Areal topography measurement of metal additive surfaces using focus variation microscopy

Lewis Newton,Nicola Senin,Carlos Gomez,Reinhard Danzl,Franz Helmli,Liam Blunt,Richard Leach

doi:10.1016/j.addma.2018.11.013

Abstract

In this work, the performance of a focus variation instrument for measurement of areal topography of metal additive surfaces was investigated. Samples were produced using both laser and electron beam powder bed fusion processes with some of the most common additive materials: Al-Si-10Mg, Inconel 718 and Ti-6Al-4V. Surfaces parallel and orthogonal to the build direction were investigated. Measurement performance was qualified by visually inspecting the topographic models obtained from measurement and quantified by computing the number of non-measured data points, by estimating local repeatability error in topography height determination and by computing the value of the areal field texture parameter Sa. Variations captured through such indicators were investigated as focus variation-specific measurement control parameters were varied. Changes in magnification, illumination type, vertical resolution and lateral resolution were investigated. The experimental campaign was created through full factorial design of experiments, and regression models were used to link the selected measurement process control parameters to the measured performance indicators. The results indicate that focus variation microscopy measurement of metal additive surfaces is robust to changes of the measurement control parameters when the Sa texture parameter is considered, with variations confined to sub-micrometre scales and within 5% of the average parameter value for the same surface and objective. The number of non-measured points and the local repeatability error were more affected by the choice of measurement control parameters. However, such changes could be predicted by the regression models, and proved consistent once material, type of additive process and orientation of the measured surface are set.

Highlights

The layer-by-layer approach offered by additive manufacturing (AM) allows for the creation of complex geometries, reducing the need for assembly and increasing design freedom [1]
These maps indicate some of the measurement challenges encountered when applying the focus variation (FV) technology to AM surfaces featuring dark, poorly contrasted recesses and specular oversaturated plateaus
As height detection in FV is based on contrast, such surfaces offer a number of challenges

Summary

Introduction

The layer-by-layer approach offered by additive manufacturing (AM) allows for the creation of complex geometries, reducing the need for assembly and increasing design freedom [1]. For surfaces produced via PBF, measurement challenges are related to non-uniformity of optical properties, with highly reflective smooth regions appearing together with poorly contrasted, dark recesses, high aspect-ratio features, high slope angles and most critically undercuts [13,14], all of which vary dependent on relative build orientation, powder size and AM process used. For EBPBF, the as-built side surfaces are especially dominated by powder adhesion due to the need for the process to semi sinter a powder region around the part geometry into a ‘cake’; this acts to offer support and increase thermal conductivity during the build process [17]. When the computations are complete, a series of contrast values (known as a contrast curve) is available for each x, y location, spanning the entire set of vertically stacked images. Additional algorithms are used to determine the maximum contrast value for each curve, and its z location

Methods

Results

Discussion

Conclusion