Shanghai University uses CT to diagnose 3D printed metals-revealing the formation mechanism of internal pore defects _ p

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Original Title: Shanghai University Uses CT to Diagnose 3D Printed Metals — Revealing the Formation Mechanism of Internal Hole Defects SLM technology is widely used in the manufacture of titanium alloys, among which Ti6Al4V alloy printed by SLM is widely used in biomedical and aerospace applications. However, the existence of pores in the printing process has become a significant obstacle to the further expansion of the application of this technology. X-ray computed tomography (XCT) technology can be used to analyze and observe the spatial distribution and morphological characteristics of pore defects. The existence of these pores will greatly affect the tensile and fatigue properties of Ti6Al4V alloy samples manufactured by SLM printing. Therefore, the formation of defects and its formation mechanism were discussed. The results show that there are two main types of porosity defects, one is the unfused porosity caused by low volumetric energy density (VED), and the other is the porosity formed by keyhole effect caused by high VED. When the volumetric energy density (VED) is 58.88 J/mmexp (3), the density of the alloy can reach 99.995%. In addition, the quantitative characteristics of pore defects, such as size distribution, morphology and orientation, were observed by XCT to reveal their formation mechanism. At the same time, the tensile properties of Ti6Al4V printed by SLM were also evaluated under different VED values, and the relationship between the tensile properties and the pore changes was studied. In general, the tensile strength and toughness of the alloys decrease significantly with increasing porosity. The observation and analysis of the fracture surface based on XCT and SEM techniques show that the undissolved defects and their porosity are the fundamental causes of the early fracture. It can be concluded that XCT technology can provide a direct and accurate check for laser additive manufacturing technology to perform comprehensive quality control. Figure 1 Graphical abstract of research results As we all know, additive manufacturing technology (AM, also known as 3D printing) can produce metal parts with complex shapes, shorten the manufacturing cycle of products and improve the utilization of materials. Among many 3D printing metals, SLM technology is widely used in aerospace,titanium round bar, automotive and medical industries. Currently, SLMs are widely used in the manufacture of various metal parts, metal matrix composites, refractory materials, and even ceramic materials. Figure 2 a Schematic diagram of an industrial CXT and B Main components of a 3D XCT Expand the full text Although 3D printing technology has made some remarkable achievements in scientific research and application practice, the physical mechanism behind it and the mechanism of metallurgical process are still not fully and completely understood. This is due to the complex light-powder interaction in the printing process. Rapid melting and solidification processes,6al4v titanium bar, complex phase transformation processes, microstructure evolution, etc. SLM technology is a way to complete the manufacturing of solid parts by using laser beam and electron beam to melt point by point, scan line by line, and melt layer by layer. At this time, the metal powder undergoes a cyclic temperature rise, rapid cooling and solidification process. Therefore, the formation of pore defects is influenced by factors such as laser parameters, powder characteristics and temperature gradients, and the ambient atmosphere. In the products printed by SLM, different types of defects can be found, such as spheroidization, cracks and pores. In addition, ti6al4v ,nickel titanium wire, the existence of pores will significantly affect the performance of SLM products, such as material rigidity, hardness, tensile properties and fatigue properties. During the printing process, the spatial distribution and morphology of pores in the product will cause the anisotropy of the product properties and mechanical property.
Fig. 3 Typical 3D volume of Ti6Al4V alloy printed by SLM and the morphological characteristics of pores obtained by different VED Fully understanding the formation mechanism of defects and metallurgical defects is an important task to improve the performance of products in the application and research of SLM printing, which requires continuous breakthroughs in experimental research and theory. Fig. 4 Results of XCT reconstruction of Ti6Al4V alloy powder Illustration: a 3D surface analysis of Ti6Al4V alloy powder by XCT; B cross-section analysis results between powder particles; C powder size distribution obtained by XCT analysis of Ti6Al4V powder; d measured sphericity results of the powder The formation and mechanism of pores during SLM printing have been reported in many literatures. At present, most of the research on porosity defects is to continue to destroy the sample, and then dissect and analyze it to characterize it. None of these studies provided sufficient information about the true shape and specific distribution of the stomata. The quantitative analysis of pores and the systematic study of three-dimensional morphology are still relatively scarce. Fig. 5 Morphological characteristics and formation mechanism of pores at different VED Nowadays, the use of non-destructive means, such as ultrasonic, infrared, eddy current testing and X-ray, for the inspection and analysis of SLM products is gradually increasing. Among the numerous non-destructive inspection technologies, XCT technology can obtain high-resolution three-dimensional images. The results of XCT can ensure that the internal features of 3D printing products can be reconstructed in 3D, such as pores, cracks, spheroidization, inclusions and so on, which can be expressed in three-dimensional form. The quantification of pores, the volume fraction of morphological features, and the size distribution of pores can be calculated by 3D image analysis. Through XCT analysis, it is found that the scanning speed and spot size can image the formation of pores in Ti6Al4V alloy printed by SLM. When printing Co-Cr alloy samples with DMLS, it is found that different methods can be used to measure the accuracy of porosity of pores, and the pore size and distribution of pores are studied by XCT system. Fig. 6 is a result of an image of a designated area of a stretched sample reconstructed by XCT Illustration: (a-c) before tension, (d-f) after fracture; (a, d) VED = 32.7 ，（ｂ，ｅ）ＶＥＤ＝５８.８ ；（ｃ，ｆ）ＶＥＤ＝１３２.４ ; (VED in units of J/mm exp (3)), and the red region indicates the region where the break will occur. Researchers at Shanghai University studied the effects of SLM process parameters on the printing of Ti6Al4V alloy to reveal the formation mechanism of typical defects under different parameters. These typical defects are studied by XCT and reveal the formation mechanism of defects, the basic morphological characteristics of pores, the basic topological characteristics, and the spatial distribution of pores in the sample. Finally, the effects of different VEDs on the strength and toughness of the fabricated samples were also systematically investigated. Fig. 7 shows the relationship between the calculated sphericity distribution and the pore equivalent diameter of the sample Main conclusions: Through research, it is found that several different types of pores are obtained under different process parameters and different VED, which is the key to determine the formation of defect types in Ti6Al4V prepared by SLM technology. According to the type of pore formation, the process window of Ti6Al4V prepared by SLM process can be divided into two categories. Irregularly shaped pores are formed at low VED. While at high VED, round pores are formed due to the keyhole effect and gas trapping. The pore diameters observed were mostly less than 10 microns and more than 100 microns. The morphology of the stomata varies from round to oval and irregular shapes, which is mainly caused by too high or insufficient energy density. For energy-insufficient VEDs, the pores are irregular and generally larger than 100 microns in size, and are often perpendicular to the fabrication direction and through a single fabrication layer. Conversely, too much energy results in a round shape with a size of less than 60 microns.
The effects of processing parameters on the densification and mechanical properties were analyzed, and the effects of defect characteristics and microstructure on the mechanical properties of TC4 alloy SLM formed parts were studied. Source: Study of pore defect and mechanical properties in selective laser melted Ti6Al4V alloy based on X-ray computed tomography，Materials Science and Engineering: A Available online 8 August 2020,titanium plate gr7, 139981, https://doi.org/10.1016/j.msea.2020.139981 back to Sohu, see more Responsible Editor:. yunchtitanium.com