Microstructural characterisation and mechanical properties of laser beam welded dissimilar joint

Abstract

The automotive industry has been identified as one of the greatest contributors of greenhouse gases (GHG) which result in global warming. Therefore, there is an urgent need for the industry to adopt innovative means of producing lightweight vehicles while maintaining passenger safety. Decreasing the total body mass of vehicles has proven to achieve fuel efficiency leading to a reduction of harmful gas emissions. The most effective technique for achieving lightweight vehicles is by joining dissimilar materials, predominantly aluminium alloys, with different steel grades. The main challenge encountered with production of hybrid/dissimilar vehicles by fusion welding is the creation of weak intermetallic compounds (IMCs) that undermine the mechanical response and corrosion resistance of dissimilar weld joints. Laser welding technique is regarded as the most successful method for producing dissimilar weld due to attributes such as low deformation, adjustable heat input and narrow heat affected zone (HAZ). As a result, the procedure was used for the current project.

Laser welding speed and laser welding power were varied to examine the influence of heat input on the characteristics of dissimilar weld joints between Z225 and Al 5754 with regards to microstructure, corrosion properties and mechanical properties. The optical microscope analysis, SEM/EDS, XRD, electrochemical corrosion, microhardness and tensile shear strength examinations were used to study the relationship between the microstructure, corrosion properties and the mechanical response of the dissimilar weld joints.

The results revealed that weld bead dimensions were increased by an increase in heat input,consequently increasing the interfacial layer thickness. The greatest microhardness values were recorded at the lowest speed (1.0 mm/s) owing to the development of IMC phases throughout the weld metal. The EDS composition predicted the presence of FeAl, FeAl2, Fe2Al5, Fe3Al14, twophased mixture of FeAl and FeAl2 and Zn-Al phase. The XRD analysis however only confirmed the presence of FeAl, FeAl2 and Fe2Al5 whose quantities increased with increasing heat input. The Z225/Al 5754 dissimilar weld joints with thicker interfacial layers exhibited better corrosion properties. The maximum tensile strength and percentage elongation of 61 MPa and 1.08 % respectively were obtained from laser power of 1500 W and 1.5 mm/s. Nevertheless, the only sample which passed the tensile strength test was produced at 1.0 mm/s and 1500 W, as it failed at the Al5754 HAZ whereas all others failed at the weld joint interface. The highest hardness, best corrosion and tensile properties were obtained from 1.0 mm/s and 1500 W. Therefore, this combination of parameters was regarded the optimum parameters for combining Z225 and Al 5754 for car body parts.