Additive Manufacturing: Processing, Microstructure and Material Properties of Titanium-based Materials: Session I
Sponsored by: TMS Titanium Committee
Program Organizers: Ulf Ackelid, Freemelt AB; Ola Harrysson, North Carolina State University; Peeyush Nandwana, Oak Ridge National Laboratory; Rongpei Shi, Harbin Institute of Technology
Monday 8:00 AM
October 18, 2021
Room: A120
Location: Greater Columbus Convention Center
Session Chair: Ola Harrysson, North Carolina State University
8:00 AM
A Machine Learning Model to Predict Tensile Properties of Ti6Al4V Parts Prepared by Selective Laser Melting with Hot Isostatic Pressing: Zhaotong Yang1; Mei Yang1; Richard Sisson1; Yanhua Li1; Jianyu Liang1; 1Worcester Polytechnic Institute
A machine learning model is established to predict the influence of Hot isostatic pressing (HIP) parameters on the tensile properties of Ti-6Al-4V parts prepared by selective laser melting (SLM). The database was established by collecting published reports on HIP treatment of SLM Ti6Al4V from 2010 to 2019. Using the established model, it is possible to prescribe HIP parameters and predict properties after HIP for SLM Ti-6Al-4V parts with high confidence. It is found that the YS and UTS are sensitive to the HIP parameters including temperature and holding time. The initial YS and UTS of as-printed parts are weaker influencing factors for YS and UTS of final parts. The model suggests that a HIP process with holding temperature lower than 970°C and time around 2 hours is desirable for SLMed Ti-6Al-4V. However, the prediction of strain to failure shows lower accuracy because of the variation of defects among different samples.
8:20 AM
An Infill Strategy for Eliminating Local Hot Spots in Ti64 Laser Powder Bed Fusion: Evan Diewald1; Christian Gobert1; Jack Beuth1; 1Carnegie Mellon University
In laser powder bed fusion (LPBF) porosity and inconsistent microstructure can be related to erratic thermal conditions caused, in part, by suboptimal infill scanning strategies. This talk presents an approach for reducing heat buildup in LPBF that can be implemented within most commercial machines. The “Powder Moat” strategy, where a thin wall is built outside the boundaries of the intended part, effectively eliminates in-plane hotspots by inducing a predictable delay after each raster. It also greatly reduces local melt pool geometric anomalies at raster ends. A semi-analytical model is used to generate process maps of delay times and moat thicknesses as a function of laser power and velocity for Ti64. The approach is validated through high speed imaging of melt pools in small portions of builds and through conventional infrared imaging of entire builds. This strategy allows for effective process planning based on melt pool dimensions derived from single beads.
8:40 AM
Effect of Powder Feedstock Size on the Characteristics of Ti6Al4V Lightweight Features from Laser Powder Bed Fusion Additive Manufacturing: An Experimental Study: Sayed Saghaian1; Jonah Hermes1; Li Yang1; 1University of Louisville
Due to the small characteristic dimensions, lightweight features such as thin walls and thin struts exhibit higher sensitivity to the powder feedstock characteristics. In this work, an experimental study was carried out to evaluate the effect of powder particle size on various characteristics of the lightweight features made of Ti6Al4V using laser powder bed fusion (L-PBF) process. Three different powder feedstock with different particle size distribution characteristics were used, and both thin wall and thin strut features of various characteristic dimensions were fabricated under different beam energy and scan speed combinations. Geometrical characteristics (e.g. dimension, surface roughness) and material characteristics (e.g. microstructure, porosity) of the fabricated features were analyzed systematically. The results aim to provide a preliminary knowledge base for the quality characteristics of Ti6Al4V lightweight features made by L-PBF and their dependency on the characteristics of the powder bed, which contributes to the design for L-PBF lightweight structure theory.
9:00 AM Invited
Processing to Microstructure to Properties in Titanium: Anthony Rollett1; 1Carnegie Mellon University
A brief overview will be given for 3D printing of Ti-6Al-4V. Adaptation of printing parameters in both electron beam and laser powder bed to non-standard powders has been demonstrated, including the use of strongly non-spherical particles as, e.g., in HDH powder. The underlying approach is to identify a process window i.e., interdependent parameter ranges in power, speed, hatch etc. that result in >99% density. Lack-of-fusion provides one limit based on melt pool overlap. Synchrotron-based high speed imaging has revealed the prevalence of keyholes and quantified their extent including, crucially, the boundary for keyhole porosity formation. Microstructures vary between martensitic and basketweave; here again, high speed synchrotron diffraction experiments help to quantify many details. Laser hot wire deposition with millimeter-scale tracks produces banded microstructures that arise from locally long dwell at the transus and consequent variations in properties, all of which can be modeled. Textures are columnar and therefore anisotropic.
