Masters Theses

Date of Award

5-2015

Degree Type

Thesis

Degree Name

Master of Science

Major

Mechanical Engineering

Major Professor

Sudarsanam S. Babu

Committee Members

Ryan R. Dehoff, Taigang G. Nieh

Abstract

Selective laser melting (SLM) is an additive manufacturing technique with which parts of nearly arbitrary shape and complexity can be produced. The parts are generated layer for layer directly out of a 3D CAD model completely without forming tools.

In this work, the selective laser technique is applied to a CoCrMo alloy. Due to its high corrosion resistance, wear-resistance, and heat resistance, this alloy is mainly used for medical or high temperature applications.

A systematic approach is used to investigate the influence of the process parameters laser power and exposure time (pulsed laser) on density, melt pool dimensions, melt pool shape, deformation due to residual stresses, microstructure, and mechanical properties. The density analysis as well as the melt pool analysis is carried out by means of optical light microscopy. A qualitative method is used to evaluate the deformation due to residual stress. The microstructure is evaluated using scanning electron microscopy and electron backscatter diffraction. A tensile test is performed to evaluate the mechanical properties.

Parts with a density greater than 99% are obtained where the volume energy exceeds 61 J/mm³. The melt pools have an elliptic shape, a width of 120 to 140 um, and a height of 70 to 80 um, depending on the volume energy. So-called twincantilevers are used to measure the deformation of builds due to residual stresses. The deformation in z-direction is on average 2.1 mm. The microstructure consists of elongated grains parallel to the build direction with different crystal orientations. Anisotropy is confirmed via tensile test. A maximum yield stress of 1000 MPa and tensile strength of 1200 MPa are found, while the average yield stress is found to be 680 MPa with a tensile strength of 1100 MPa. Furthermore, it is established, that the position of the samples on the base plate has a huge influence on the density and, hence, the mechanical properties.

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