Effect of phase transformations on laser forming of Ti–6Al–4V alloy

Abstract

Surface deformation behavior of beta solution treated and overaged Ti-6Al-4V during laser shock processingIn laser forming, phase transformations in the heat-affected zone take place under steep thermal cycles, and have a significant effect on the flow behavior of Ti-6Al-4V alloy and the laser-forming process. The flow-stress data of a material are generally provided as only dependent on strain, strain rate, and temperature, while phase transformations are determined by both temperature and temperature history. Therefore, effect of phase transformations on the flow behavior of materials in thermomechanical processing is not given necessary considerations. In the present work, both the ␣ → ␤ transformation during heating and the decomposition of ␤ phase, producing martensite ␣Ј or lamellae ␣ dependent on cooling rate, are numerically investigated. The spatial distribution of volume fractions of phases is obtained by coupling thermal and phase transformation kinetic modeling. Consequently, the flow stress of Ti-6Al-4V alloy is calculated by the rule of mixtures based on the phase ratio and the flow stress of each single phase, which is also a function of temperature, strain, and strain rate. According to the obtained flow-stress data, the laser-forming process of Ti-6Al-4V alloy is modeled by finite element method, and the deformation is predicted. A series of carefully controlled experiments are conducted to validate the theoretically predicted results.

Ductility improvement due to martensite α′ decomposition in porous Ti–6Al–4V parts produced by selective laser melting for orthopedic implants

“…It is known that Ti-6Al-4V is an αþβ alloy and that the volumetric fraction of the α phase, which is about 90% at room temperature (Henry et al, 1995;Facchini et al, 2010), decreases with increasing temperature until it is completely transformed into β phase when the temperature reaches β-transus. According to the literature (Ahmed and Rack, 1998;Vrancken et al, 2012;Pederson et al, 2003;Fan et al, 2005;Veiga et al, 2012), the β-transus temperature of Ti-6Al-4V is about 970750 1C. Based on dilatometric experiments, Homporová et al (2011) reported that the α-β transformation in Ti-6Al-4V alloy with duplex microstructure occurs between 750 and 1050 1C.…”

A model for Ti–6Al–4V microstructure evolution for arbitrary temperature changes

“…Only few papers address the topic of the Koistinen-Marburger coefficient, coefkm. Fan et al (2005) evaluated the coefficient to 0.003 after experiments by Malinov et al (2001a). The author of the current thesis evaluated the same parameter from spot weld experiments (Elmer et al 2005b) to be 0.005.…”

“…Diverse modelling methods are applied to model phase transformations and some of the transformation kinetics associated have been evaluated for titanium alloys [19]. The phase transformation kinetics during either isothermal or cooling processes have been successfully described with the JMAK model , Avrami [26] and Kolmogorov [33]) for titanium alloys [34], especially for Ti-6Al-4V [11,19,35]. Nevertheless, phase transformations during heating have been studied by a few authors [28,36].…”

Prediction and measurement of residual stresses and distortions in fibre laser welded Ti-6Al-4V considering phase transformation

“…Malinov et al [19,20] studied the phase transformation kinetics under isothermal condition using resistivity technique and continuous cooling conditions using differential scanning calorimetry at different cooling rates using the Johnson Mehl Avrami and Kolmogorov (JMAK) equation [21][22][23] and the additivity rule to determine the fraction of β transformed to α. Ahmed and Rack [24] studied the effects of cooling rate from above the beta transus on phase transformation during cooling and determined that martensitic formation forms at high cooling rates above 410 K/s, while Gil Mur et al [25] studied the decomposition of martensite into α and β during tempering heat treatment and observed fully martensitic structure at much lower rates. Kelly and Kampe [26,27], Crespo et al [28] and Murgau et al [29] modelled microstructural evolution in Ti-6Al-4V during a laser metal deposition process and Fan et al [30] numerically investigated the effect of phase transformations on laser forming of Ti-6Al-4V using JMA kinetic parameters for isothermal transformation from Malinov et al [19] but without applying additivity principles to diffusion controlled transformation. Elmer et al [7] measured experimentally using the synchrotron X-ray diffraction technique the transformation kinetics during gas tungsten arc welding (GTAW) of Ti-6Al-4V.…”