Cause of angular distortion in fusion welding: asymmetric cross-sectional profile along thickness

Angular distortion is a common problem in fusion welding, especially when it comes to thick plates. Despite the fact that various processes and influencing factors have been discussed, the cause of the angular distortion has not been clearly revealed. In this research, the asymmetry of cross-sectional profile along thickness is considered of great importance to the angular distortion. A theoretical model concerning the melting-solidification process in fusion welding was established. An expression of the angular distortion was formulated and then validated by experiments of laser welding 316L stainless steel. The results show that the asymmetric cross-sectional profile is a major contributory factor towards the angular distortion mechanism. The asymmetry of cross-section profile along thickness causes the difference between two bending moments in the lower and upper parts of the joint. This is the difference that drives the angular distortion of the welded part. Besides, the asymmetry of cross-section profile is likely to be influenced by various processes and parameters, thereby changing the angular distortion

The size effect on forming quality of Ti–6Al–4V solid struts fabricated via laser powder bed fusion

Laser powder bed fusion (LPBF) is useful for manufacturing complex structures; however, factors affecting the forming quality have not been clearly researched. This study aimed to clarify the influence of geometric characteristic size on the forming quality of solid struts. Ti–6Al–4V struts with a square section on the side length (0.4 to 1.4 mm) were fabricated with different scan speeds. Micro-computed tomography was used to detect the struts’ profile error and defect distribution. Scanning electron microscopy and light microscopy were used to characterize the samples’ microstructure. Nanoindentation tests were conducted to evaluate the mechanical properties. The experimental results illustrated that geometric characteristic size influenced the struts’ physical characteristics by affecting the cooling condition. This size effect became obvious when the geometric characteristic size and the scan speed were both relatively small. The solid struts with smaller geometric characteristic size had more obvious size error. When the geometric characteristic size was smaller than 1 mm, the nanohardness and elastic modulus increased with the increase in scan speed, and decreased with the decline of the geometric characteristic size. Therefore, a relatively high scan speed should be selected for LPBF—the manufacturing of a porous structure, whose struts have small geometric characteristic size

Design and compressive behavior of controllable irregular porous scaffolds: based on Veronoi-tessellation and for additive manufacturing

Adjustment of the mechanical properties (apparent elastic modulus and compressive strength) in porous scaffolds is important for artificial implants and bone tissue engineering. In this study, a top-down design method based on Voronoi-Tessellation was proposed. This method was successful in obtaining the porous structures with specified and functionally graded porosity. The porous specimens were prepared by selective laser melting technology. Quasi-static compressive tests were conducted as well. The experiment results revealed that the mechanical properties were affected by both porosity and irregularity. The irregularity coefficient proposed in this study can achieve good accommodation and balance of “irregularity” and “controllability”. The method proposed in this study provides an efficient approach for the bionic design and topological optimization of scaffolds

Assumption of constraining force to explain distortion in laser additive manufacturing

Distortion is a common but unrevealed problem in metal additive manufacturing, due to the rapid melting in metallurgy and the intricate thermal-mechanical processes involved. We explain the distortion mechanism and major influencing factors by assumption of constraining force, which is assumed between the added layer and substrate. The constraining force was set to act on the substrate in a static structural finite element analysis (FEA) model. The results were compared with those of a thermal-mechanical FEA model and experiments. The constraining force and the associated static structural FEA showed trends in distortion and stress distribution similar to those shown by thermal-mechanical FEA and experiments. It can be concluded that the constraining force acting on the substrate is a major contributory factor towards the distortion mechanism. The constraining force seems to be primarily related to the material properties, temperature, and cross-sectional area of the added layer

Distortion of thin-walled structure fabricated by selective laser melting based on assumption of constraining force-induced distortion

Metal additive manufacturing has shown great potential in aerospace, medical, and automobile industries; however, distortion of metal part has been an obstacle in widespread application of metal additive manufacturing. The mechanism of thin-walled structure distortion remains unrevealed. In this study, the origin of distortion of thin-walled structure was discussed, based on the previously proposed assumption of constraining force-induced distortion. The relation between the microstructure and macro-distortion has been linked via the constraining force. The influence of scan directions and structure sizes on the distortion was also studied, and the approaches to decrease the thin-walled structure were discussed. Use of the alternant scan strategy has been validated as an effective approach if the structure sizes cannot be adjusted

Mechanical properties of in-situ synthesis of Ti-Ti3Al metal composite prepared by selective laser melting

Titanium composite strengthened by Ti3Al precipitations is considered to be one of the excellent materials that is widely used in engineering. In this work, we prepared a kind of Ti-Ti3Al metallic composite by in-situ synthesis technology during the SLM (selective laser melting) process, and analyzed its microstructure, wear resistance, microhardness, and compression properties. The results showed that the Ti-Ti3Al composite, prepared by in-situ synthesis technology based on SLM, had more homogeneous Ti3Al-enhanced phase dispersion strengthening structure. The grain size of the workpiece was about 1 μm, and that of the Ti3Al particle was about 200 nm. Granular Ti3Al was precipitated after the aluminum-containing workpiece formed, with a relatively uniform distribution. Regarding the mechanical properties, the hardness (539 HV) and the wear resistance were significantly improved when compared with the Cp-Ti workpiece. The compressive strength of the workpiece increased from 886.32 MPa to 1568 MPa, and the tensile strength of the workpiece increased from 531 MPa to 567 MPa after adding aluminum. In the future, the combination of in-situ synthesis technology and SLM technology can be used to flexibly adjust the properties of Ti-based materials

