A review of dynamic recrystallization phenomena in metallic materials

The ability to map plastic deformation around high strain gradient microstructural features is central in studying phenomena such as fatigue and stress corrosion cracking. A method for the visualization of plastic deformation in electron back-scattered diffraction (EBSD) data has been developed and is described in this article. This technique is based on mapping the intragrain misorientation in polycrystalline metals. The algorithm maps the scalar misorientation between a local minimum misorientation reference pixel and every other pixel within an individual grain. A map around the corner of a Vickers indentation in 304 stainless steel was used as a test case. Several algorithms for EBSD mapping were then applied to the deformation distributions around air fatigue and stress corrosion cracks in 304 stainless steel. Using this technique, clear visualization of a deformation zone around high strain gradient microstructural features (crack tips, indentations, etc.) is possible with standard EBSD data.

Misorientation mapping for visualization of plastic deformation via electron back-scattered diffraction.

In recent years, there is an increased in used of titanium alloys for some parts of mass-produced automobiles and aerospace. However, titanium alloys are characterized by difficult machinability, high melting temperature, high strength, low thermal conductivity, and high reactivity to oxygen, which overshadowed conventional manufacturing processes. To this end, there is a pressing need for more efficient technologies for the manufacture of low-cost titanium structures. Over the years, several joining techniques have been considered for fabrication of titanium alloys. Nevertheless, laser beam welding presents a viable option for welding of titanium due its versatility, high specific heat input, and flexibility. To date, under optimum processing conditions, the strength of the laser-welded titanium alloys can be close to the original material; however, there are still some processing problems such as lower elongation and corrosion resistance coupled with inferior fatigue properties. In this document, the laser beam welding of similar and dissimilar titanium alloys is reviewed, focusing on the influence of the processing parameters, microstructure-property relationship, metallurgical defects, and possible remedies.

A review on laser beam welding of titanium alloys

In this study, Ti 6Al 4V was fabricated via wire and arc additive manufacturing (WAAM) using different deposition strategies. The effects of the travel direction and interlayer dwell time on the evolution of the prior-β grain morphology, microstructure, microhardness, and room-temperature tensile properties of the as-deposited walls were systematically investigated. The thermal and stress/strain field during the deposition process were also simulated to elucidate the macro/microstructural evolution during the deposition. When the interlayer dwell time was less than 24 s, there was a transition from equiaxed prior-β grains (EBGs) to columnar prior-β grains (CBGs) from the bottom to the top of the deposited wall. Vertical and tilted CBGs were obtained with bidirectional and unidirectional travel directions, respectively. With a dwell time of 120 s and unidirectional travel directions, full-equiaxed prior-β grains were obtained within the wall. The EBGs were formed not from the solidification of the molten pool but from the recrystallization induced by the large heat accumulation and high stress/strain during the WAAM process. The recrystallization nucleation rate of the EBG zone was the highest with a dwell time of 24 s. There were tilted layer bands when bidirectional travel strategies were used. There was a fine lamellar α in the layer band-free region and a basketweave α in the layer band region. The increase in the interlayer dwell time led to a decrease in the width of the α lath in the layer band region, but little changed in the layer band-free region. In addition, the hardness, yield strength, and tensile strength increased with the increase in the dwell time from 0 to 120 s.

Effects of deposition strategies on macro/microstructure and mechanical properties of wire and arc additive manufactured Ti6Al4V

Since its inception, laser beam welding as a high-quality fusion joining process has ascertained itself as an established and state of art technology exhibiting tremendous growth in a broad range of industries. This article provides a current state of understanding and detailed review of laser welding of titanium (Ti) alloys with corresponding dissimilar counterparts including steel, aluminium, magnesium, nickel, niobium, copper, etc. Particular emphasis is placed on the influence of critical processing parameters on the metallurgical features, tensile strength, hardness variation, percentage elongation and residual stress. Process modifications to improve dissimilar laser weldability by virtue of techniques such as laser offsetting, split beam, welding-brazing, hybrid welding and materials modifications by means of the introduction of single or multiple interlayers, fillers and pre-cut grooves are exploited. Detailed and comprehensive investigations on the phenomena governing the formation and distribution of the intermetallic phase, material flow mechanisms, their relations with laser parameters and their corresponding impact on the microstructural, geometrical and mechanical aspects of the welds are thoroughly examined. The critical issues related to the evolution of defects and the corresponding remedial measures applied are explored and the characteristics of fracture features reported in the literature are summarised in thematic tables. The purpose of this review is tantamount to emphasise the benefits and the growing trend of laser welding of Ti alloys in the academic sector to better exploit the process in the industry so that the applications are explored to a greater extent.

Current research and development status of dissimilar materials laser welding of titanium and its alloys

Formability and microstructure evolution for hot gas forming of laser-welded TA15 titanium alloy tubes

Study on the mixed dynamic recrystallization mechanism during the globularization process of laser-welded TA15 Ti-alloy joint under hot tensile deformation

Recrystallization behavior during hot tensile deformation of TA15 titanium alloy sheet with substantial prior deformed substructures

Understanding of the deformation inhomogeneity and slip mode of titanium alloys during the thermo-mechanical process is crucial for effectively guiding the forming process and controlling the property of final products. To this end, a crystal plasticity finite element model (CPFEM) was proposed to address these issues of polycrystalline TA15 titanium alloy sheets during the hot tensile process along the rolling direction (RD) at 750 ℃ in this paper. Based on the analysis of Schmid factors (SFs), the weighted averaged SF of basal and prismatic slip systems (WABPSF) was adopted as the criterion of ‘soft’ or’hard’ grains. In term of the simulated results, the localized stress concentration tends to spread at the parts of hard grains adjacent to soft grains, yet the distributions of strain manifest in a sequential fashion. As for the slip mode of TA15 rolling sheets during the hot tensile process along RD, multiple slip systems can be activated as long as the resolved shear stress is larger than the critical resolved shear stress (CRSS). And the prismatic slip systems act as the predominant slip mode, and exhibit an incremental role on the tensile deformation along RD. Meanwhile, the simulated results indicate that the activation of basal slip systems would result in the distinct reduction of basal SF.

Jie Zhao

Papers

Formability and microstructure evolution for hot gas forming of laser-welded TA15 titanium alloy tubes

Study on the mixed dynamic recrystallization mechanism during the globularization process of laser-welded TA15 Ti-alloy joint under hot tensile deformation

Recrystallization behavior during hot tensile deformation of TA15 titanium alloy sheet with substantial prior deformed substructures

Analysis of deformation inhomogeneity and slip mode of TA15 titanium alloy sheets during the hot tensile process based on crystal plasticity model

Experimental and modelling study of an approach to enhance gas bulging formability of TA15 titanium alloy tube based on dynamic recrystallization