Dissimilar friction welding of 6061-T6 aluminum and AISI 1018 steel: Properties and microstructural characterization

Abstract

Joining of dissimilar materials is of increasing interest for a wide range of industrial applications. The automotive industry, in particular, views dissimilar materials joining as a gateway for the implementation of lightweight materials. Specifically, the introduction of aluminum alloy parts into a steel car body requires the development of reliable, efficient and economic joining processes. Since aluminum and steel demonstrate different physical, mechanical and metallurgical properties, identification of proper welding processes and practices can be problematic. In this work, inertia friction welding has been used to create joints between a 6061-T6 aluminum alloy and a AISI 1018 steel using various parameters. The joints were evaluated by mechanical testing and metallurgical analysis. Microstructural analyses were done using metallography, microhardness testing, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray elemental mapping, focused ion beam (FIB) with ultra high resolution SEM and transmission electron microscopy (TEM) in TEM and STEM modes. Results of these analysis first suggested that joint strengths on the order of 250 MPa could be achieved. In addition, failures were seen in the plasticized layer on the aluminum side of the joint. Further, bond lines were characterized by a thin layer of formed Al–Fe intermetallic. This intermetallic layer averaged roughly 250 nm thick and compositionally appears related to the FeAl and Fe₂Al₅ phases.

Introduction

Dissimilar metal joining offers the potential to utilize the advantages of different materials often providing unique solutions to engineering requirements. The main reasons for dissimilar joining are due to the combination of good mechanical properties of one material and either low specific weight or good corrosion resistance or good electrical properties of second material. Consequently, joining processes for dissimilar materials have received considerable attention in the recent years. Much of this activity has focused on the transportation industries such as aerospace, aviation, shipbuilding, railway transportation. This is especially in the automotive industry due to the potential weight reduction of both vehicle components and structures. The need to expand the use of lightweight structures in the automotive industry has increased interest in the use of both aluminum and magnesium as structural materials. However, the cost of aluminum compared to steel restricts its application for automobile parts. As a result aluminum is more economical when it can be used in hybrid structures with steel. In order to incorporate these hybrid structures, proper joining methods for aluminum-to-steel dissimilar combinations are necessary. Steel and aluminum have been successfully joined for production applications using both mechanical fastening and adhesive bonding. Although these approaches have demonstrated fitness for the intended applications, they suffer from low specific strengths and are largely limited to lap geometries. In considering structural joints between steel and aluminum, welding processes should be taken into account [1], [2], [3], [4], [5], [6], [7], [8], [9], [10].

Research conducted on the welding of aluminum to steel ranges from solid state to fusion welding processes. Specific processes investigated include as resistance spot [4], [5], [6], [7], [8], [9], [10], [11], [12], friction stir [3], [13], [14], [15], friction stir knead [16], friction stir spot [17], [18], [19], diffusion [20], magnetic pulse [21], [22], magnetic pressure seam [23], electromagnetic impact [24], and explosion welding processes [25], [26], and gas metal arc welding (GMA)–cold metal transfer (CMT) [27], laser beam [28], [29], [30], [31], laser assisted pressure [32], laser hybrid [33], laser roll [34], and braze assisted [35], [36], [37] welding processes.

Mechanical bonding between steel and aluminum is challenged due to significant differences in both physical and metallurgical properties. For example, large differences in thermal properties such as expansion coefficient, conductivity, and specific heat can lead to residual stresses. Metallurgically, joints between aluminum and steel can result in multiple intermetallic phases that generally form by solid state reaction. These intermetallics generally result in mechanical degradation of the joint. The formation of these phases is mainly driven by interdiffusion of the species and is highly dependent on the specific time and temperature history of the welding process. The extended thermal cycles (higher temperatures/longer times) associated with fusion welding processes generally result in the formation of thick intermetallic compound (IMC) layers at the joint interface. The formation of these layers is generally considered the root cause for the property degradation seen with these types of joints. Solid state welding techniques facilitate joint formation at lower temperatures and often at very short times. The use of solid state joining process generally is associated with reduced formation of these intermetallic phases. One such method, friction welding, has affectively been used for joining aluminum to steel in production environments. Properly applied, friction welding (and its variant inertia friction welding) allows joining at relatively low temperatures with an overall short thermal cycle. Nonetheless, friction welding of aluminum alloys and low alloy steels remains challenging. Recent research has addressed friction welding material combinations including pure and alloyed aluminum to austenitic stainless steel, aluminum alloys to carbon steel, aluminum etc. however, the application of friction welding aluminum alloys with low alloy steels remains a dominant interest. Joints involving this material combination are expected to see increasing numbers of industrial applications [3], [8], [9], [10], [11], [12], [14], [33], [38], [39], [40], [41], [42].

This paper investigates the joint properties and metallurgical characteristics of dissimilar inertia friction welds between AISI 1018 steel and 6061-T6 aluminum. Particular emphasis and study concentrated on interface microstructure characterization by means of microhardness mapping, SEM, EDS, X-ray elemental mapping, FIB, (S)TEM and (S)TEM–EDS line analysis. This study reflects the considerable demand and importance for industrial application of such dissimilar joints between aluminum and steel, particularly in transportation industry.