Characterization of Laser Deposited Alloy 718
Additive Manufacturing (AM) is a method of producing three-dimensional objects using additive processes. It allows great flexibility in the processes and reduces the design-to-production time. Laser Metal Deposition (LMD) is one of AM methods under development and is based on the deposition technology. LMD has advantages in grain growth control, material functional grading, lower material storage requirement and more spatial freedom. Considering the outstanding features, it is important to study the characteristics of LMD products, which in this study is Alloy 718 for aerospace applications.
Single-wall Laser LMD samples are built with varied process parameters using gas-atomized Alloy 718 powders. Two experiments were carried out with focuses on 1) evaluations and comparisons of the microstructural characteristics, porosity and hardness of the samples are performed; 2) The effect of heat treatments including solution treatment and aging on the microstructure as well as the hardness.
The results of the experiments revealed directional solidification features and typical phases such as γ matrix, Laves phase and carbide. 0.06% average porosity and a majority of < 20 µm size are measured from the LMD samples. Only spherical gaseous pores are found while no lack-of-fusion pore is found. A hardness Vickers of 246 in average hardness is measured from the LMD samples. In the heat treated samples, δ phases were found; By direct-aging at 750 ℃ for 10 to 15 hours, the samples reach a maximum hardness of around 382 HV. The same hardness was reached by 1 hour of solution treatment at 950 ℃ followed by 5 hours aging at 750 ℃.
The effects of processing parameters on the characteristics of LMD processed Alloy 718 are compared and discussed. A 2-dimentional map of porosity distribution along the length of the sample is made and the patterns are investigated along both the length and the height of the sample. It is found in the sample that the starting part of the deposit is higher in number of pores while the finishing part is larger in pore size. It is also found that the top layer of the deposit has the highest porosity level, pore number and pore size. Moreover, the hardness gradient along the build-up direction is evaluated and discussed. No significant hardness gradient was found. The precipitation hardening effect of LMD process and possible improvements are also discussed.
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