Comparison of Optical and Stylus Methods for Surface Texture Characterisation in Industrial Quality Assurance of Post-Processed Laser Metal Additive Ti-6Al-4V

Abstract

Additive manufacturing technologies enable lightweight, functionally integrated designs and development of biomimetic structures. They contribute to the reduction in material waste and decrease in overall process duration. A major challenge for the qualification for aerospace applications is the surface quality. Considering Ti-64 laser powder bed fusion (LPBF) parts, particle agglomerations and resulting re-entrant features are characteristic of the upper surface layer. Wet-chemical post-processing of the components ensures reproducible surface quality for improved fatigue behaviour and application of functional coatings. The 3D SurFin® and chemical milling treatments result in smoother surface finishes with characteristic properties. In order to characterise these surfaces, three methods for surface texture measurement (contact and non-contact) were applied, namely confocal microscopy, fringe projection and stylus profilometry. The aim of this work was to show their suitability for measurement of laser powder bed fusion as-built and post-processed surfaces and compare results across the evaluated surface conditions. A user-oriented rating of the methods, summarising advantages and disadvantages of the used instruments specifically and the methods in general, is provided. Confocal microscopy reaches the highest resolution amongst the methods, but measurements take a long time. The raw data exhibit large measurement artefacts for as-built and chemically milled conditions, requiring proper data post-processing. The stylus method can only capture 2D profiles and the measurement was restricted by particle agglomerations and craters. However, the method (process and instrument) is entirely standardised and handheld devices are inexpensive, making it accessible for a large group of users. The fringe projection method was the quickest and easiest regarding measurement and data post-processing. Due to large areal coverage, reproduction of location when performing repeat measurements is possible. The spatial resolution is lower than for confocal microscopy but is still considered sufficiently high to characterise the investigated surface conditions.

Influence of Contour Scan Variation on Surface, Bulk and Mechanical Properties of LPBF-Processed AlSi7Mg0.6

“…The majority of studies including the effect of surface condition on fatigue performance of the LPBF-processed AlSi7Mg0.6 material, as well as other aluminium alloys and other typical LPBF powder materials (e.g., Ti-64, 316L steel or Inconel 718), evaluate the application of different surface post-processing strategies, e.g., [7,10,15,42,45,[53][54][55].…”

The effects of varying wall thickness on the surface roughness of Ti-6Al-4V by electron beam powder bed fusion

“…These roughness parameters were selected based on their prevalence in the literature on surface roughness with EB-PBF [78][79][80] and according to the recommendations of ISO/ASTM 52902 [76]. For each sample, three surface profiles were measured (figure 3(a)) and characterized.…”

Precision of diamond turning sinusoidal structures as measurement standards used to assess topography fidelity

“…These include optical interaction in the case of optical instruments, and finite stylus size and deformation due to contact on softer materials. The literature has discussed comparisons of the performance of roughness measuring instruments for many years, for example in 2010 by Leach et al [3] and in 2023 by Buchenau et al [4].…”