Microstructural Characterization of Laser Weld of Hot-Stamped Al-Si Coated 22MnB5 and Modification of Weld Properties by Hybrid Welding.

Hana Šebestová,Libor Mrňa,Petr Horník,Pavel Doležal,Šárka Mikmeková,Jan Novotný

doi:10.3390/ma14143943

Abstract

The presence of Al-Si coating on 22MnB5 leads to the formation of large ferritic bands in the dominantly martensitic microstructure of butt laser welds. Rapid cooling of laser weld metal is responsible for insufficient diffusion of coating elements into the steel and incomplete homogenization of weld fusion zone. The Al-rich regions promote the formation of ferritic solid solution. Soft ferritic bands cause weld joint weakening. Laser welds reached only 64% of base metal’s ultimate tensile strength, and they always fractured in the fusion zone during the tensile tests. We implemented hybrid laser-TIG welding technology to reduce weld cooling rate by the addition of heat of the arc. The effect of arc current on weld microstructure and mechanical properties was investigated. Thanks to the slower cooling, the large ferritic bands were eliminated. The hybrid welds reached greater ultimate tensile strength compared to laser welds. The location of the fracture moved from the fusion zone to a tempered heat-affected zone characterized by a drop in microhardness. The minimum of microhardness was independent of heat input in this region.

Highlights

Press-hardened ultra-high-strength steels are excellent materials for lightweight design, especially in the automotive industry
About a 6% decrease in fusion zone (FZ) microhardness towards the base metal (BM) microhardness is identified in laser weld
The microhardness of FZ of hybrid welds made with 40 A and 60 A was higher compared to laser weld

Summary

Introduction

Press-hardened ultra-high-strength steels are excellent materials for lightweight design, especially in the automotive industry. Gu et al [8] studied a hot-stamped steel equivalent to 22MnB5 They described the effect of welding speed on microstructure and properties of butt joints performed with Nd:YAG laser. The argon atmosphere resulted in a higher content of delta ferrite in FZ He et al [12] experimentally studied the effect of laser power, welding speed, and coating condition on the microstructure and mechanical properties of 22MnB5 lap joints. Norman et al [15] used hybrid laser-arc technology with a filler wire to weld the 22MnB5 with and without coating They monitored the welding process with a high-speed camera and studied the formation of oxides on the melt pool surface affecting the dilution of a melt pool, leading to the formation of Al-rich intermetallic phases. The TIG welding was performed in a direct current electrode negative (DCEN) regime

Methods of Weld Inspections

Base Metal Properties

Microhardness Measurement

Strength Properties Measurement

Conclusions