Metallisation of additive manufactured polyamide 12 by low pressure cold spray

Abstract

Cold Spray is a solid deposition process that can expand the field of application of polymeric materials thanks to the build-up of a metallic coating that improves their electrical properties and the wear resistance, among others. The novelty of the Cold Spray deposition on polymers has as drawback the lack of processing models that allow to choose the manufacturing parameters and forecast the coatings and substrates properties. In this work, the influence of the processing parameters for Aluminium-Zinc coating build-up via Low Pressure Cold Spray on polyamide 12 additively manufactured via Fused Filament Fabrication was investigated. Several sets of processing parameters combinations were systematically modified to establish the deposition window, leading to a final set that allowed to determine the physical, microstructural, and mechanical properties of the coatings, and the physical and thermal properties of the polymeric substrate. For the fixed gas pressure of 0.45 MPa and gas temperature of 500 °C, the scanning speed and the powder mass flow rate had a strong influence. Thick coatings (around 850 μm in size) were achieved at high powder mass flow rate (26 g/min) at the expense of low bond strength (between 1 and 3 MPa). The surface roughness acquired the highest values in those samples where erosion took place, occurring this phenomenon at small standoff distances and low powder mass flow rate, and no finding any relationship with the transverse gun speed. The generation of an interface layer of mixed metallic particles polymeric substrate plays a key role in the subsequent metallic coating growth, with thicknesses that ranged from 30 μm to 200 μm. In the substrate, the crystallinity degree of the polyamide underneath the coating increased, evidence of a heat affected zone where temperatures around the crystallisation temperature were reached, that also promoted stretching of the polymeric substrate due to dilatational phenomena. All this accompanied of an overall decrease in the thickness of the PA12 coupon after coating build up deposition with the high powder mass flow rate. This evidence indicates that a large plastic deformation of the polymeric substrate at the most energetic combination of processing parameters took place.