In situ investigation of phase transformations in Ti-6Al-4V under additive manufacturing conditions combining laser melting and high-speed micro-X-ray diffraction

Abstract

We present combined in situ X-ray diffraction and high-speed imaging to monitor the phase evolution upon cyclic rapid laser heating and cooling mimicking the direct energy deposition of Ti-6Al-4V in real time. Additive manufacturing of the industrially relevant alloy Ti-6Al-4V is known to create a multitude of phases and microstructures depending on processing technology and parameters. Current setups are limited by an averaged measurement through the solid and liquid parts. In this work the combination of a micro-focused intense X-ray beam, a fast detector and unidirectional cooling provide the spatial and temporal resolution to separate contributions from solid and liquid phases in limited volumes. Upon rapid heating and cooling, the β ↔ α′ phase transformation is observed repeatedly. At room temperature, single phase α′ is observed. Secondary β-formation upon formation of α′ is attributed to V partitioning to the β-phase leading to temporary stabilization. Lattice strains in the α′-phase are found to be sensitive to the α′ → β phase transformation. Based on lattice strain of the β-phase, the martensite start temperature is estimated at 923 K in these experiments. Off-axis high speed imaging confirms a technically relevant solidification front velocity and cooling rate of 10.3 mm/s and 4500 K/s, respectively.

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

“…First, it does not take into account undercooling that might lower either the solidification or solid state transition temperatures. As reported in another study 47 , undercooling lowers the melting temperature by ∼40 K, and the error introduced by a change of this magnitude is accounted for by the error bars. Second, it does not explicitly take into account the instantaneous thermal gradients within the probed volume.…”

“…Similarly, thermocouples, as used in some studies, only measure temperatures relatively far from the hottest regions during this process and are inherently slow 4,46 . A number of recent studies have employed in situ X-ray probes to study the subsurface dynamics during AM processing, but they have either focused on imaging, or have been conducted in different spatial or temporal domains than that considered here 15,47,48 . Here, we demonstrate the use of in situ X-ray diffraction to monitor the subsurface structural evolution of the crystalline phases in Ti-64 as it cools below the solidification temperature to the β-transus following laser melting.…”

State of the Art in Directed Energy Deposition: From Additive Manufacturing to Materials Design

“…Recent developments in quality monitoring includes high-energy X-ray synchrotron studies of DED. These encompass: High energy synchrotron X-ray source and high speed imaging camera used in tandem to detect the in situ melt pool geometries and deduce the phase transformations of Ti-6Al-4V [108]; a piezo driven powder delivery in conjunction with a laser heat source to investigate the powder-melt pool interaction during printing of Ti-6Al-4V [109]. These studies provide insights into the DED process physics, but are still far from mimicking all the components in a real DED system.…”

“…Detects phase transformations and melt pool dynamics [73,108,109] Repetitive process controller Used to optimize layer height during the process [119] 6. Determination of Optimal Process Parameters for Laser Based Powder-Fed DED DED is an emerging field in the area of metal AM, and our goal was to create efficient process maps which provide a holistic picture of the DED process parameters.…”

Revisiting fundamental welding concepts to improve additive manufacturing: From theory to practice

“…Therefore, the heat introduced to deposit each layer generates successive heat affected zones on previously deposited layers, in a process similar to multipass fusion welding [7][8][9][10]. As in welding, additive manufactured parts experience non-equilibrium solidification [11][12][13], due to fast cooling rates observed. The effect of multiple thermal cycles over these non-equilibrium structures may promote the formation of new phases and precipitates if the permanence time at a specific temperature for the solid-state transformation to occur is reached.…”