Effect of processing parameters on pore structures, grain features, and mechanical properties in Ti-6Al-4V by laser powder bed fusion

doi:10.1016/j.addma.2022.102915

Additive Manufacturing

Volume 56, August 2022, 102915

https://doi.org/10.1016/j.addma.2022.102915 Get rights and content

Abstract

In this study, the process-structure-property linkages in laser powder bed fusion of Ti-6Al-4V were investigated experimentally and quantified via correlation analysis. Forty-two different combinations of laser power and scan speed were examined to probe lack-of-fusion, keyholing, and beading-up defect regions in addition to dense regions of the processing space. The resultant structures of the fabricated components were characterized through X-ray computed tomography for visualization and characterization of internal pores and optical metallography to examine prior-β grain features. Mechanical properties were assessed via uniaxial tension tests and Vickers microhardness measurements. Correlation analyses, used to quantify the statistical bivariate relations between feature pairs describing processing conditions, microstructural features, and mechanical properties, revealed the relationship between processing parameters and defect pore morphology, which in turn correlated with mechanical properties. Additionally, the work expands on process maps for avoiding porosity by defining correlations between processing conditions and resultant properties, providing a means for tailoring properties and structures via processing in laser powder bed fusion additive manufactured materials.

Graphical Abstract

Introduction

Ti-6Al-4V is widely studied in additive manufacturing (AM), as it has broad applicability to aerospace [1], [2], biomedical [3], [4], marine, chemical and energy, and defense industries, due to its high mechanical strength, low density, corrosion resistance, and biocompatibility [1], [2], [3], [4], [5], [6]. Ti-6Al-4V processed via AM is often reported to have higher strength but lower ductility than in traditionally processed conditions due to differences in microstructural features including α-lath size, prior-β grain size/morphology, and defects [7], [8]. The present study focuses on identifying relationships among different processing conditions, microstructures and defects, and mechanical properties in Ti-6Al-4V fabricated via laser power bed fusion (L-PBF) AM.

L-PBF AM is a layer-by-layer process to fabricate three-dimensional components, where for each layer, the laser scans and melts powder in the pattern of the current 2D slice of the 3D component [5], [6]. Numerous parameters can have an impact on the structure, over several length scales, of the final component, including hatch spacing (the distance between parallel laser passes), h, powder layer thickness, t, laser power, P, and laser scanning velocity, v, [9], as well as other variables (e.g., parameters describing the beam source) that are closely related to the laser energy absorptivity [10], [11]. These parameters of P, v, h, and t are typically reported in L-PBF studies that aim to correlate processing with structural features, such as defect morphology and grain/sub-grain features, as the variation of these parameters influences cooling rate and thermal history within components [6], [9], [12], [13].

Process maps can be used to identify combinations of parameters that result in certain features of interest. For example, Gockel and Beuth [14] introduced a process map of electron-beam power versus scan speed for electron beam wire feed AM process Ti-6Al-4V in which they defined boundaries separating different prior-β grain size and shapes based on estimated solidification modes, supporting the use of P-v process maps for microstructural control. Gordon et al. [9] extended the use of processing maps to delineate boundaries for different processing defects in L-PBF, and used synchrotron-based micro-tomography to characterize morphology of pores under a range of processing regimes, defining boundaries to differentiate lack of fusion (LOF) and keyholing defects [15].

The present study aimed to determine the four processing regimes that result in dense builds, and builds that contain defects due to LOF, keyholing, and beading-up. These regimes are accessed by different combinations of processing parameters that affect melt pool size and shape during processing [13]. The formation of both LOF and keyholing pores is directly related to the laser energy used, where irregularly shaped LOF defects appear with insufficient laser energy density and/or insufficient overlap between adjacent melt pools [13]. In contrast, keyhole pores appear when the energy density is high, resulting in a deep melt pool in which the molten metal may vaporize and leave behind trapped gas pores of a relatively round shape [12]. While LOF and keyholing pores are typically discussed in terms of melt pool sizes, beading-up or balling is attributed to melt pool shape: if the melt pool is sufficiently elongated along the direction of laser scanning, then a Rayleigh-Plateau instability of the metallic fluid occurs due to surface tension, such that the long melt pool separates into multiple melt pools, resulting in bumps in the solidified material [16], [17], [18].

A total of 42 processing parameter sets, guided by various studies in literature [13], [15], [19], were designed to probe dense, keyholing, lack of fusion, and beading-up regimes in fabricated tensile specimens, to generate samples with varying defect structures, microstructures, and mechanical properties. To delineate different regions, boundaries can be defined based on two metrics: (1) laser energy density, which corresponds to melt pool size, and (2) laser power multiplied by scanning velocity, which corresponds to melt pool shape. Mathematical details on the process map design are given in Section 2.1.

