Interface microstructure and tensile properties of a third generation aluminium-steel butt weld produced using the Hybrid Metal Extrusion &amp; Bonding (HYB) process

Lise Sandnes,Randi Holmestad,Per Erik Vullum,Filippo Berto,Øystein Grong,Tina Bergh

doi:10.1016/j.msea.2021.140975

Interface microstructure and tensile properties of a third generation aluminium-steel butt weld produced using the Hybrid Metal Extrusion & Bonding (HYB) process

Lise Sandnes, Randi Holmestad + Show 4 more

Open Access

https://doi.org/10.1016/j.msea.2021.140975

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Abstract

Abstract The Hybrid Metal Extrusion & Bonding (HYB) process is a patented solid state joining method for metals which utilizes filler material additions to consolidate the weld. In the present investigation the interface microstructure and tensile properties of a 4 mm thick joint, belonging to the third generation Al-steel HYB butt welds, are characterized. The mechanical testing shows that the HYB weld exhibits excellent tensile properties, displaying ultimate tensile strength values in the range from 238 to 266 MPa. Moreover, supplementary digital image correlation analysis of the strain evolution occurring during tensile testing reveals that all plastic deformation is localized to the soft heat-affected zone on the aluminium side of the joint, leading to necking and final fracture in the aluminium. Scanning and transmission electron microscopy examinations of the Al-steel interface show that bonding occurs by a combination of microscale mechanical interlocking and intermetallic compound formation. The intermetallic layer has a thickness varying between 0.1 and 1 μm and is composed of Al–Fe–Si crystals. This makes the butt joint highly resistant against interfacial cracking. The subsequent benchmarking against commercial fusion and solid state welding methods reveals that the tensile properties of the Al-steel HYB weld even surpass those reported for comparable friction stir welds. At the same time, the HYB process allows butt welding to be performed at much higher speeds compared to friction stir welding, without compromising the mechanical integrity of the weldment.

Highlights

There is an increasing interest in welding of aluminium (Al) alloys to steel for industrial applications due to the growing demand for weight reduction in engineering design
The Hybrid Metal Extrusion & Bonding (HYB) process allows butt welding to be performed at much higher speeds compared to friction stir welding, without compromising the mechanical integrity of the weldment
To ensure reliable readings for the yield strength (YS) and the fracture strain, it is important that the virtual extensometer is placed outside the steel and within the central parts of the extrusion zone and the heat-affected zone (HAZ) on the Al side of the joint

Summary

Introduction

There is an increasing interest in welding of aluminium (Al) alloys to steel for industrial applications due to the growing demand for weight reduction in engineering design. The formation and growth of these IMCs depends both on the time and temperature during welding, as well as the presence of constituent elements influencing the reaction kinetics [6,7,8,9,10,11,12,13]. Both the chemical composition, morphology and thickness of the resulting IMC layer affect the mechanical integrity of the joint [12,13,14,15,16]. Only low heat input processes, such as FSW and cold metal transfer welding (CMTW), are capable of producing Al-steel welds suitable for industrial use [33,34]

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