it is not possible to remove the part from the build platform without distortion. A stress relief step is necessary to equalize the stress gradients caused by pro- cessing thermal gradients to allow removal of the part from the build platformwithout distortion or part cracking. As the first exposure to elevated temperatures following the build, recrystallization or transformation of the structure can occur during the stress relief heat treatment, so caremust be taken todevelopanappropriate stress relief cycle thatwill produce HEAT TREATMENT OF ADDITIVELY MANUFACTURED METAL COMPONENTS For parts made by direct metal laser melting, heat treatment is important to set the final microstructure, which impacts mechanical properties. Andrew Wessman, Eric Ott,* and Rajendra Kelkar GE Additive Technology Center, West Chester Township, Ohio 6 Additively manufactured power door opening system (PDOS) bracket. Courtesy of GE Additive. *Member of ASM International