Subsurface Cooling Rates and Microstructural Response during Laser Based Metal Additive Manufacturing

Abstract

Laser powder bed fusion (LpBf) is a method of additive manufacturing characterized by the rapid scanning of a high powered laser over a thin bed of metallic powder to create a single layer, which may then be built upon to form larger structures. Much of the melting, resolidification, and subsequent cooling take place at much higher rates and with much higher thermal gradients than in traditional metallurgical processes, with much of this occurring below the surface. We have used in situ high speed X-ray diffraction to extract subsurface cooling rates following resolidification from the melt and above the β-transus in titanium alloy Ti-6Al-4V. We observe an inverse relationship with laser power and bulk cooling rates. the measured cooling rates are seen to correlate to the level of residual strain borne by the minority β-ti phase with increased strain at slower cooling rates. the α-ti phase shows a lattice contraction which is invariant with cooling rate. We also observe a broadening of the diffraction peaks which is greater for the β-ti phase at slower cooling rates and a change in the relative phase fraction following LpBf. these results provide a direct measure of the subsurface thermal history and demonstrate its importance to the ultimate quality of additively manufactured materials. In LPBF, a common metal additive manufacturing (AM) process, a thin layer of precursor powder is selectively melted by tracing a high power laser to create a single, solid layer, followed by the spread of the next layer of powder for melting, and so on to build a part layer-by-layer 1-14. This approach offers a number of distinct advantages over conventional metal fabrication, including rapid prototyping, efficient material utilization, fabrication of complex geometries incompatible with machining or molding techniques, and applicability to materials which may exhibit machining difficulties, such as titanium 4,8-12,14. However, unlike conventional manufacturing techniques, the LPBF process is characterized by rapid melting and resolidification of small volumes of material that results in large thermal gradients both temporally and spatially, and consequently cooling rates that can be orders of magnitude higher than in conventional casting 10,15,16. Such large localized heating and cooling rates largely control the resulting microstructure, grain size and orientation, can lead to large residual stresses in some materials, and may also result in compositional segregation or formation of non-equilibrium trapped phases 4,17-20. Therefore quantifying these cooling rates and correlating them to the characteristics of the final build is an important step towards informing theoretical and process models that can predict and potentially control such stresses and inhomogeneities that are generally undesirable, especially in critical components. For this study, we focused on Ti-6Al-4V (Ti-64, 90%Ti, 6% Al, 4%V-by weight) as our material system. The high strength, light weight, and excellent corrosion resistance inherent to titanium and its ...

Progress on In Situ and Operando X-ray Imaging of Solidification Processes

“…The heating/cooling rates of the LPBF process are approximately 10 5 –10 6 K s −1 which is three orders of magnitude faster than that of traditional casting, and the thermal gradient established inside the molten pool can reach between 10 3 and 10 4 K mm −1 . To tackle such demanding scientific challenges, research teams across the globe developed different types of additive manufacturing simulators [ 13 , 26 , 97 ] combined with high-speed X-ray imaging and diffraction facilities at Advanced Photon Source (APS) [ 151 ], Diamond Light Source [ 26 , 58 , 152 ], European Synchrotron Radiation Facility [ 153 ], Stanford Light Source [ 13 , 154 , 155 ], and Swiss Light Source [ 42 ], to probe and elucidate the molten pool dynamics during LPBF.…”

“…In situ and operando X-ray diffraction or X-ray imaging in the reciprocal spacing has been used to capture the dynamic phase transformation, extract the subsurface cooling rates, and evolution of strains during LPBF of Ti-6Al-4V [ 42 , 155 , 156 ]. Cang et al [ 156 ] studied the phase transformation during a single spot LPBF of Ti-6Al-4V, showing the evolution powder underwent melting, solidification (with a cooling rate of 1 × 10 5 K s −1 ), and reported that β → α’ solid-state phase transformation is diffusionless and has a transformation rate above 10 4 μm s −1 .…”

“…Cang et al [ 156 ] studied the phase transformation during a single spot LPBF of Ti-6Al-4V, showing the evolution powder underwent melting, solidification (with a cooling rate of 1 × 10 5 K s −1 ), and reported that β → α’ solid-state phase transformation is diffusionless and has a transformation rate above 10 4 μm s −1 . Thampy et al [ 155 ] studied the effect of laser powers on the changes in lattice spacing during LPBF of a single layer Ti-6Al-4V track. They reported that the lattice parameter and residual strain of the β-Ti reduced with increasing cooling rate from 1.5 × 10 4 to 7.5 × 10 4 K s −1 , similar to that reported ref [ 42 ] whereas the α/α’ phase is invariant with cooling rate.…”

Non- and Quasi-Equilibrium Multi-Phase Field Methods Coupled with CALPHAD Database for Rapid-Solidification Microstructural Evolution in Laser Powder Bed Additive Manufacturing Condition

“…These conditions lead to a microstructure that results in exceptional mechanical properties [2][3][4]. Many researchers have reported that the solidification microstructure obtained by LPBF was produced using cooling ranges from 10 4 K/s to 10 6 K/s and temperature gradients from 10 5 K/s to 10 7 K/s [5][6][7]. Rapid solidification is desired to design the LPBF process parameters to obtain a precise microstructure.…”

Evaluation of the corrosion resistance of an additive manufacturing steel using electrochemical techniques

“…Additive manufacturing (AM) is a process in which a part or machine element is constructed from a threedimensional model with the help of CAD-CAM software, by adding successive layers. [1][2][3][4][5][6]. Although these technologies are constantly evolving [7], there is still a limited state of the art on the effects that these materials can have under working conditions, especially corrosion.…”