Aluminum–aluminum compound castings by electroless deposited zinc layers

Abstract

Sandblasted aluminum sheets (Al 99.5) were coated with a basic zincate solution and integrated into high pressure die casting components (AlSi9Cu3(Fe)) with wall thickness ranging from 3 mm to 6 mm. Investigations of the Al–Al-interface showed that sound castings could be realized. Microprobe analysis was used to elucidate the dissolution and diffusion of the Zn coating as a function of the processing parameters. Due to the very small total amount of zinc we did not find a zone of brittle intermetallic phases at the interface. Shear testing of the compounds confirmed the presence of a firmly bonded compound without embrittlement by intermetallic phases.

Introduction

One interesting possibility to join different materials is by compound casting where two metals – one solid, the other liquid – are combined. This was shown successfully by Fragner et al. (2008) as well as Papis et al. (2009) by adding thin layers of zinc to Al-wrought sheets and cast little amounts of Al alloys onto the sheets using small tube furnaces and inert gas. The challenge of compound casting was the realization of a firmly bonded interface which developed by a diffusion or reaction between both metals leading to a continuous transition from one material into the other. In addition, the transition zone should be free of brittle phases which might develop due to additional material supporting bonding.

Generally, joining of two aluminum alloys is difficult due to the natural aluminum oxide layer. This very thin but thermodynamically stable layer has a melting point which is much higher than that of aluminum and is not dissolved during the casting process but completely prevents the formation of a metallic bonding. Thus, the concept of Papis et al. (2008) to realize a firm bonding was to remove the oxide layer and to replace it by a reactive coating. The coating is thought to prevent re-oxidation and to melt or dissolve during the casting process in order to form a continuous metallic transition between both components after cooling.

Zinc is well suited as coating material due to its low melting temperature of 420 °C combined with its high solubility in aluminum at elevated temperatures. A well established approach to generate adhesive coatings is the so-called zincate treatment – well described by Golby and Dennis (1981) and Robertson et al. (1995) where the aluminum oxide layer is simultaneously dissolved and replaced by a thin zinc layer in an electroless chemical process. Golby and Dennis (1981) investigated the adhesion of Ni foiles on zincate treated Al surfaces using peel of tests. They found out, that especially pretreatment procedures like sandblasting with SiC grit influences the adhesion properties of the coatings in a great manner. Robertson et al. (1995) therefore describes the building mechanisms of zinc particles during the zincate treatment on rotating Al-plates and uses electrochemical methods to describe the process. The zincate treatment is used for improving adhesion optical and corrosive reasons. This was shown by Monteiro et al. (1991) investigating the coating technique using Al-sheets and performing parameter studies to gain information about the coating morphology. Also Hino et al. (2009) managed to investigate the building mechanism for zinc nuclei and developed very good models for the creation of zinc particles depending on the location on the surface as well as morphologies during following galvanizing steps for zinc. The application of the zincate pretreatment does not only result in high quality coatings by dissolving oxides, but also exhibits the possibility to improve the corrosion resistance of aluminum, especially against sodium hydroxide.

A promising approach to join light metals was presented by Fragner et al. (2008) and Papis et al., 2008, Papis et al., 2009 who used a combination of zincate treatment (<1 μm) and electrolytically deposited Zn layer (5–10 μm). Couples of AlMg1 substrate and various Al alloys were successfully produced by means of a laboratory-scale compound casting process under controlled thermal conditions. Defect-free interfaces could be realized by applying a droplet of the liquid alloy onto a preheated Zn coated substrate in an Ar6.0 atmosphere with subsequent slow cooling. The firmly bonded interface developed by formation of intermetallic phases and a diffusion zone.

The very short heating and cooling times during high pressure die casting represent a challenge for Al–Al-compound casting. Previous work performed by Rübner et al. (2011) has shown that using a combination of zincate treatment and electrolytically deposited Zn layer could be successful but did not lead to a sound process due to different reasons. The very high and locally varying melt velocities sometimes completely washed away the Zn-coating. In addition, the comparatively thick Zn coating led to a strong embrittlement of the interface where intermetallic phases are formed. These problems may be solved if the zincate treatment alone already leads to a sound bonding where only a very thin Zn layer (<1 μm) is deposited on the aluminum insert. In this case, washing away of the coating or the formation of brittle intermetallic phases should be suppressed. As Rübner et al. (2011) showed was the initial approach using a standard zincate treatment not successful at all. In this case the shear strength of the interface was nearly zero. Thus, Schwankl et al., 2013a started a systematic investigation to improve the quality of the Zn coating i.e. the area fraction covered by Zn. These investigations showed that all steps during the zincate treatment are important especially the nature of the zincate solution and the sand blasting process parameters.

Aim of this paper is to show that an optimized zincate treatment of aluminum inserts leads to sound Al–Al-compounds during high pressure die casting. In order to simulate different temperature–time histories the mold temperature and the wall thickness of the component are varied. Microprobe analysis shows the Zn profile at the interface. There is a strong correlation between the thickness of the casting and the quality of the interface which is characterized by mechanical shear tests.