Investigation of strengthening mechanism of commercially pure titanium joints fabricated by autogenously laser beam welding and laser-MIG hybrid welding processes

Ruifeng Li,Tianzhu Sun,Shujin Chen,Feng Zhang,Yingtao Tian,Bin Liu

doi:10.1007/s00170-018-2922-9

Abstract

In this study, in order to achieve a better understanding of the strengthening mechanism in the commercially pure (CP) Ti welds, autogenously laser beam and laser-MIG hybrid welding of 4.2 mm thick CP-Ti plates were performed and the correlation between microstructure, texture distribution and the mechanical properties were systematically investigated. Microstructural coarsening and increase in microhardness were observed in the HAZ and WZ. The tensile test results suggested the base metal was the weak point of the joint for both welding conditions. The EBSD observations confirmed that a large number of left{10overline{1}2right} and left{11overline{2}2right} twin grains occurred in the HAZ and WZ of both welded joints, while a higher concentration of these twin grains were found in the laser-MIG hybrid joints. High concentration of the twin grain boundaries can act as barrier to stop dislocation slip during deformation and therefore contribute to the strengthening of the welds. The existence of very small twin grains and acicular α phase in HAZ and WZ would equivalently reduce the averaged grain size and therefore induce an increase in strength based on Hall-Petch’s law. In addition, the averaged Schmid factor of BM is higher than that of the WZ and HAZ in both welding joints suggesting that the grain boundary sliding will take place preferably in BM during deformation so that the necking and fracture occurred in base metal during tensile tests of both welding joint specimens.

Highlights

Pure titanium (CP-Ti) has been widely used in many industries, such as aerospace, defense, petrol-chemical, nuclear energy and medical applications, due to its high specific strength, excellent corrosion resistance, high temperature performance and excellent biocompatibility [1,2,3]
The weld zone needs to be well protected to prevent titanium from absorbing these gas elements resulting in interstitial hardening which is harmful for the mechanical performance of the welds [7,8,9,10]
Most studies showed that the hardness and strength of the laser welded commercially pure (CP)-Ti joints, which comprises of relatively larger grained weld zone (WZ) and heat affected zone (HAZ) zones, were higher than that of the base metal (BM), which consists of relatively smaller grains [5, 11, 12, 14, 17]

Summary

Introduction

Pure titanium (CP-Ti) has been widely used in many industries, such as aerospace, defense, petrol-chemical, nuclear energy and medical applications, due to its high specific strength, excellent corrosion resistance, high temperature performance and excellent biocompatibility [1,2,3]. Most studies showed that the hardness and strength of the laser welded CP-Ti joints, which comprises of relatively larger grained WZ and HAZ zones, were higher than that of the BM, which consists of relatively smaller grains [5, 11, 12, 14, 17] This is controversial to the Hall-Petch’s law and stimulated plenty of research interests. Wang revealed that it was the grain orientation, rather than microstructure, affecting the microhardness distribution of the welded joint and a correlation between mechanical properties and texture was pointed out [22] It can be seen in the literature that there is no general agreement on the strengthening mechanism of CP-Ti welds and further study is still needed. The influences of minor elements, such as oxygen and nitrogen, on the weld properties were not taken into account in this study

Material and methods

Macro- and microstructure

EBSD analysis

X-ray diffraction

Tensile strength and fracture morphology

Conclusions