Selective Laser Melting of a Near‐α Ti6242S Alloy for High‐Performance Automotive Parts

Abstract

This study aims to investigate additively manufactured Ti6242S specimens compared with the widely used Ti64 alloy with a special focus on microstructure and mechanical properties as well as the impact of subsequent heat treatments. As the Ti6242S alloy, which belongs to the family of near‐α Ti‐alloys, is often used at higher service temperatures, uniaxial tensile tests are performed at a room temperature up to 500 °C. By means of optical and electron microscopy, it is found that the as‐built microstructure consists of acicular α′ martensite, which decomposes to α + β during the subsequent heat treatment. A special focus on the prior microstructure shows that the Ti6242S alloy has a small β grain size, which influences the resulting α′ microstructure after the β → α′ phase transformation. Furthermore, the mechanical properties at room temperature as well as elevated temperatures exceed the values for selective laser melted Ti64 and conventionally cast Ti6242 material. The heat‐treated Ti6242S specimens exhibit an ultimate tensile strength of about 1213 MPa including a ductility of 11.3% at room temperature. These values may path the way to a substitution of the widely used Ti64 alloy by the near‐α Ti6242S alloy, especially for highly loaded components at elevated temperatures.

Optimization of the Post-Process Heat Treatment Strategy for a Near-α Titanium Base Alloy Produced by Laser Powder Bed Fusion

“…Figure 1 displays the investigated one-step (β-annealing), two-step (duplex annealing) and three-step (triplex annealing) heat treatments investigated within this study. Based on thermodynamic calculations as reported in reference [ 11 ], a schematic diagram of the rising β phase fraction at higher temperatures is shown on the right of Figure 1 .…”

“…Recent research activities have focused on the investigation of near-α titanium base alloys, which reveal similarities to the popular α + β alloys such as Ti-6Al-4V (m.%, Ti64), yet containing a lower fraction of β stabilizing elements [ 7 , 8 , 9 , 10 ]. Especially the near-α Ti-6Al-2Sn-4Zr-2Mo-Si (in m.%, Ti6242S) alloy produced by LPBF has gained attention due to several advantages such as higher tensile strength and superior creep resistance in comparison to the Ti64 alloy [ 11 ]. Additive manufacturing, e.g., LPBF, is regarded as a promising technology when compared to conventional manufacturing techniques, which is caused by less material waste via the direct production of highly complex geometries [ 12 ].…”

“…Additive manufacturing, e.g., LPBF, is regarded as a promising technology when compared to conventional manufacturing techniques, which is caused by less material waste via the direct production of highly complex geometries [ 12 ]. For example, titanium alloys are a popular choice for LPBF in industrial applications for producing biomedical implants [ 13 ], lightweight aerospace parts [ 14 ], as well as complex parts of the exhaust system in the automobile sector [ 11 ]. Choosing LPBF as a manufacturing technique can significantly improve the mechanical properties of components.…”

“…The high cooling rates of this manufacturing process evoke a non-equilibrium material condition and promote a high density of lattice defects and residual stresses, which may lead to the formation of cracks and delamination during the LPBF process [ 16 , 17 ]. The microstructure of the Ti6242S components consists of long columnar parent β-grains (~83 µm width), several hundred µm length which contain fine acicular αʹ martensite (~1 µm minor axis length and ~8 µm major axis length) as recorded in [ 11 ].…”

“…Recent investigations on an-LPBF manufactured Ti6242S alloy have shown that the heat treatment response led to the precipitation of β particles at α-grain boundaries and within α grains. This behavior was ascribed to the low diffusivity of the β-stabilizing element Mo in this alloy [ 11 ]. The Ti6242S alloy is also known for its high creep resistance caused by its low self-diffusion coefficient [ 5 , 7 ].…”