# Effects of Defocus Distance and Weld Spacing on Microstructure and Properties of Femtosecond Laser Welded Quartz Glass-TC4 Alloy Joints with Residual Stress Analysis

## TL;DR

This study explores how laser defocus distance and weld spacing affect the strength and stress in quartz glass-TC4 alloy joints made with femtosecond lasers.

## Contribution

The paper first analyzes the relationship between joint residual strain and femtosecond laser weld spacing.

## Key findings

- A defocus distance of +0.1 mm and weld spacing of 40 μm achieved maximum shear strength of 14.4 MPa.
- Weld spacing below 30 μm causes non-uniform shrinkage, inducing tensile stress and affecting joint strength.
- Defocus distance mainly affects stress along the laser impact direction, while weld spacing influences stress along the spacing direction.

## Abstract

This study develops an optimized femtosecond laser welding process for joining quartz glass and TC4 titanium alloy (Ti-6Al-4V) under non-optical contact conditions, specifically addressing the manufacturing needs of specialized photoelectric effect research containers. The joint primarily consists of parallel laser-welded zones (WZ) interspersed with base material. The defocus distance of the femtosecond laser predominantly influences the depth and phase composition of the WZ, while the weld spacing influences the crack distribution in the joint region. The maximum shear strength of 14.4 MPa was achieved at a defocusing distance of +0.1 mm (below the interface) and a weld spacing of 40 μm. The XRD stress measurements indicate that the defocusing distance mainly affects the stress along the direction of laser impact (DLI), whereas the weld spacing primarily influences the stress along the direction of spacing (DS). GPA results demonstrate that when the spacing is less than 30 μm, the non-uniform shrinkage inside the WZ induces tensile stress in the joint, leading to significant fluctuations in DS residual stress and consequently affecting the joint’s shear strength. This study investigates the effects of process parameters on the mechanical properties of dissimilar joints and, for the first time, analyzes the relationship between joint residual strain and femtosecond laser weld spacing, providing valuable insights for optimizing femtosecond laser welding processes.