The macroscopic mechanical performance of additive manufactured structures is essential for the design and application of multiscale microlattice structure. Performance is affected by microstructure and geometrical imperfection, which are strongly influenced by the size of the struts in selective laser melting (SLM) lattice structures. In this paper, the effect of size on microstructure, geometrical imperfection, and mechanical properties was systemically studied by conducting experimental tests. A series of AlSi10Mg rod-shaped samples with various diameters were fabricated using SLM. The uniaxial tensile test results show that with the decrease in build diameter, strength and Young's modulus of strut decreased by 30% more than the stable state. The main reasons for this degradation were investigated through microscopic observation and micro X-ray computed tomography (μ-CT). In contrast with large-sized strut, the inherent porosity (1.87%) and section geometrical deviation (3%) of ponysize strut is greater because of the effect of thermal transform and hydrogen evolution, and the grain size is 0.5 μm. The discrepancy in microstructure, geometrical imperfection, and mechanical properties induced by size effect should be considered for the design and evaluation of SLM-fabricated complex structures.

Study of Size Effect on Microstructure and Mechanical Properties of AlSi10Mg Samples Made by Selective Laser Melting

This study is focused on the 3D printing of NiTi shape memory alloy (SMA) in cuboidal shaped samples via the Laser Powder Bed Fusion (L-PBF) process using a dissimilar material build plate of 316L stainless steel. Four processing parameters were investigated at three levels for the optimization of the process. Those parameters were the laser beam power, scanning speed, laser beam spot size and hatch spacing. The produced samples exhibited varying levels of bonding with the substrate. Compression testing, Archimedes density measurement and EDX chemical composition were carried out. A design of experiment model was developed relating the input process parameters to the output properties. The effect of the calculated volumetric energy density and the sample melt-pool temperatures on the output measures were also investigated for the strong bonding and weldability with the substrate metal.

Laser beam powder bed fusion of nitinol shape memory alloy (SMA)

A machine learning guided investigation of quality repeatability in metal laser powder bed fusion additive manufacturing

Despite the recent progress in additive manufacturing (AM) process and technology, challenges in the repeatability and reproducibility of AM parts still hinders the adoption of this technique in many industries. This is particularly difficult when a part is qualified on a particular part on a certain machine using optimised parameters. If a manufacturer wishes to expand production to multiple machines, the ability to translate these optimised parameters to different machines much be understood. In this study, four different metal L-PBF printers were used to produce 316L tensile testing samples using the same processing parameters and metal powder supplied from a single batch from the same supplier. In addition to the analysis of the correlation between the input parameters and the output measures, this study reports that despite the same set process parameters, there is significant variations were found in the mechanical performance and properties of the AM samples produced on the different L-PBF metal additive manufacturing machines. For the range of the input processing parameters and the resulting input volumetric energy density applied of 21–37 J/mm3, values of (4–42)%, (200–716) MPa, and (52–214) GPa were obtained for the elongation, ultimate tensile strength and elastic modulus on additively manufactured 316L samples respectively.

Comparison of the porosity and mechanical performance of 316L stainless steel manufactured on different laser powder bed fusion metal additive manufacturing machines

Porous metal lattice structures have a very high potential in biomedical applications, setting as innovative new generation prosthetic devices Laser powder bed fusion (L-PBF) is one of the most widely used additive manufacturing (AM) techniques involved in the production of Ti6Al4V lattice structures The mechanical and failure behavior of lattice structures is strongly affected by geometrical imperfections and defects occurring during L-PBF process Due to the influence of multiple process parameters and to their combined effect, the mechanical properties of these structures are not yet properly understood Despite the major commitment to characterize and better comprehend lattice structures, little attention has been paid to the impact that single struts have on the overall lattice properties In this work, the authors have investigated the tensile strength and fatigue behavior of thin L-PBF Ti6Al4V lattice struts at different building orientations (0°, 15°, 45°, and 90°) This investigation has been focused on the effect that microstructural defects (particularly porosity) and actual surface geometry (including surface texture and geometrical errors such as varying cross-section shape and size) have on the mechanical performances of the struts in relation to their building direction The results have shown that there is a tendency, particularly for low printing angles, of fatigue life to decrease with decreasing of the building angle This is mainly due to the surge in surface texture and loss in cross-sectional regularity On the other hand, the monotonic tensile test results have shown a low sensitivity to these factors The strut failure behavior has been examined employing dynamic digital image correlation (DIC) of tensile tests and scanning electron imaging (SEM) of the fracture surfaces

Additively manufactured Ti-6Al-4V thin struts via laser powder bed fusion: Effect of building orientation on geometrical accuracy and mechanical properties.

