Key Engineering Materials Vol. 964

Abstract: Anisotropy in tensile behaviour, plastic flow behaviour, and low cycle fatigue (LCF) behaviour of additively manufactured (AM) maraging steel in different build orientations are presented and compared with conventionally manufactured maraging steel. Also, the effect of heat treatment (namely, solution treatment and ageing) on tensile behaviour and low-cycle fatigue behaviour were studied. The AM maraging steel showed more anisotropy in as-built (AB) condition and moderate anisotropy in heat-treated (HT) condition. Experimental engineering stress-engineering strain and true stress-true strain data of AB AM maraging steel and HT conditions have been analysed using Hollomon, Ludwik, Swift, Ludwigson, and Voce plastic flow relationships. It is also observed that the 0° oriented specimen exhibits better tensile and LCF behaviour as compared to the 90° oriented specimen in AB and HT conditions.

Abstract: Additive manufacturing (AM) provides numerous advantages compared to conventional manufacturing methods, such as high design freedom and low material waste. Among the available materials, precipitation-hardenable aluminum alloys are highly attractive for AM due to their high specific strength and low density. Precise control of the processing conditions during AM and post heat treatment (HT) is required to tailor the final mechanical properties. Consequently, many variables, such as the chemical composition and process and HT parameters, must be considered to design suitable alloys for AM. Experimental investigations are, however, limited in variation of these variables. Therefore, computational alloy design approaches allowing for a faster evaluation of many possible variations must be developed. This work presents a high-throughput approach to determine the precipitation kinetics and thermodynamic properties based on the CALculation of PHAse Diagrams (CALPHAD) method. The developed approach is successfully validated for an Al-Mg-Si-Ti-Fe alloy and is applied to screen 243 combinations of chemical compositions and HT parameters. The results confirm the microstructural stability of the Al-Mg-Si-Ti-Fe system to small composition variations.

Abstract: Laser Powder Bed Fusion (L-PBF) might be a promising solution for producing complex wear-resistant parts from NiCrBSi self-fluxing nickel alloys. This study investigates a large set of parameters with a baseplate heated up to 500 °C. Furthermore, laser remelting is also explored to further increase surface density and reduce cracking. Material health is deeply investigated by image analysis to quantify different defects (lack of fusion, porosity and cracks). Spatters likely induce lacks of fusion, while the low toughness and high hardness values cause cracks. The lack of fusion surface fraction and crack length decrease with the preheating temperature while the crack width increases. A surface density of 99 % is obtained with careful process optimisation leading to a laser power of 100 W, a laser speed of 750 mm.s^-1 and a preheating temperature of 500 °C.

Abstract: Over the last decade, the development of 3D printing technology has led to both increased interest and availability of these machines. The steady growth and demand have made 3D printing an affordable and consumer-friendly craft. Due to the shrinking barrier-of-entry, the technology is increasingly integrated into wider areas of science and research, as well as the manufacturing industry in general. The demand for individualized medical solutions is continuously expanding in multiple fields, such as medical training and patient-specific surgical guides—exposing a need for further development of these technologies for mainstream applications. The research areas of material science, life science, biotechnology, and medical applications have greatly benefited from the increasing availability of this technology—especially in regard to 3D printing high viscosity substrates which are biological, biocompatible, or bioresorbable. In this area of research and development, mainly micro-extrusion syringe-based systems are used. However, these extruders are limited to the volume of the utilized syringe. The presented work primary objective was to develop a multi-material 3D printer capable of processing substrates with a wide variety of viscosities, with a particular focus on biological-based substrates. The entire project was developed with an open-source mindset—with the intent of furthering future research possibilities within this space. A coreXY printer (0.5x0.5x0.5 m) was developed, including an automatic tool changing mechanism. A peristaltic pump and ink jet extruder were developed and manufactured to improve their processability for additive manufacturing and extrusion of hydrogels and as well as liquids.

Abstract: Laser cladding of a Ni based powder on cast iron is performed by a 4kW continuous diode laser. For this, a robot programming method named “Wavering” is used. Indeed, this method allows to cover large surfaces (width higher than 5 mm and thickness higher than 4 mm). The cast iron substrate used during this work is employed for its thermal properties in glass mold Industry. However, it has drawbacks which are weak wear, corrosion, and abrasion resistance. Conventional techniques used to protect the molds, like Plasma Transferred Arc (PTA), affect the molds microstructure, but also the thermal and mechanical properties. The laser cladding of the Ni based alloy allows to protect the molds without affecting the cast iron thermal properties (low Heat Affected Zone). The purpose of this research is to produce a well bonded Ni based melted powder without pores or cracks on large and curvilinear surfaces. First, the coating, adapted to this substrate geometry, is optimized. Then, an investigation of the impact of the processing parameters (power, scanning speed and wavering frequency) on the microstructure is carried out. Besides, the mechanical behaviour is analysed by microhardness. In addition, the evolution of the Heat Affected Zone (HAZ) according to the processing parameters is observed. Two kinds of areas are inspected in terms of microstructure: a stable area obtained after a single pass and an interferenced area linked to superimposed passes. Those analyses led to a cladding parameter optimization to obtain perfect bonding, to avoid porosity propagation and to limit the HAZ emergence on curvilinear surfaces. Finally, in comparison with the PTA technique, it appears that laser cladding process with wavering method leads to a good coating of curvilinear surfaces.

Abstract: AA7075 is one of the most resistant aluminium alloys, so it is frequently used in very demanding industries as aeronautics or defence. However, the 7075 alloy falls into the non-weldable category thus hardly processable through additive manufacturing processes, and specially on laser-based DED (Directed Energy Deposition). The low absorption together with cracking behaviour remain a challenge for the industrialisation of these processes. Alloying with minor elements or addition of nano-reinforcement have been proven as a successful approach to increase its manufacturability. In this work, the feasibility of printing 7075 with nano-TiC as additive was evaluated. Two compositions with 0.5 and 2% in weight were developed by dry mixing. The powders were characterized by scanning electron microscopy (SEM) and flowability was compared with the unreinforced alloy. With the optimal laser process parameters, 3D coupons were printed to be characterized microstructurally, thermally, and mechanically. Process monitoring using thermal and high-speed cameras was carried out to gain insight into the thermal behaviour of the melt-pool and resulting process stability. After printing, aspect ratio of single tracks was measured, and dilution was also evaluated. Although addition of 0.5% of n-TiC promotes a slight improvement on the alloy, allowing it to be mechanically tested, it still presents some defects as porosity. By increasing the content up to 2%, both the quality and the mechanical performance were enhanced significantly.

Abstract: Alloy design allows to define specific compositions of materials having certain technological requirements as boundary conditions. Moreover increasing interest raised in the last decades on materials for Additive Manufacturing (AM). These new technologies consider a new approach, i.e. bottom up, for component manufacturing and suitable materials in different forms, generally liquid or solid (powders, filaments). One of the most interesting advantages of AM is the potentiality of realizing components with complex geometries and in a single or in few pieces to be assembled. For this reason, in this work a ferritic allow has been designed with the scope of realizing heat exchangers for highly corrosive alkaline environments. Water-ammonia solution are in-fact used in absorption machines for refrigeration and several heat exchangers are required for its thermodynamic cycle. After defining a specific composition, also on the base of thermodynamic simulations, the alloy has been produced by Vacuum Induction Melting (VIM). After suitable thermal treatments microstructural, mechanical and thermal characterizations have been carried out.