In-process range-resolved interferometric (RRI) 3D layer height measurements for wire + arc additive manufacturing (WAAM)

Abstract

In this work a range resolved interferometry (RRI) instrument for absolute distance measurements is integrated into a wire + arc additive manufacturing (WAAM) system to provide in-process monitoring of layer height, and prospects for volume and profile monitoring are discussed. Interferometry as a coherent optical technique offers a straightforward in-process measurement even in the harsh welding environment, as compared to non-coherent techniques based either on laser profiling or camera vision systems. RRI can be accomplished at significantly lower cost, and with higher depth of field (up to 10s of cm) than existing optical coherence tomography based weld monitoring. In this experiment titanium feedstock was used to create a 150mm long, 13.5mm high weld-wall comprised of 11 welded layers. The RRI in-process measurements are in very good agreement with both mid-process, on-machine micrometer measurements taken by hand after each weld, and post-process laser scanning measurements of the completed wall. The high depth of field allows direct referencing of the layer height measurements to the build plate making the measurement independent of the motion system and build plate bending, considerably lowering uncertainties. This, together with the capability for cost-effective in-process measurements in harsh environments, should make the proposed approach very interesting for routine use in WAAM systems.

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“…In Figure 2 , the data output is the galvanometer scan angle and the signal amplitude (corresponding to the distance along the laser beam from the fibre tip at each instance). According to [ 6 ] the RRI head unit can convert polar coordinates into Cartesian coordinates using the geometrical relations given in equations ( 3 )–( 5 ) using the distance between the galvanometer mirror and the reflection and the galvanometer angle: …”

“…It is possible to accurately calibrate the galvanometer length by turning the mirror directly back towards the fibre tip with the RRI instrument. According to [ 6 ] if the galvanometer mirror angle θ is zero and the beam is angled vertically, then the equation describes a two-dimensional case in which components on the y -axis are absent (i.e., the 3D diagram shown in Figure 2 ). Thus, the RRI instrument outputs a point cloud that describes reflections occurring at specific spatial locations and times q c , w c , t c , each given by an array of values [ 6 ].…”

“…Following the methodology, study, and design carried out by [6], RRI's instrument provides a 4.8 kHz data rate. In Figure 2, the data output is the galvanometer scan angle and the signal amplitude (corresponding to the distance along the laser beam from the fbre tip at each instance).…”

“…We measured 7.8 θ angular amplitude with an angular frequency of 28 Hz and 3.9 kHz data rate. The RRI instrument falls below detected noise levels on steeper sides due to too much light reflection [ 6 ]. Because the angled sidewalls scatter more light, the RRI instrument will have better coverage of the sidewalls for materials with less specular reflection.…”

“…The central point of illumination and detection provided by interferometric techniques (including RRI) makes it easier to integrate them into mounting structures. As an interferometric technique similar to RRI [ 4 – 6 ], optical coherence tomography (OCT) is widely used in monitoring. In principle, OCT can achieve a resolution of 0.01 mm (compared to RRI's maximum working range of 10 cm).…”