Fabrication & characterisation of protective HEA films on Mg substrate by mechanical alloying, cold spraying & post-surface laser annealing processing
Dr. Rocco Lupoi
Tel: +353 1 896 1729
Dr. Sedat Özbilen
European Commission, Marie S. Curie Individual European Fellowship 2020
Mg coating/repair applications will be developed for aerospace sector, machinery & automotive industry by depositing HEA (High entropy alloys) films on Mg with MA (Mechanical alloying) + CS (Cold spray) + post-SLA (Surface laser annealing) processing which is the overriding aim of the project. In this respect, HEAs represent a new class of materials that present novel phase structures/properties. Therefore there is a growing interest in recent years on HEA films/coatings. They have been produced by many different production techniques such as magnetron sputtering, laser cladding, thermal spraying, electrodeposition, plasma-transferred arc cladding, and recently processed with CS technology: improvement on this is proposed to be carried out in the current project. To combat the problem of corrosion in Mg alloys for aerospace sector, machinery & automotive industry, many protective coating techniques have been developed. It is generally accepted that it is necessary to apply a surface protective coating to Mg components/products if they are to survive in a harsh working environment. Indeed, over the years, intensive research efforts have been devoted to developing better coatings for combating the poor corrosion/wear properties of Mg alloys to meet the challenges demanded by industry. Protective coatings can be fabricated on Mg alloys using laser surface processing techniques, but the main problems of the high chemical reactivity, the relatively low melting/boiling points of Mg alloys, & the formation of brittle intermetallic compounds in the coating cannot be easily overcome. Another common problem encountered in the laser cladding of a protective coating on Mg substrates is that significant dilution from the substrate often occurs, & this can adversely affect the corrosion resistance of the coating. To alleviate these technical problems, a 3-step approach will be followed in the project, which will involve CS deposition & post-SLA of MAed HEA powder feedstock on Mg substrate to circumvent the inherent problems. Such a technique can avoid excessive melting/boiling of the Mg substrate. So far, only a limited number of studies have focused on HEA usage in surface modification. Therefore, this project focuses on the study of the fabrication & characterization of the protective HEA coatings on Mg substrate by MA+CS & post SLA processing, a topic never researched so far. There is no publication in the open literature regarding the title of the present project, confirming the idea is novel, industry relevant & with high potential.
Extensive research has recently been carried out on HEAs, & attractive properties have been achieved that are suitable for applications as structural/functional materials. Thus, HEAs are considered as potential candidate materials for many challenging industrial applications such as protection of Mg parts currently protected with Al CS coatings. They therefore warrant further study from a scientific/commercial point of view. Although bulk HEAs possess good properties, the high cost, caused by using a large number of high-purity elemental materials, limits their industrial applications. As is known, coating deposition is an effective approach to broaden the application areas of high-cost materials due to its material saving. CS process during which the coating material is not melted can be a unique technology for depositing HEA coatings. To do this, nano-structured HEA feedstock powder materials must be prepared for use in CS & MA synthesis is suitable for this purpose. CS deposits are essentially free of thermally induced defects commonly observed in traditional thermal spray deposits, such as oxidation, evaporation, gas release, shrinkage porosity, & thermally induced residual stresses. Because of these advantages over other spray deposition processes, CS has generated a great deal of interest within the manufacturing community for repair of advanced coatings, & additive manufacturing applications. CS systems must focus on improving control of the particle flow & pre-processing the feedstock powder to increase deformation & bonding during deposition. Furthermore, a significant advantage of CS is an improvement of working conditions which becomes more secure due to the low temperature in gas spray. The primary aim of CS is the fabrication of thin metallic coatings. Present study will show it to be a promising, cost-effective, & environmentally acceptable technology to impart surface protection & restore dimensional tolerances to Mg alloy components, for example, on helicopters & fixed-wing aircraft together with automotive industry parts. Therefore, the present project is a new & original idea incorporating totally an innovative approach to the use of HEA coatings which has industrial applications. The technical interest in CS is twofold: first as a coating process for applications in surface technology, & second as a solid-state additive manufacturing process, offering an alternative to selective laser melting. It is known that CS produces severe non-uniform plastic deformation on the impacting particles, resulting an inhomogeneous micro-structure in the deposits. Bulk-scale mechanical properties vary with CS process parameters. CS deposits` ductility is usually low, but can be recovered to near bulk level by post-SLA. In optimized post SLA, the deposit quality can be enhanced, particularly the bond strength, by increasing bonding area between deposited particles & reducing porosity in the microstructure. Therefore, by using variable but controlled MA process and CS coating parameters (HEA feedstock powder & HEA MAed powder characteristics, CS technology geometric parameters, CS deposition parameters), variations in the micro-structure and property in HEA coatings will be attained. Microstructural characterization of HEA feedstock powders, MAed powders, CS coatings and CS+post-SLA coatings will be carried out by SEM, TEM, XRD, porosity measurements. Wear, oxidation, corrosion tests, hardness and elongation measurements will be made to determine chemical and mechanical properties of coatings. With this way relationships among deposition parameters-microstructure-properties will be determined in addition to the feasibility study of producing protective (wear and oxidation resistant) HEA coatings on Mg-parts by combined MA+CS & post-SLA process.