Plasma Transferred Arc deposition of powdered high performances alloys: process parameters optimisation as a function of alloy and geometrical configuration

Abstract

The deposition of high wear- and corrosion-resistant alloys through Plasma Transferred Arc (PTA) is an easily automated process combining the possibility to obtain very thick coatings with high deposition speeds, low thermal distortion of the part and negligible dilution levels, thanks to a very high energy-concentration [S. Kalpakjian, S.R. Schmid, Manufacturing Engineering and Technology—4th edition, Addison-Wesley Publishing, 2001, p. 789–790, Scripta Materialia 37 (1997) 721, Applied Surface Science 201 (2002) 154, Surface and Coatings Technology 71 (1995) 196, Journal of Materials Processing Technology 128 (2002) 169, M. Bonacini, Plasma ad arco trasferito per riporti saldati con superleghe in polvere, “Saldatura e taglio termico verso il 2000” IIS Meeting, October 3rd 1998, Milan-Italy, Wear 250 (2001) 611, Wear 249 (2002) 846, Composites Science and Technology 58 (1998) 299]. Literature studies regard mostly wear or high temperature behaviour of the deposited alloys and plasma hardening treatment without powder [Applied Surface Science 201 (2002) 154, Surface and Coatings Technology 71 (1995) 196, Journal of Materials Processing Technology 128 (2002) 169, M. Bonacini, Plasma ad arco trasferito per riporti saldati con superleghe in polvere, “Saldatura e taglio termico verso il 2000” IIS Meeting, October 3rd 1998, Milan-Italy, Wear 250 (2001) 611, Surface and Coatings Technology 106 (1998) 156, Wear 249 (2002) 846, Surface and Coatings Technology 92 (1997) 157, Wear 181–183 (1995) 810, Wear 225–229 (1999) 1114], rather than an optimisation of process parameters, also in critical geometric configurations. An experimental campaign has been carried out on the deposition of two nickel- and a cobalt-base superalloys: Hastelloy 276, Inconel 625 and Stellite 6. Benchmarks of C-Mn steel have been chosen to test geometrical configurations that are critical for the application (corners and grooves). Specimen characterisation through liquid penetration inspection, optical- and scanning electron microscopy and microhardness tests proved that process parameters optimisation depends only on the geometrical configuration and not on the deposited alloy. This result suggests the importance of an accurate design of the reciprocal positioning and movement between the torch and the part to be coated, as a function of the deposition geometry.

Introduction

In Plasma Transferred Arc (PTA) deposition of metal powders plasma is set up within the torch between the tungsten electrode and the nozzle, through a low-current pilot arc; then the arc transfers from the orifice to the work piece, becoming part of the electrical circuit, as shown in Fig. 1. Metal powder is fed into the collimated plasma beam, ensuring high energy-concentration: thus narrow and deep welds can be obtained [1], [2], [3], [4], [5], [6], [7], [8]. The process is characterised by extremely high temperatures (up to 20,000–30,000 °C [1]), excellent arc stability, low thermal distortion of the part, high welding speeds (torch/part relative speed: 2–16 mm/s) [1], [2], [3], [4], [5], [6], [7], low environmental impact (low oxides emissions, low percentage of ultra-fine powder) [6]. Plasma high thermal intensity allows high deposition rates and single layer deposits, with evident time savings.

Among the wide choice of metal powders for PTA deposition, Ni- and Co-base alloys are commonly used for high wear- and corrosion-resistant coatings, able to increase a component service life and ensure high temperature performances [3], [4], [7], [13], [14], [15]. Powders are obtained through protective atmosphere atomisation, ensuring a narrow grain size distribution and absence of oxidation, which could cause porosity in the deposited layer [6]. The spherical geometry obtained allows regularity of alimentation, whereas the negligible ultra-fine percentage of the powders, with grains dimensions between 50 and 150 μm [10], ensures easier manipulation with respect to other deposition techniques (oxyfuel gas welding, plasma spray, etc.) as shown in Table 1.

PTA high energetic efficiency, combined with very low dilution levels, yields to single-layer thick coatings that exhibit microstructural properties characteristic of a bulk material, independent of the substrate properties [2], [3], [9], [12]. Dilution, that is the percentage of the base material melted into the coating, is within 5–10% [3]. Additional characteristics of the deposits are a fine microstructure due to high cooling rates [2], [11], excellent shape constancy of the weld, and high adhesion between coating and support.

The industrial applications of these high-performance coatings are quite wide: moulds for glass and ceramics; automotive valves; chemical and petrol-chemical valves; lamination cylinders; plastics extrusion screws and dies. The deposits can be obtained with completely automated unmanned machines. The strategic potential of the PTA process is very little sustained by literature data useful to outline its technological limits. In particular, most of reference studies regard wear- and corrosion performances of the coatings, but do not consider the optimisation of the involved parameters, as a function of the deposition geometry. So, in accordance with the belief that process optimisation is as determinant to the coating properties as the alloys selection [3], a critical benchmark geometry has been chosen to determine process parameters influence on the quality of the deposited coating.