Surface and Coatings Technology, 70 (1995) 221-229 221 Laser alloying of aluminium alloys with chromium A. Almeida, M. Anjos, R. Vilar, R. Li and M. G. S. Ferreira Instituto Superior Tkcnico, Av. Rovisco Pals, 1000 Lisboa (Portugal) W. M. Steen and K. G. Watkins University of Liverpool, PO Box 147, Liverpool L69 3BX ( UK) Abstract The microstructure and corrosion resistance of laser-alloyed aluminium and ANSI 7175 aluminium alloy with chromium were investigated. Surface layers alloyed with chromium contain relatively large amounts of intermetallic compounds dispersed in a matrix of ct-A1. The intermetallic compound particles present needle-like morphologies, organized in a dense network or distributed radially. A17Cr, AlllCr 2 and ct-A1phases have been identified by X-ray diffraction. The alloyed layers may contain cracks, pores, inclusions and undissolved chromium particles, depending on the chromium concentration and the particle size. However, homogeneous layers were produced by a two-step process, consisting of laser alloying followed by remelting. The second treatment eliminates porosity and refines the structure. The hardness attains a Vickers hardness of 155 HV in chromium-alloyed aluminium and exceeds 300 HV in chromium-alloyed 7175. The corrosion behaviour of the above alloys was assessed using anodic polarization techniques. Laser alloying of aluminium and 7175 with chromium improves the pitting corrosion resistance of the alloys. The effect depends on the chromium content of the alloyed layers and is more significant in 7175 alloy. 1. Introduction High performance aluminium alloys have been the object of intensive research in the past few years. The research efforts have been concentrated on the develop- ment of new compositions with improved properties as well as on the development of novel processing tech- niques, capable of improving the behaviour of conven- tional alloys and producing new alloys with outstanding properties. Rapid solidification (RS) processing has played a major role in recent developments in this field, since it promotes a general refinement of the microstruc- ture, extension of solubility of critical alloying elements in the ~-A1 terminal solid solution and the formation of metastable phases, including metallic glasses and quasi- crystals [ 1]. Since these microstructural modifications are often responsible for an increase in the wear and corrosion resistance and high temperature strength of aluminium alloys, RS is being seriously considered for the development of high performance alloys. Amongst RS techniques, laser surface alloying (LSA) is particu- larly efficient for producing surface layers with improved wear and corrosion resistance on aluminium alloys, since it combines the controlled modification both of the microstructure and of the chemical composition to tailor surface properties to the application requirements. A1-Cr alloys produced by RS techniques were investi- gated by several authors and it was demonstrated that this system exhibits a considerable tendency to form supersaturated solid solutions. Warlimont et al. [2] showed that more than 6 at.% Cr can be retained in solid solution in samples quenched from the liquid state at cooling rates in the range 106-10 s K s-1, as compared with an equilibrium solid solubility of 0.38 at.% [3]. Detailed microstructural studies made by Furrer and Warlimont [4] and Bendersky et al. [5] on compositions up to 15 wt.% Cr showed that the extension of solid solubility is limited to 6at.%. The authors explain this limitation by the fact that solid solutions with higher chromium content decompose by a dis- continuous precipitation reaction that develops during cooling, even when the cooling rate was as high as l0 s K s-1. Similar work was reported on other systems, such as A1-Cr-(Ni, Mo), A1-Mn-Cr, A1-Cr-Fe and A1-Cr-Zr [6-10]. It was also shown that aluminium- transition metal alloys obtained by RS present excellent stability when exposed to moderately high temperatures [6], a very attractive feature for automobile and aircraft applications. In spite of the promising characteristics of A1-Cr alloys obtained by RS techniques, no detailed work has until now been reported on laser alloying of aluminium alloys with chromium. This paper reports results of a study that aims to evaluate the possibility of producing corrosion-resistant surface layers by alloying aluminium and high resistance aluminium alloys with chromium. The substrates were commercial pure aluminium and A1-Zn-Mg alloys of the 7000 series. The latter were chosen because these 0257-8972/95/$9.50 SSDI 0257-8972(94)02263-P © 1995 Elsevier Science S.A. All rights reserved