Effect of Heat Treatment on Laser-PBF Ti-6Al-4V Microstructure

Ti-6Al-4V is widely used in the aerospace industry because of its high specific strength, excellent heat resistance and corrosion resistance. On the other hand, the manufacturing cost is higher than other alloys due to its high cutting and material cost. AM (Additive Manufacturing), which can provide them with enhanced functionality through high flexibility of design and improvement of material yield by near-net shape, is expected to be a solution to the above problem. In particular, the Laser-PBF (Powder Bed Fusion) method enables high-precision building of complex shapes, so this method is being applied to products.<br/><br/>However, since process of AM is a new technology, there are many unclear points regarding the relationship between manufacturing conditions, quality, and mechanical properties, and it is important to clarify the fundamental phenomena in order to obtain stable material properties. The purpose of this study is to control the material properties by optimizing the microstructure. The effect of different process parameters (power, scan speed, hatch spacing, layer thickness) and heat treatment conditions on the microstructure was evaluated experimentally. The test specimens were built by L-PBF method under several conditions of process parameters. After observing the as-built microstructure, heat treatment was carried out under several conditions (with reference to the β transformation temperature). Microstructures after building and heat treatment were compared to evaluate the effect of different building conditions on the microstructure after heat treatment. Also, the heat treatment condition in which the improvement in the fatigue property was expected from the microstructure after the heat treatment was shown.<br/><br/>As a result of the above tests, it was suggested that the influence of process parameters on the as-built microstructure was small compared with the change of heat treatment conditions, but the as-built microstructure may have a slight influence on the microstructure after heat treatment. In addition, it was clarified that the selection of the heat treatment condition at the temperature in which the α colony formation by the coarsening of the α lath did not occur was an effective means for the fatigue property improvement.