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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.
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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.