PSI - Issue 14

Anigani Sudarshan Reddy et al. / Procedia Structural Integrity 14 (2019) 449–466 Author name / Structural Integrity Procedia 00 (2018) 000–000

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advantage in design and manufacturing, in both industrial as well as academic sectors by Michael Schmidt et al. (2017) or Erwin Rauch et al. (2018). Direct metal laser sintering (DMLS) is one of the popular LAM techniques in which 3D parts produced from powders by the selective application of laser energy to powder beds by Thomas Grunberger et al. (2015). In this process, a series of complex process parameters like laser power, scan speed, layer thickness and hatch distance are optimized to produce dense parts having complex geometries by Pavel Hanzl et al. (2015) or Robert Mines et al. (2010) or Santhosh Kumar Rao Chandrasekara et al. (2017). Starting with serving as an ideal prototype generator, this technology is now capable of being suitable for full-fledged batch production of parts to cater to the demands of the aerospace, power, healthcare, tool and die and other general engineering applications by Eleonora Atzeni et al. (2012). Testing and characterization of each build plate is mandatory for establishing repeatability and consistency of mechanical properties during scaled up production. The mechanical properties are typically characterized for each build plate via standard ASTM test specimens having dimensions measuring 100 mm length and 6 mm diameter. A minimum of six specimens is needed in each build plate, to test the mechanical properties along and transverse to the build direction, for establishing the basic room temperature mechanical properties, during production. If the part is one that sees high temperature, then additional samples are needed to be built for corresponding high temperature mechanical testing (along both longitudinal and transverse directions). These are time consuming and expensive. Very few studies have established the feasibility of using small scale test specimens for LAM, in a systematic manner. Some of them refer to directly printing small scale test specimens during the build process by Van Zyl et al. (2016), however, while this is attractive in terms of building the test specimens directly without having to further machine into dogbone test specimens, the LAM process does not always ensure that such small scale specimens with thickness < 1 mm and overall dimensions within 25 mm, can be obtained without any distortion or microstructural change during buildup of such thin tensile specimen geometries. Moreover, further post processing such as removal of supports as well as removal from the build plate may not be able to guarantee a specimen that is distortion free. Secondly, while the DMLS technology is seen very attractive for building complex parts, it is most suited in applications requiring hybrid components, especially during repair and refurbishment, or as a functionally gradient material, added on to a conventionally manufactured part by Mary Kathryn Thompson et al. (2016) or Vanekar et al. (2017) or Ruth Jiang et al. (2017) or Vayre, B et al. (2012) or Tammas Williams et al. (2017) or Simon Ford et al. (2016) or Germain Sossou et al. (2018). Although these parts may not be more than a few millimeters in size added on to a conventional cast or wrought part, the current process of having these qualified comprises making use of standard ASTM test specimens. In order to optimize the process parameters to obtain dense parts and to validate build to build consistency in terms of mechanical properties as well as strength of the joint in hybrid component having an LAM part built on a conventionally manufactured part, a number of test samples have to be printed and tested as per the ASTM standards, adding to the time and cost. Thirdly, while optimizing the process parameters for any new alloy or part geometry and mandatory validation of each chemistry or process via mechanical properties, having a small scale testing capability will enable a faster cycle time for the optimization trials as well as lead to a higher productivity. In order to reduce the time and costs involved in testing as well as to enable testing for hybrid parts, this study comprises a systematic evaluation of the suitability of small scale tensile specimens for laser additive manufacturing DMLS process in five different alloys representative of the all the materials that are currently being used for part production using the powder bed fusion process. Nickel (IN718), Cobalt (CoCrMo), Steels (maraging steel and SS316L) and Titanium (Ti6Al4V) alloys, were evaluated for their room temperature tensile behavior using small scale test specimens, in the as printed as well as heat treated condition. The test specimens used in this study were machined out of DMLS blocks and comparisons were made with standard ASTM test specimens to establish the small scale test specimens as being suitable for AM alloys. 2. Experimental details DMLS alloys processed as standalone specimens will be referred to as “monolithic” specimens in this study. DMLS alloys deposited or welded on to conventionally manufactured materials will be referred to as “hybrid” on a “substrate”. Table 1 is a list of samples used in this study, as a monolithic DMLS and as a hybrid DMLS, (DMLS