An Experimental Investigation of Residual Stress Development during Selective Laser Melting of Ti-6Al-4V

Selective laser melting (SLM) is an additive manufacturing (AM) process that gives rise to large thermal gradients and rapid cooling rates that lead to the development of undesirable residual stress and distortion. In this work, a number of different techniques (i.e., x-ray-diffraction, hole-drilling, layer-removal, and contour) were utilized to establish the effect of process parameters on residual stress development during SLM of Ti-6Al-4V. The measurements indicated that higher laser power, slower scan speed, smaller stripe width, reduced substrate overhang, and reduced build plan area each reduce the level of residual stress. In addition, the correlation between microstructure, crystallographic texture, and residual stress were investigated using electron backscatter diffraction (EBSD) and backscatter electron (BSE) imaging. The experimental results from this work provide a quantitative foundation for future simulations of residual stress evolution during SLM and provide an informed understanding of residual stress development that can be used for process planning and improvement

Characterizing and Modeling the Precursors to Coarse Grain Formation during Beta-Annealing of Ti-6Al-4V

Coarse prior β grains exceeding 3 mm in diameter have been sporadically observed following β annealing of α+β forged titanium alloys. Recent work has shown that the occurrence of coarse grains may be due in part to the stabilization of a {001} texture during hot working that was further enhanced in intensity at the expense of other texture components during the early stages of β annealing. With the majority of the material comprised of low misorientation subgrains of a single texture component, the nuclei for coarse grains was the minority fraction of grains that were highly misoriented, and therefore had boundaries with higher energy and mobility, compared to the average grain. In this work, Ti-6Al-4V bar was side-pressed to various reductions in the α+β phase field to further investigate the role of texture and the effects of strain, strain-path, and deformation heating on the propensity to form abnormally large grains during β-annealing. The experiments were interpreted in the context of a continuum finite element model and viscoplastic self-consistent crystal plasticity simulations. Based on the results from experiment and modeling, we make recommendations with respect to the α+β forging process to avoid the occurrence of excessively coarse β grains

Characterizing and Modeling the Precursors to Coarse Grain Formation during Beta-Annealing of Ti-6Al-4V

Coarse prior β grains exceeding 3 mm in diameter have been sporadically observed following β annealing of α+β forged titanium alloys. Recent work has shown that the occurrence of coarse grains may be due in part to the stabilization of a {001} texture during hot working that was further enhanced in intensity at the expense of other texture components during the early stages of β annealing. With the majority of the material comprised of low misorientation subgrains of a single texture component, the nuclei for coarse grains was the minority fraction of grains that were highly misoriented, and therefore had boundaries with higher energy and mobility, compared to the average grain. In this work, Ti-6Al-4V bar was side-pressed to various reductions in the α+β phase field to further investigate the role of texture and the effects of strain, strain-path, and deformation heating on the propensity to form abnormally large grains during β-annealing. The experiments were interpreted in the context of a continuum finite element model and viscoplastic self-consistent crystal plasticity simulations. Based on the results from experiment and modeling, we make recommendations with respect to the α+β forging process to avoid the occurrence of excessively coarse β grains

Transient Plastic Flow and Phase Dissolution During Hot Compression of α/β Titanium Alloys

International audienc

The Application of Differential Scanning Calorimetry to Investigate Precipitation Behavior in Nickel-Base Superalloys Under Continuous Cooling and Heating Conditions

A suite of experimental tools and fast-acting, numerical-simulation techniques was used to quantify the precipitation behavior of three nickel-base superalloys: IN-100, LSHR, and 718. Experimental methods comprised differential scanning calorimetry (DSC) to establish the specific heat as a function of temperature and selected direct-resistance heating trials (using a Gleeble® machine) to obtain samples for microstructural analysis. For the DSC experiments, each alloy was cooled at a prescribed constant rate (between 5 and 20 K/min) after an initial soak/equilibration in the high-temperature, single-phase (supersolvus) temperature regime. On-heating DSC trials beginning at ambient temperature were also performed on alloy 718 in three different starting conditions: super-δ-solvus solution treated and water quenched (denoted as ST), solution treated and aged (STA), and solution treated and overaged (STOA). DSC results, revealing the thermal signatures associated with the kinetics of precipitation of γ′ (IN-100, LSHR) or γ′ and γ″ (718), were interpreted using a previously-developed fast-acting routine that treats concurrent nucleation, growth, coarsening, and dissolution. For these simulations, special attention was paid to various thermo-kinetic input parameters including equilibrium solvus-approach curves, bulk free energies of transformation, matrix-precipitate interface energies, and effective diffusivities. For the γ-γ′ superalloys (IN-100 and LSHR), estimates of precipitate volume fraction as a function of temperature from the specific-heat data revealed semi-quantitative agreement with simulation predictions. For the γ-γ′-γ″ superalloy (718), simulation predictions of precipitate volume fractions were converted to specific heat as a function of temperature and showed semi-quantitative agreement with the direct measurements