Combination of Scanning Strategies and Optimization Experiments for Laser Beam Powder Bed Fusion of Ti-6Al-4V Titanium Alloys

Downskin Surface Roughness Prediction with Machine Learning for As-Built CM247LC Fabricated Via Powder Bed Fusion with a Laser Beam

Powder bed fusion with a laser beam (PBF-LB) is a widely used metal additive manufacturing method for fabricating complex three-dimensional components with a variety of metallic powders. However, metal parts fabricated by PBF-LB often present surface quality problems because of the layer-wise building process and the occurrence of partially unmelted powder particles. To reduce the surface roughness, surface post-processing is required, which incurs additional time and cost. In particular, the downskin surface generally has the worst surface roughness among the fabricated components. The rough surface reduces the lifetime and quality of the holed part owing to cracks, corrosion, and wear. In this study, for fast and efficient improvement of the downskin surface roughness of CM247LC fabricated by PBF-LB, machine learning algorithms, namely support vector regression (SVR), random forest (RF), and multilayer perceptron (MLP), were introduced to predict downskin surface roughness in the process parameter selection step. Three PBF-LB process parameters (laser power, scanning speed, and hatching distance) and the overhang angle were selected as the input variables for the machine learning models for predicting downskin surface roughness. Test samples were prepared and used for training and evaluation of the proposed machine learning algorithms, with RF showing the most promising results. Early results were confirmed when model predictions were compared to the actual measured roughness of a fabricated vane part, with average deviations of 13.7%, 4.3%, and 22.5% observed for SVR, RF, and MLP, respectively. The results showed that the proposed machine learning models could accurately predict the downskin surface roughness in the process parameter selection step without the use of any sensor, with RF showing the highest prediction accuracy.

Effect of Surface Textures and Fabrication Methods on Shear Bond Strength Between Titanium Framework and Auto-Polymerizing Acrylic Repair Resin

The aim of the study was to evaluate the effect of airborne particle abrasion (using different sizes of alumina particles) on the shear bond strength (SBS) between cast and milled titanium metal frameworks and auto-polymerizing acrylic repair resin. Forty flat cylindrical titanium disks were divided into two main divisions: cast and milled titanium. The two divisions were further subdivided into four groups based on metal surface treatment. Three particle sizes of aluminum oxide air abrasive powders (50µm, 110µm, and 250µm) were used for metal surface treatment by airborne particle abrasion. One group was the control group with no surface treatment. Auto-polymerizing acrylic repair resin was applied to all titanium disks. The specimens were subjected to SBS testing using a universal testing machine (Instron Corporation, Norwood, Massachusetts, United States). Surface evaluation was performed using a scanning electron microscope. One-way ANOVA was used for statistical analysis. The results showed a significant increase in SBS after airborne particle abrasion of both milled and cast titanium groups (p<0.001). The SBS was directly proportional to the size of the aluminum oxide particles. The milled titanium group showed higher SBS values than the cast group when the surface was not treated with alumina particles (p < 0.001) and when the surface was treated with the smaller particle sizes of 50 µm, whereas the cast group demonstrated higher SBS values than the milled group (p < 0.01) when the particle size was increased to 110 µm and 250 µm. It could be concluded that SBS between titanium metal frameworks and auto-polymerizing repair acrylic resin was directly related to the size of the alumina airborne particle abrasives. The fabrication method of the titanium framework also influenced the SBS as the untreated milled frameworks demonstrated favorable SBS values compared to the untreated cast frameworks.

Influence of Density Gradient on the Compression of Functionally Graded BCC Lattice Structure

In this paper, five grading functional gradient lattice structures with a different density perpendicular to the loading direction were proposed, and the surface morphology, deformation behavior, and compression properties of the functional gradient lattice structures prepared by selective laser melting (SLM) with Ti-6Al-4V as the building material were investigated. The results show that the characteristics of the laser energy distribution of the SLM molding process make the spherical metal powder adhere to the surface of the lattice structure struts, resulting in the actual relative density of the lattice structure being higher than the designed theoretical relative density, but the maximum error does not exceed 3.33%. With the same relative density, all lattice structures with density gradients perpendicular to the loading direction have better mechanical properties than the uniform lattice structure, in particular, the elastic modulus of LF, the yield strength of LINEAR, and the first maximum compression strength of INDEX are 28.99%, 16.77%, and 14.46% higher than that of the UNIFORM. In addition, the energy absorption per unit volume of the INDEX and LINEAR is 38.38% and 48.29% higher, respectively, than that of the UNIFORM. Fracture morphology analysis shows that the fracture morphology of these lattice structures shows dimples and smooth planes, indicating that the lattice structure exhibits a mixed brittle and ductile failure mechanism under compressive loading. Finite element analysis results show that when the loading direction is perpendicular to the density gradient-forming direction, the higher density part of the lattice structure is the main bearing part, and the greater the density difference between the two ends of the lattice structure, the greater the elastic modulus.

Experimental Research on Variable Parameter Forming Process for Forming Specimen of TC4 Titanium Alloy by Selective Laser Melting

To optimize the microstructure and properties of TC4 specimens formed by selective laser melting (SLM), the test program of formed specimens by the variable parameter forming process (VPFP) was designed based on the quantitative parameter forming process (QPFP). The purpose of this study is to explore the influence of the VPFP on the surface morphology, tensile properties, and microstructure of the specimens. The test results show that the surface morphology and tensile properties of the specimens were better formed by the VPFP. The internal holes of the specimens formed by the VPFP were small in volume and occupied a relatively small proportion, and the density could reach 99.7%. When the laser power was 300 W-260 W and equally divided into six hierarchies, the tensile strength could reach 1185.214 MPa by VPFP, but the elongation had no obvious change. The number of secondary acicular martensite α' phases was decreased in the microstructure of the specimens formed with VPFP. With the superposition of the hierarchy, the length of the primary acicular martensite α' phase became shorter, the width became larger, and the width of the columnar crystal β phase became smaller. The VPFP is used to change the inherent method of forming specimens with the same parameters, which provides a new idea for SLM-forming structures; the test provides data and yields a theoretical research basis for forming the specimens process method.