The effect of surface roughness on mechanical behavior of commercially pure titanium implants produced by selective laser melting

Abstract

Implants produced by selective laser melting (SLM) have differentiating surface roughness caused by the process itself and applied surface treatments. Since surface quality and mechanical properties are critical parameters for implants and surface roughness is a known factor for stress concentration, it is aimed to investigate the effect of surface roughness on commercially pure titanium implants produced by SLM. Surface roughness is affected by several process parameters but this study focuses on the effect of position and orientation on surface roughness. First step is specified as determining the roughness measurement method ideal for this work. Therefore, 3 different roughness measurement methods as in confocal microscopy (CM), tactile profilometer and scanning electron microcopy (SEM) are compared by using the most common surface roughness parameter, Ra. Secondly, in order to determine the effect of position and orientation on surface roughness and determine roughness ranges across the build chamber, a build is designed with parts at 15 different positions with 6 different orientations that are specified considering the location of laser. Finally, for mechanical characterization, tensile bars are designed considering both the standards stated for tensile and fatigue test. Some parts are also post processed to see the post processing effect. For every position, 5 samples are subjected to tensile testing with 5 MPa pre-load until the part failure so that the average yield stress and UTS for every position are determined. Calculated average yield stresses are used to calculate fatigue test input. 5 stress levels are determined and 2 samples for low cycle, 3 samples for high cycle fatigue data are tested with load controlled, tension-tension fatigue test set-up with 60Hz frequency and R of 0.1. Surface finishing is differentiated by MPP and SB in addition to as-built form. Some samples are post processed with the standard post process of Materialise. Sand blasting and anodization are applied while for SB samples only sandblasting is applied. Then samples are tested to see pp effect on surface roughness and mechanical behavior. When the results are analyzed considering the roughness range and sample amount, the ideal roughness measurement method is determined as tactile profilometer because of its capability, repeatability, practical application and effectiveness if time and cost. The roughness range across the build plate is quantified for different positions and measured Ra values are in the range of 7 to 24 µm across the build plate. It is also concluded that at the right bottom side of the build chamber, and for the orientations perpendicular to the laser beam, surface roughness increases. Relation between Rz and thickness is specified and an equation is suggested to eliminate the effect of roughness on thickness. Since the thickness is effective on cross section calculations used for mechanical characterization, suggested equation is used to recalculate stress values measured with tensile and fatigue tests. Even though measured tensile and fatigue results indicate that increased surface roughness has a negative effect on tensile strength and fatigue life, recalculated tensile and fatigue results display no difference occurs with varying surface roughness. Therefore, it is shown that surface roughness has an effect on thickness hence has an effect on cross section that is affecting fatigue and tensile test results but it does not have a real significant effect on mechanical behavior of SLM printed cpTi parts.