Additive manufacturing of monolithic microwave dielectric ceramic filters via digital light processing

Microwave dielectric ceramics are employed in filters as electromagnetic wave propagation media. Based on additive manufacturing (AM) techniques, microwave dielectric ceramic filters with complex and precise structures can be fabricated to satisfy filtering requirements. Digital light processing (DLP) is a promising AM technique that is capable of producing filters with high accuracy and efficiency. In this paper, monolithic filters made from Al2O3 and TiO2, with a molar ratio of 9:1 (0.9 Al2O3-0.1 TiO2), were fabricated by DLP. The difference in the dielectric properties between the as-sintered and post-annealed samples at different temperatures was studied. The experimental results showed that when sintered at 1550 °C for 2 h and post annealed at 1000 °C for 5 h, 0.9 Al2O3-0.1 TiO2 exhibited excellent dielectric properties: εr = 12.4, Q × f = 111,000 GHz, τf = +1.2 ppm/°C. After comparing the measured results with the simulated ones in the passband from 6.5 to 9 GHz, it was concluded that the insertion loss (IL) and return loss (RL) of the filter meet the design requirements

Finite element analysis of mechanical behavior, permeability of irregular porous scaffolds and lattice-based porous scaffolds

In view of the low elastic modulus of the porous structure, it has attracted extensive attention in the field of artificial tissue implants for bone tissue engineering, and it has become important to find a porous structure suitable for human bone tissue. In this study, we constructed three type regular porous structure (cube, diamond, rhombohedral dodecahedron) and an irregular porous structure based on Voronoi tessellation. Firstly, the structural characteristics of porous structures were studied. After permeation simulation and compression simulation, we found that the structural characteristics (porosity, pore size, specific surface area) of four porous structures have a strong positive correlation with permeability. With the increase of porosity, the effective elastic modulus of the four porous structures decreases gradually. When the porosity is 80%, the effective elastic modulus and permeability of the four porous structures can basically meet the requirements of human bone implants. Irregular porous scaffolds exhibit relatively limited anisotropy in terms of mechanical properties and permeability. In view of the similarity between the structure and the human bone, the irregular porous structure exhibits superior development and application potential compared to the regular porous structure

Study on defect-free debinding green body of ceramic formed by DLP technology

Zirconia and hydroxyapatite(HA) are two typical implant materials, which have the advantages of excellent mechanical strength and good biological activity respectively. It was found that composite material had good biocompatibility and mechanical strength compared to the single material. In this paper, the porous scaffolds of ZrO2/HA composite were formed by digital light processing (DLP) technology and their performance were evaluated. Cell experiments showed that the addition of HA had a positive effect on cell proliferation and differentiation. Mechanical tests showed that the composite scaffold with 10 wt% HA had the best compressive capacity due to the pinning and bridging effect of a small amount of HA grains. When scaffolds were immersed in the simulated body fluid (SBF), the compressive strengths of the composite scaffolds decreased within the first 14 days and gradually increased after 14 days. The reason for this phenomenon was the degradation of calcium phosphate components and the deposition of apatite. By the 28th day, the compressive strengths of all the composite scaffolds increased to over 20 MPa, close to that of the zirconia scaffolds during the same period (25 MPa). The compressive strengths of all scaffolds met the requirement of cancellous bone during the entire soaking period, and the composite scaffolds have potential application value in bone repair

Additive manufacturing of bio-inspired ceramic bone scaffolds: structural design, mechanical properties and biocompatibility

Ideal bone scaffolds require good biocompatibility and moderate mechanical properties, so as to promote the proliferation and differentiation of osteoblast cells, and achieve the good bone repair. Inspired by the porous structure of cancellous bone, 15 groups of bone scaffolds with variable irregularity (Ir1-5) and porosity (35.53-61.75%) were designed and fabricated by ceramic digital light processing (DLP) using 20 wt.% hydroxyapatite doped zirconia as the matrix material. The effects of structural parameters and material on mechanical properties and biocompatibility were studied. The shrinkage test results showed that the density of scaffolds was mainly affected by the porosity. The mechanical test results showed that Ir2 and Ir3 scaffolds had better compressive behaviors, and the compressive strength could be increased by 30% by regulating the irregularity. All scaffolds showed comparable mechanical properties to that of cancellous bone. Cell experiments showed that the effect of structure on cell proliferation, differentiation, and mineralization was most evident at the early stage of implantation. Meanwhile, the biocompatibility variation with the irregularity was consistent with mechanical properties. This study proved that a bio-inspired bone scaffold with excellent comprehensive properties could be obtained through reasonable design