Effect of Substrate Alloy Type on the Microstructure of the Substrate and Deposited Material Interface in Aluminium Wire Arc Additive Manufacturing

Wire + Arc Additive Manufacture is an Additive Manufacturing process that requires a substrate to initiate the deposition process. In order to reduce material waste, build and lead time, and improve process efficiency, it is desirable to include this substrate in the final part design. This approach is a valid option only if the interface between the substrate and the deposited metal properties conform to the design specifications. The effect of substrate type on the interface microstructure in an aluminium part was investigated. Microstructure and micro-hardness measurements show the effect of substrate alloy and temper on the interface between the substrate and deposited material. Microcracks in the as-deposited condition were only found in one substrate. The deposited material hardness is always lower than the substrate hardness. However, this difference can be minimised by heat treatment and even eliminated when the substrate and wire are made of the same alloy.

Effect of γ′ Phase Elements on Oxidation Behavior of Nanocrystalline Coatings at 1050 °C

To study the effect of γ′ phase elements on the oxidation behavior of nanocrystalline coatings, two comparable nanocrystalline coating systems were established and prepared by magnetron sputtering. The oxidation experiments of the nanocrystalline coatings on the K38G and N5 superalloys were carried at 1050 °C for 100 h, respectively. The chemical composition of the above coatings is the same as the substrate alloy, including the γ′ elements, such as Al, Ta, and Ti. After serving at a high temperature for certain periods, their oxides participated and then affected the oxidation behavior of the coatings. The Al2O3 scale can be formed on the N5 coating, which cannot be formed on the K38G coating. Tantalum and titanium oxides can be detected on the oxide scale, which ruin its purity and integrity.

Studies on the Oxidation Behavior of a Designed Nanocrystalline Coating on K38 Alloy at 1050 °C

A new framework for a nanocrystalline coating system is established and prepared to study the oxidation behavior with a significant difference in elemental composition. K38 superalloy is selected as a substrate alloy and the composition of the 2nd-generation single-crystal superalloy Rene N5 is used as the sputtered nanocrystalline coating. The oxidation behavior of the newly designed nanocrystalline coating is comparatively studied with the original K38 coating and its substrate alloy at 1050 °C for 500 h. Moreover, microstructure evolution on the interface is used for studying the influence of element interdiffusion behavior on the substrate alloy. Results show that the nanocrystalline coatings increase the oxidation performance of alloys at 1050 °C for 100 h. The sputtered SN-N5 nanocrystalline coating exhibits the best oxidation resistance among the three groups of specimens for 500 h. Interdiffusion occurred and is observed on the SN-N5 coating after long-term oxidation. However, no topologically close-packed phases participated in the substrate alloy.

Thermo-Mechanical and Low Cycle Fatigue Failure Behavior Relevant to Temperature Regime in a TBCed Superalloy Specimen

Steady state and non-steady state thermo-mechanical fatigue failure is great concern in this work. At first steady state thermo-mechanical fatigue failure behavior was investigated using the round-bar TBC specimens, after getting basic data of mechanical properties of the bond/top coats and the substrate alloy. The failure behavior was compared with that during isothermal low cycle fatigue (LCF). Next non-steady state TMF tests were carried out in which non-steady state thermal stress was significant in the TBC specimen, compared with the properties under the steady state TMF. The experimental work clearly demonstrated that the TMF failure lives were significantly changed depending on the temperature regime during TMF and LCF. Of particular importance was found in the non-steady state TMF tests. The non-steady state TMF cycling promoted the delamination of ceramic top coat, resulting in a significant reduction in fatigue life.

Durability of Low Melt Alloys as Thermal Interface Materials

This paper focuses on developing a reliable thermal interface material (TIM) using low melt alloys (LMAs) containing gallium (Ga), indium (In), bismuth (Bi), and tin (Sn). The investigation described herein involved the in situ thermal performance of the LMAs as well as performance evaluation after accelerated life cycle testing, which included isothermal aging at 130°C and thermal cycling from −40°C to 80°C. Three alloys (75.5Ga &24.5In, 100Ga, and 51In, 32.5Bi &16.5Sn) were chosen for testing the thermal performance. Testing methodologies used follow ASTM D5470 protocols and the results are compared with some commercially available TIMs. The LMAs-substrate interaction was investigated by applying the alloys using different surface treatments (copper and tungsten). Measurements show that the alloys did survive extended aging and cycling depending upon the substrate-alloy combinations.

Microstructural Features and Oxidation Behaviour of NiCrAlY Coatings Obtained by HVOF Process

NiCrAlY coating system has been widely used for the advanced gas turbines to provide protection against high temperature oxidation and corrosion. Various methods have been used to develop these superalloy coatings. In present investigation, NiCrAlY superalloy coatings have been deposited on the superalloy substrate (Superni76) using commercially available NiCrAlY powder and High Velocity Oxide Fuel (HVOF) process. These coatings have been characterised in terms of their microhardness, porosity, microstructure features and surface roughness. The coatings have been oxidized cyclically (1hour heating and 20minutes cooling) in air at 900οC. The weight change curves have been plotted and the parabolic rate constant has been evaluated. The oxides formed after oxidation has been studied by using various techniques like optical microscopy, FESEM/EDAX and XRD. It has been observed Al¬2O3, NiCr2O4 and Cr2O3 formed upon oxidation of the coatings provide protection to the substrate alloy.

The Effect of Coatings on the Cyclic Oxidation Behaviour of Ti6Al4V at 750°C

For improvement of the cyclic oxidation resistance of the Ti6Al4V alloy, multi-layered Ti50Al50N/Ti70Al30N and mono-layered Ti60Al30Si10N were deposited on the alloy. It has been found that both coatings provide an effective protection for substrate alloy and the cyclic oxidation resistance of the TiAlSiN coating is better. After cyclic oxidation, it has been found that N diffused into the substrate alloy.

Failure Mechanism of Crack on Oxide–Alloy Interface: An Elastic–Plastic Analysis

Spallation failure of oxide scale in high-temperature environment, usually occurring at the oxide–alloy interface, primarily originates from the interfacial defects such as cracks. At the same time, the substrate alloy usually experience plastic deformation during high-temperature oxidation process. In this study, we extend our previous work on stress-diffusion interaction in the oxidation of Fe–Cr alloys by including the inelastic deformation of alloys and use it to study the growth mechanism of a crack lying along oxide–alloy interface. The results predict that the plasticity of alloy helps to prevent the crack from growing. It is also found that faster diffusion of species will lead to higher level of interfacial failure driving force. Reduction of Cr ion diffusion in oxide by introduction of the reactive element in the alloy will help to prevent interfacial crack growth.