9:40 AM
Effects of Process Parameters on Fatigue Behavior and Defect Characteristics in LPBF Ti-6Al-4V: Austin Ngo1; David Scannapieco1; Hunter Taylor2; Ryan Wicker2; Joseph Pauza3; Anthony Rollett3; John Lewandowski1; 1Case Western Reserve University; 2University of Texas at El Paso; 3Carnegie Mellon University
Four-point bending fatigue testing was conducted on machined and polished LPBF Ti-6Al-4V mechanical testing specimens built with varying process parameters. Optimized and sub-optimal process parameter sets were selected to fabricate baseline builds and defect-induced builds, respectively. S-N fatigue data was generated for each process parameter set, and specimen fracture surfaces were imaged using OM and SEM. Fractographic analyses consisted of quantifying all defects on the fracture surface, identifying ‘killer’ defects responsible for fatigue failure, and estimating fracture toughness from the crack length at catastrophe. Fracture surface height profiles were generated via laser scanning and compared to visual analyses of crack progression. Different types of process defects (i.e. lack of fusion, keyhole) were found to be more prevalent based on particular process parameter sets, which further influenced S-N curves. The effects of process parameters on defects and resulting S-N fatigue will be discussed in the context of a Kitagawa-Murakami-type approach.
10:00 AM Break
10:20 AM
Fatigue Fracture Surface Defect Quantification for LPBF Additively Manufactured Ti-6Al-4V: David Scannapieco1; Austin Ngo1; Collin Sharpe1; Hunter Taylor1; Ryan Wicker1; Joseph Pauza1; Anthony Rollett1; John Lewandowski1; 1Case Western Reserve University
Keyhole, lack of fusion (LoF), and the process window are AM parameter regimes that form unique defects in as-built Ti-6Al-4V. The impact of each process regime’s defects on S-N fatigue and fracture surface morphology has been quantified by measuring size, shape, and orientation of every defect on multiple fatigue fracture surfaces. This novel dataset exceeds 10,000 defects and enables comparison between the effects of different process regimes on S-N and fracture surface characteristics. This work shows the LoF regime to exhibit the largest defects, often interconnected, and highest defect density. Extreme value analysis (EVA) was used to evaluate the size and probability of critical defects on the fracture surfaces. The fracture surfaces revealed an increase in defect density on going from the fatigue to the overload region, likely due to an increased plastic zone size and sampling volume. The implications of these results on S-N fatigue behavior will be discussed.
10:40 AM
Mechanical Strength and Fatigue Performance of Laser Powder Bed Fusion Processed Hydride-dehydride Ti-6Al-4V Powders: Mohammadreza Asherloo1; Melody Delpazir1; Ziheng Wu2; Muktesh Paliwal3; Anthony Rollett2; Amir Mostafaei1; 1Illinois Institute of Technology; 2Carnegie Mellon University; 3Kymera International - Reading Alloys
Hydride-dehydride (HDH) Ti-6Al-4V with non-spherical powder morphology is not widely used in laser powder bed fusion (L-PBF). The effect of non-spherical particles on powder flowability, packing density, and dynamics of laser-powder interaction should be considered to achieve near-fully dense, mechanically sound parts. HDH Ti-6Al-4V powder with size distribution of 50-120 μm is L-PBF processed and optimized processing parameters are used to fabricate cuboids and mechanical testing specimens with a density greater than 99.8 %. Hot isostatic pressing (HIP) is applied on as-built specimens to remove residual porosity. Microstructural observations and phase analyses show the formation of columnar β grains with acicular α/α’ phases in as-built condition, while the HIP results in partial decomposition of α’ to α+β inside the prior β grains. Results such as mechanical strength, hardness and fatigue life indicate that the cost-effective, non-spherical HDH powders used in this study are a viable raw material for use in the L-PBF process.