Ti-6Al-4V is a popular alloy due to its high strength-to-weight ratio and excellent corrosion resistance. Many applications of additively manufactured Ti-6Al-4V using selective laser melting (SLM) have reached technology readiness. However, issues linked with metallurgical differences in parts manufactured by conventional processes and SLM persist. Very few studies have focused on relating the process parameters to the macroscopic and microscopic properties of parts with different size features. Therefore, the aim of this study was to investigate the effect of the size of features on the density, hardness, microstructural evolution, and mechanical properties of Ti-6Al-4V parts fabricated using a fixed set of parameters. It was found that there is an acceptable range of sizes that can be produced using a fixed set of parameters. Beyond a specific window, the relative density decreased. Upon decreasing the size of a cuboid from (5 × 5 × 5 mm) to (1 × 1 × 5 mm), porosity increased from 0.3% to 4.8%. Within a suitable size range, the microstructure was not significantly affected by size; however, a major change was observed outside the acceptable size window. The size of features played a significant role in the variation of mechanical properties. Under tensile loading, decreasing the gauge size, the ultimate and yield strengths deteriorated. This investigation, therefore, presents an understanding of the correlation between the feature size and process parameters in terms of the microscopic and macroscopic properties of Ti-6Al-4V parts manufactured using SLM. This study also highlights the fact that any set of optimized process parameters will only be valid within a specific size window.

The Effects of Feature Sizes in Selectively Laser Melted Ti-6Al-4V Parts on the Validity of Optimised Process Parameters.

Laser powder bed fusion (L-PBF) additive manufacturing has reached wide-scale technology readiness for various sectors. However, some challenges posed by the complex nature of the process persist. Limited studies investigated the correlation between the micro- and macroscopic properties of L-PBF AlSi10Mg parts and the features’ sizes with the build orientation in mind. Therefore, this study presents a comprehensive view on the “size effect” for samples larger than those available in the literature (up to 12 mm) on the defects, microstructure evolution, and mechanical properties in two build orientations using a fixed set of process parameters. Microstructural differences were observed between the build orientations, but no considerable difference with size change was detected. The porosity content was inversely proportional to the feature size irrespective of the build orientation, leading to an increase in ductility that was more evident in the horizontal specimens (~44%). This was attributed to an in-situ heat treatment. Although specimens oriented parallel to the build direction showed no significant size-effect in terms of the mechanical properties (hardness and tensile), anisotropy was evident. Based on the findings presented in the study and the scientific explanations discussed corroborated by thermal imaging during processing, it is concluded that although any set of ‘optimised’ process parameters will only be valid for a specific size range, the severity of the size-effect changes dynamically based on the range.

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Incorporating the Roles of Feature Size and Build Orientation in the Process-Structure-Property Relationship for Additively Manufactured Parts

Together, advanced technologies and metallic materials rapidly progress in the field of Laser Powder Bed Fusion (PBF-LB) additive manufacturing (AM). Although PBF-LB drastically shortens the design-to-manufacture time, post-processing operations on metal parts, including heat-treatments, are often labour intensive and time-consuming. Therefore, we assess the feasibility of conductingin-situ heat-treatments during PBF-LB of AlSi10Mg parts. A build plate heated at set temperatures up to 500 °C was employed while printing for 10–13 h to tune the material's microstructure. For comparison, a batch of samples was printed at 220 °C and 500 °C with a printing time of 4–6 h, which was subsequently solution heat-treated. The experimental results showed that increasing the printing time from 5 h to 11 h while printing at 220 °C resulted in an in-situ direct aging heat-treatment with an actual increment of hardness and strength. Printing at 300 °C induced a significant Al lattice relaxation and consequent in-situ stress relieving of the as-built parts. Lastly, the study shows that in-situsolution heat-treatment is possible when using build plate temperatures between 450 °C and 500 °C for 11 h, although a reduction in ductility (−36%) occurs, as compared to conventionally solution heat-treated counterparts. 

Chinmay Phutela

Papers

The Effects of Feature Sizes in Selectively Laser Melted Ti-6Al-4V Parts on the Validity of Optimised Process Parameters.

Incorporating the Roles of Feature Size and Build Orientation in the Process-Structure-Property Relationship for Additively Manufactured Parts

Tuning the microstructure and mechanical properties of AlSi10Mg alloy via in-situ heat-treatments in laser powder bed fusion