The present study focuses on developing process maps for L-PBF Ti-6Al-4V by experimentally identifying the impact of processing parameters on: (a) microstructure, in terms of prior-β grain size and morphology; (b) processing-related defects, in terms of frequency, volume fraction, and morphology; and (c) mechanical properties. The specimens were characterized using non-destructive X-ray computed tomography to obtain size, distribution, and morphological information on internal pores. Uniaxial tensile tests assessed ultimate tensile strength (UTS) and elongation to failure of all samples. The microstructures of samples were imaged to characterize prior-β grain sizes and morphologies, and their Vickers microhardness values were measured. These data were used to construct processing-structure-property maps, and correlation analyses were performed to determine which processing descriptors, microstructural features, and mechanical properties were most strongly correlated in both dense samples and those with pores. Thus, in addition to providing comprehensive maps that show the effect of varying processing parameters on microstructure, including porosity, and mechanical properties, the statistical analyses presented in this study uncover the linear and nonlinear correlations among processing features, microstructural features, and mechanical properties.

Section snippets

Process map design

To probe a range of processing regimes intended to produce a range of microstructures as well as dense samples and those with keyholing, lack of fusion, and beading-up defects, a process map was designed in which laser power and laser scan speed were varied for a constant layer thickness and hatch spacing. These four parameters are important to consider in processing maps because they each influence the total energy imparted by the laser to the powder bed. Specifically, the volumetric energy

Processing defects

To characterize the observed processing defects, the average pore equivalent diameter, total projected area of pores within the gauge region, total pore quantities, and average pores sphericity were computed from the XCT scans for each processing condition studied. Characteristic defect structures, as visualized via X-ray computed tomography, are shown in Fig. 3. The pore sphericity was measured via Hakon Wadell sphericity, $ψ_{p}$ [45], defined as: $ψ_{p} = \frac{A_{s}}{A_{p}} = \frac{π^{\frac{1}{3}} {(6 V_{p})}^{\frac{2}{3}}}{A_{p}}$ where $A_{p}$ and $V_{p}$ are the measured

Conclusions

The effect of a wide range of processing parameters on microstructure, defect structure, and mechanical properties for L-PBF Ti-6Al-4V were studied experimentally via optical microscopy, XCT, and mechanical testing. Two sample types were classified – dense (samples with less than 1% porosity) and porous (samples with more than 1% porosity) – and statistical correlation analyses were performed on each sample type to quantitatively investigate both linear and nonlinear relationships among

CRediT authorship contribution statement

Timothy W. Simpson: Writing – review & editing. Allison M. Beese: Writing – review & editing, Supervision, Resources, Project administration, Methodology, Funding acquisition, Conceptualization. Lu Yin: Investigation, Formal analysis. Qixiang Luo: Writing – original draft, Visualization, Validation, Software, Methodology, Investigation, Formal analysis, Data curation.

Acknowledgments

The financial support provided by a Pennsylvania State University College of Engineering Multidisciplinary Seed Grant is gratefully acknowledged. The samples were fabricated using 3D System ProX-320 laser PBF system in Penn State’s Center for Innovative Materials Processing (CIMP-3D). The authors are grateful for the help of the Center for Quantitative Imaging (CQI) at Penn State for XCT scanning, for Taylor P. Rosen’s assistance in Vickers microhardness measurements, and for Tianming Zhao’

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Research paperEffect of processing parameters on pore structures, grain features, and mechanical properties in Ti-6Al-4V by laser powder bed fusion

Abstract

Graphical Abstract

Introduction

Section snippets

Process map design

Processing defects

Conclusions

CRediT authorship contribution statement

Acknowledgments

Mater. Sci. Eng. A

Prog. Mater. Sci.

Addit. Manuf.

Addit. Manuf.

Scr. Mater.

J. Alloy. Compd.

Addit. Manuf.

Mater. Sci. Eng. A.

Acta Mater.

Opt. Laser Technol.

Mater. Sci. Eng. A.

Acta Mater.

Mater. Sci. Eng. A

Turk. J. Emerg. Med.

Acta Met.

Titanium - A Technical Guide

Biomedical titanium alloys and their additive manufacturing

Rare Met

Additive fabrication technologies applied to medicine and health care: a review

Int. J. Adv. Manuf. Technol.

Additive manufacturing of Ti6Al4V alloy: a review

Mater. Des.

Defect structure process maps for laser powder bed fusion additive manufacturing

Addit. Manuf.

Lasers in additive manufacturing: a review

Int. J. Precis. Eng. Manuf. - Green. Technol.

Overview of laser absorptivity measurement techniques for material processing

Lasers Manuf. Mater. Process.

Welding Metallurgy

Synchrotron-based X-ray microtomography characterization of the effect of processing variables on porosity formation in laser power-bed additive manufacturing of Ti-6Al-4V

JOM

Research paper
Effect of processing parameters on pore structures, grain features, and mechanical properties in Ti-6Al-4V by laser powder bed fusion