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Development of a Weldability Testing Strategy for Laser Powder-Bed Fusion Applications

Kemerling, Brandon L

Abstract Details

2018, Doctor of Philosophy, Ohio State University, Welding Engineering.
Laser powder-bed fusion (L-BPF) is an additive manufacturing process which uses a focused laser energy source and metallic powder particles to manufacture components in a layer-by-layer fashion. The process has received tremendous interest and is gaining traction in the aerospace, automotive and biomedical industries for its unique ability to produce complex and easily-customizable metal parts. Despite the advantages offered by L-PBF, weldability issues can arise in the form of cracking, distortion and layer delamination in manufactured parts due to thermally-induced stress accumulation. In this study, a standard testing procedure is developed for evaluating the weldability issues associated with L-PBF processing. The test coupon used to evaluate weldability consisted of a “double Block-O” design that can be built in a relatively short time period (less than 1 hour) and develops sufficient restraint to promote cracking in susceptible materials. The design also allows the quantification of residual stress and the assessment of residual stress levels based on build strategy. Three materials were used in conjunction with the double Block-O design to assess L-PBF weldability: 304L stainless steel, Ni-base alloy Waspaloy, and aluminum alloy 6061. Most of the preliminary work and residual stress analysis were conducted using 304L. Waspaloy and aluminum alloy 6061 were used to determine the severity of the test with respect to cracking. Residual stress in Type 304L builds was evaluated using finite element analysis (SYSWELD) and neutron diffraction in the test geometry as a function of laser scan strategy. Residual stresses measured via neutron diffraction reveal variations in the stress profile for the x and y directions as a function of laser scan strategy. The stress profiles in the z direction are found to be relatively insensitive to variations in laser scan strategy. The scan strategy which resulted in the largest stress accumulation within the test geometry is adopted as the standard scan strategy for the testing procedure. Good agreement is observed between the computational and experimental stress measurements. Multi-scale characterization techniques are employed to evaluate the as-built 304L material and highlight the microstructural features present in L-PBF builds. This is supplemented with in-situ process monitoring data obtained using a profilometer, infrared camera and photodetector. The ability to detect weldability issues in real-time with these process monitoring sensors is proven through correlations with the characterization results. Weldability testing results for Waspaloy show evidence of intergranular cracking along solidification grain boundaries within the build material. Dendritic fracture surfaces are observed via scanning electron microscopy and the solidification cracking mechanism is identified. The cracks are preferentially oriented along the L-PBF build direction and range in length from 15 to 100 microns. Weldability testing results for aluminum alloy 6061 show severe evidence of cracking within the build material. Solidification cracks are observed and evidence of crack propagation is shown throughout the material at regions of porosity and lack of fusion defects. The cracks are preferentially oriented along the build direction and range in length from 20 to 1,000 microns. Weldability test results are quantified using average crack length and number of cracks per mm2 as criteria. These metrics are then used to rank alloys with respect to their weldability performance in L-PBF applications. The alloys tested in this study are ranked in the order of 304L, Waspaloy and 6061 from most weldable to least weldable. The testing of additional alloys will allow users to evaluate materials susceptibility to weldability issues during L-PBF processing. This information may ultimately be used for guidance in material selection and in developing new alloys for L-PBF applications.
John Lippold, PhD (Advisor)
Antonio Ramirez, PhD (Committee Member)
Carolin Fink, PhD (Committee Member)
267 p.

Recommended Citations

Citations

  • Kemerling, B. L. (2018). Development of a Weldability Testing Strategy for Laser Powder-Bed Fusion Applications [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu152380570674288

    APA Style (7th edition)

  • Kemerling, Brandon. Development of a Weldability Testing Strategy for Laser Powder-Bed Fusion Applications. 2018. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu152380570674288.

    MLA Style (8th edition)

  • Kemerling, Brandon. "Development of a Weldability Testing Strategy for Laser Powder-Bed Fusion Applications." Doctoral dissertation, Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu152380570674288

    Chicago Manual of Style (17th edition)