Strengthening of Ti-6Al-4V Alloy by Short-Time Two-Stage Heat Treatment
Ti-6Al-4V is one of the most popular and demanded titanium alloys used for applications such as aerospace, medical implants and industrial machinery. However, its typical low-strength properties make it necessary to increase its strength characteristics. To improve these properties, it must be strengthened by improving its microstructure. This can be achieved through heat treatment, by allowing microstructure changes to be made.
The microstructural modifications of the Ti-6Al-4V alloy support its strengthening process. Usually, two-stage heat treatments are used in order to obtain adequate hardening, including one austenitizing stage and one secondary stage, usually involving an aging treatment. Many studies and simulations have been conducted in order to understand the development of precipitates and to optimize the heat treatment routes.
Short-time two-stage heat treatments of the Ti-6Al-4V alloy have been studied in order to evaluate their effect on the alloy´s mechanical properties, such as its hardness and strength. The heat treatment consists of a double stage. First, the alloy is austenitized for a short time at a high temperature, promoting the formation of fine plate-like alpha phase grains with a small fraction of beta phase. Then, the alloy is quenched and aged to increase the fraction of beta phase, thus resulting in a bimodal microstructure.
The aim of this part of the study was to analyze the effect of different aging temperatures on the hardness and strength of Ti-6Al-4V alloy after a short-time two-stage heat treatment. The aging temperature was varied between 500 and 595°C, and the aging time was kept constant at two hours. The alloy samples were then tested for hardness and tensile strength, with hardness measurements performed on as-received, one-step and two-step heat treated specimens.
The microstructure of the Ti-6Al-4V alloy after the two-stage heat treatment underwent significant changes. The alloy microstructure displayed homogeneous distribution of both alpha and beta phases with fine size grains, typical of a two-stage heat treated microstructure. The hardness obtained was also in good agreement with the obtained microstructure. For example, the highest hardness values were obtained for specimens heat treated at 595°C, with a hardness of 400 HV.
The tensile strength of the specimens also varied between the different aging temperatures. The specimens aged at 595°C showed the highest tensile strength values, reaching 1600 MPa, which is around 15-20% higher than the strength values obtained for the one-step and as-received specimens. These results can be attributed to the microstructural changes promoted by the two-stage heat treatment, which allowed for improved toughness, hardenability and strength.
From this study, it can be concluded that short-time two-stage heat treatment of Ti-6Al-4V alloy has a significant effect on its hardness and strength. The optimal results were obtained when the aging temperature was kept at 595°C, allowing for improved hardness, hardenability and tensile strength. This results in a strengthened Ti-6Al-4V alloy with significantly improved mechanical properties, which can be utilized in various applications.
