Fluxless arc weld-brazing of aluminium alloy to steel

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Abstract

In this work, joining of aluminium alloy AA6061-T6 to Interstitial Free steel using pulsed gas metal arc welding process has been attempted. The effect of different surface conditions of steel (viz, galvanized, galvanealed and uncoated) and gap between the sheets on braze joint formation have been investigated. Galvanized steel surface showed good bead width, joint formation and lap shear strength compared to the other two combinations. Interface gap has not affected the wetting behaviour significantly but presence of a gap of 300 μm or so helped in escape of zinc vapour during the process there by avoiding formation of any crevice or macroporosity in the joint. Features and properties of the joint are characterized by metallography, fractography, energy dispersive spectroscopy (EDS) and lap shear tests. Load carrying capacity of Al-Galvanized steel was highest (222 N/mm of seam length) compared to other combinations, aided by better wetting due to presence of Zn on the surface and minimum porosity due to interfacial gap provided during brazing.

Introduction

Selective substitution of steel with aluminium enables light weight design of automobile bodies (Cantor et al., 2008). However, such substitution significantly depends on the availability of suitable joining technologies. Different non-fusion joining techniques such as adhesive bonding, riveting, and friction stir welding of aluminium to steel have been used in the recent past. For example, very recently adhesive bonding technology has been implemented in Honda Acura RLX for joining inner and outer door trims and attained a weight reduction of 17% (Marsh, 2012). Friction stir welding of aluminium to steel is applied to front sub frame in Honda Accord and recorded a weight saving of 25% (Lin, 2013). However, fusion joining in this dissimilar material combination is yet to be successfully adopted owing to problems caused by limited solid solubility of Fe in Al or vice-versa and due to the wide disparity in thermo-physical properties like melting temperature, thermal conductivity and coefficient of thermal expansion. Due to low solid solubility of Fe in Al and vice versa, brittle intermetallic compounds (IMC) form at the interface and renders the joint to be weak (Ozaki and Kutsuna, 2009). But, it has also been reported that if the interface IMC layer thickness is controlled (to less than 10 μm), joints with good mechanical strength are possible (Kreimeyer and Sepold, 2002). Several research groups are carrying out investigations on joining of aluminium and steel using different thermal joining techniques to maximize the mechanical properties aimed at applications in automotive industry. Dong et al. (2010) studied the effect of heat treatment on mechanical performance of the 5A02-H34 aluminium alloy to stainless steel lap joint. Cao et al. (2013) studied the effect of variation in filler wire composition on interfacial characteristics and mechanical performance of cold metal transfer brazed 6061/galvanized Q235, 7075/galvanized Q235and 5183/galvanized Q235 combinations. Dong et al. (2012) also reported the strong joint formation in GTAW welding of 5A02 aluminium alloy and galvanized Q235 steel using Al-12%Si filler wire. Matheu et al. (2006) reported improved wetting by preheating the filler metal and also reported that joints with enhanced mechanical performance can be obtained with Al–Si based filler wire. Hence, from a thorough literature survey, a brazing like process in which filler wire or low melting base material which melts and wets the unmelted solid to form a joint is suitable for fusion joining of aluminium and steel. In such brazed joints, wetting behaviour of the molten metal on the solid surface plays an important role in forming the joint and maximizing the joint strength (AWS Committee on Brazing and Soldering, 1991). The factors affecting wetting behaviour are surface condition of the unmelted base metal (in terms of chemistry and roughness), viscosity and chemical composition of the molten metal and temperature gradients in the brazing zone (Eustathopoulos et al., 1999). For example, thermal gradients can be varied by incorporating preheating to obtain suitable conditions for good wetting (Matheu et al., 2007). Gap between the surfaces to be brazed also affects wetting, as braze flows by capillary action.

Automotive bodies extensively use galvanized (GI) and galvanealed (GA) IF steel and the automotive industries are familiar with arc welding processes such as gas metal arc welding (GMAW). Indigenous ability to consistently join these two dissimilar materials of interest to automotive, that too by using an arc welding process is expected to be very useful in light weighting. In this work, pulsed GMAW brazing of aluminium alloy 6061-T6 to interstitial free (IF) steel (with different surface conditions) in sheet form is investigated to understand the effect of surface chemistry of steel to be brazed and Al-Steel interfacial gap on joint formation, interface intermetallic layer formation and consequent mechanical properties

Section snippets

Material used

Aluminium alloy A6061-T6 of 2 mm thick sheet, 1.2 mm thick IF steel. Elemental composition of the base materials is given in Table 1. IF steel was used with three different surface conditions viz., galvanized (GI), galvanealed (GA) and uncoated (U). Microstructures are given in Fig. 1. The coating layer of GI steel consist of a 13–16 μm thick layer of elemental Zn and GA steel consist of a thick layer (10–15 μm) of δ-phase (FeZn10.08) Fe–Zn intermetallic compound. Differential scanning calorimetry

Effect of chemical composition on wetting and porosity

Fig. 4 shows the actual P-MIG brazed Al/Steel joints with different surface conditions of steel. A continuous uniform bead can be formed with P-MIG brazing process. The visual surface quality is acceptable for all welds on different steel surfaces. Fig. 5a shows a typical Al/Steel brazed lap fillet joint and Fig. 5b shows the schematic of transverse cross-section of the joint indicating bead geometry features, which are used to explain the wetting behaviour. Transverse cross-sectional

Conclusions

  • (1)

    Pulsed-GMAW process can be used as a joining technique for dissimilar aluminium-steel combination

  • (2)

    Wetting and spreading behaviour is altered by the surface chemistry of steel. Presence of zinc coating on steel enhances the wetting and spreading behaviour of steel than in the form of iron zinc alloy in the case of galvanealed steel.

  • (3)

    The thickness and morphology of the intermetallic compounds formed at the interface are dependent on energy intensity and distribution.

  • (4)

    Joints made with galvanized

References (16)

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