Effect of Heat Treatments on Thermomechanical Properties of Additively Manufactured Ti6Al4V alloy at Elevated Temperatures and Correlation with Microstructure for Aerospace and Defense application /
Nouman Ali
- 94p. Soft Copy 30cm.
Additive manufacturing (AM), being the future of the aerospace manufacturing industry, is famous for manufacturing rapidly, saving material, time and capital. Among metal AM processes, Selective Laser Melting (SLM) is the most used Powder Bed Fusion (PBF) technology and known for manufacturing near net shape parts with highest density. For aerospace and terrestrial applications, Ti alloys are considered to give balanced mechanical properties and Ti6Al4V is the most famous and most employed Ti alloy, attributed as the Ti workhorse especially in the aerospace industry. Its high strength to weight ratio, high corrosion resistance, high fatigue performance and excellent fracture toughness has been proven. Whereas its low thermal conductivity and high reactivity at elevated temperatures poses challenges during conventional manufacturing process including machining and casting. Thus, to avoid these demerits, the AM of Ti6Al4V and investigation of its mechanical properties has been the focus of research for the last two decades. In this research the thermomechanical properties of selectively laser melted (SLMed) Ti6Al4V have been studied with and without heat treatments and thermal properties at higher temperatures have been investigated to find out the reasons for low thermal conductivity through microstructure analysis and relative density measurements. The effect of different heat treatments with varying temperatures i.e. 900°C, 930°C and 950°C and dwell time i.e. 2 and 4 hours on thermomechanical properties have also been examined. Moreover, the effect of SLM build and scan orientation on thermal conductivity has been analyzed. The microstructure analysis has been performed through various qualitative and quantitative techniques including Optical Microscopy (OM), Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and X-Ray Diffraction (XRD) analysis. The findings of microstructure investigations in terms of elemental composition, phase identification and grain morphology and relative density attained through different heat treatments have been correlated with heat treatment parameters and resulting thermomechanical properties. The thermal conductivity of heat treated SLMed Ti6Al4V has shown slightly improved range of values i.e. 2.47-2.85W/mK as compared to as-built stress relieved Ti6Al4V and shown linear relationship with thermal conductivity range of 2.47-4.86W/mK with an increase in temperature from RT to 300°C. The thermal conductivity of as-built stress relieved Ti6Al4V in scan orientation has given higher values of 2.98W/mK as compared to build orientation value of 2.33W/mK. Findings show that heat treatment with temperature of 930°C and dwell of 2 hours given balance mechanical properties of high ductility i.e. 14.6% and optimum tensile strength 859MPa, whereas Hot Isostatic Pressing (HIP) has significantly enhanced the thermal conductivity to 2.85W/mK. HIP treatment achieved the highest relative density as compared to all heat treatments i.e. 99.5%. Whereas the relative density increased from 97.6% to 98.98% with an increase in temperature and dwell time of heat treatments up to 930°C and decreased at 950°C i.e. 96.79% which probably resulted in decreased thermal conductivity values. xi The wide scatter in thermal conductivity i.e. 2.11W/mK to 6.5W/mK, at RT by different methods, i.e. steady state method and transient plane source (TPS) methods observed by couple of researchers has been investigated by performing measurements through both methods. Results of steady state and TPS methods showed the similar thermal conductivity of as-built stress relieved Ti6Al4V i.e. 2.33W/mK and 2.41W/mK respectively at RT. It suggests that this wide variation found in literature was probably due to variation in metal AM process parameters used by different researchers and not because of measuring methods. The TPS method has given slightly higher values because it is probably less affected by surface roughness and possible oxide layers on surface. Overall, the hot isostatically pressed (HIPed) selectively laser melted (SLMed) Ti6Al4V in scan orientation has the capability to give highest thermal conductivity values for aerospace applications but with a compromise in mechanical strength due to layered manufacturing.