# Microstructure Evolution and Fracture Mode of Laser Welding–Brazing DP780 Steel-5754 Aluminum Alloy Joints with Various Laser Spot Positions

## TL;DR

This study examines how laser spot position affects the strength and fracture behavior of steel-aluminum joints, showing that positioning the laser more on steel improves joint strength and changes where the joint breaks.

## Contribution

The study reveals how laser spot position influences microstructure and fracture mode in steel–aluminum laser welding–brazing joints.

## Key findings

- Tensile-shear strength increased from 169 MPa to 241 MPa as laser spot proportion on steel rose from 50% to 90%.
- Fracture location shifted from the interface to the aluminum alloy base metal with higher laser spot proportion on steel.
- Intermetallic compounds inhibited crack propagation when laser spot proportion on steel was high.

## Abstract

Joining steel and Al alloys can fully utilize their advantages for both base metals (BMs) and optimize automobile structures. In this study, the laser welding–brazing technique was utilized to join DP780 steel and aluminum alloy 5754 (AA5754). The mechanical properties, microstructure, and fracture locations of steel–Al joints prepared using different laser spot positions were comparatively investigated. As the proportion of the laser spot on the steel BM increased from 50% to 90%, the tensile–shear strength of the steel–Al welded joint rose from 169 MPa to 241 MPa. Meanwhile, the fracture location of the joint shifted from the interface to the BM of the aluminum alloy. The change in the laser spot position could dramatically affect the interfacial microstructure and fracture mode of the steel–Al joint. When the proportion of the laser spot on the steel BM was relatively small (50%), the growth of intermetallic compounds (IMCs) was inhibited. The metallurgical bonding effect at the steel–Al interface was poor. In this case, the interfacial zone became the primary path for the crack propagation. Thus, interface failure became the dominant failure mode of the steel–Al joint. On the contrary, metallurgical bonding at the interface was remarkably improved as the proportion of the laser spot on the BM of the steel increased (to 90%). It was determined that the IMCs could effectively hinder the propagation of cracks along the interface. Eventually, the joint fractured in the Al alloy’s BM, resulting in a qualified steel–Al joint.