| Project Team | Institution | Role | ||
| Dr Nick Birbilis | Monash University | Program Leader | ||
| Dr Dan Fabijanic | Deakin University | Research Fellow | ||
| Professor Peter Hodgson | Deakin University | Chief Investigator | ||
| Dr Kevin Ralston | Monash University | Research Fellow | ||
| Dr Adam Taylor | Deakin University | Research Fellow | ||
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Executive Summary
Project B3.1 consists of a range of research endeavours summarised as ‘Surface Engineering’, specifically the objective of the project is to engineer gradient structures to optimize durability, corrosion and oxidation resistance, and hardness and wear resistance. The project was created in the phase 2 portfolio from the reorganization/combination of two projects previously known as D4 and D5 in phase 1, and as such work has initially been a continuation and expansion of those programs. Research activities have been primarily focused on corrosion resistance but with an increase in researcher hours, and collaboration between Deakin and Monash it is anticipated that significant strides will be made on wear resistance studies.
Considerable time has been devoted to creating a holistic understanding of the relationship between grain size, processing and corrosion resistance for a number of alloys but particularly for Al alloys. This understanding is not only important for predicting whether grain refinement or a specific processing route will lead to enhanced corrosion resistance in a given electrolyte but may also prove to have far reaching impacts for other materials cantered electrochemical processes (such as the development of more efficient batteries). Additionally, exciting work using fluidised bed technology to modify/alloy the surface of light alloys for enhanced corrosion and wear resistance is ongoing with the chromising of various steels, as a proof of concept, showing remarkable improvements in corrosion resistance. The process has been shown to work well on light metals with the successful aluminising of Ti and Mg, as well as the siliconising of Al.
Project B3.1 consists of a range of research endeavours summarised as ‘Surface Engineering’, specifically the objective of the project is to engineer gradient structures to optimize durability, corrosion and oxidation resistance, and hardness and wear resistance. The project was created in the phase 2 portfolio from the reorganization/combination of two projects previously known as D4 and D5 in phase 1, and as such work has initially been a continuation and expansion of those programs. Research activities have been primarily focused on corrosion resistance but with an increase in researcher hours, and collaboration between Deakin and Monash it is anticipated that significant strides will be made on wear resistance studies.
Considerable time has been devoted to creating a holistic understanding of the relationship between grain size, processing and corrosion resistance for a number of alloys but particularly for Al alloys. This understanding is not only important for predicting whether grain refinement or a specific processing route will lead to enhanced corrosion resistance in a given electrolyte but may also prove to have far reaching impacts for other materials cantered electrochemical processes (such as the development of more efficient batteries). Additionally, exciting work using fluidised bed technology to modify/alloy the surface of light alloys for enhanced corrosion and wear resistance is ongoing with the chromising of various steels, as a proof of concept, showing remarkable improvements in corrosion resistance. The process has been shown to work well on light metals with the successful aluminising of Ti and Mg, as well as the siliconising of Al.
Project Aims/Targets
To develop light alloys with structurally or chemically modified surfaces such that these alloys will have graded properties. The surfaces of these alloys will have been engineered to have properties that are superior to the bulk with respect to corrosion and wear resistance.
Enhanced Wear Resistance
To develop light alloys with structurally or chemically modified surfaces such that these alloys will have graded properties. The surfaces of these alloys will have been engineered to have properties that are superior to the bulk with respect to corrosion and wear resistance.
Enhanced Wear Resistance
Effect of Grain Size on Corrosion Resistance Al
A significant and major research push has been made to understand the effect of grain size on corrosion resistance, particularly for high purity aluminium - which adds to the work done to date on pure magnesium. Aluminium was processed using a number of different processing routes to obtain samples with refined grains, which were subsequently heat treated with the goal of obtaining a large distribution of different grain sizes for corrosion evaluation. Processing included: cold rolling, cryo-rolling, ECAP (1, 4, and 8 passes), and SMAT, with as-cast, pure aluminium, and wire samples also evaluated for comparison. Accurate grain sizes were measured with the aid of high quality optical images obtained after electro-etching with Barkers solution. Corrosion rates were determined from polarisation experiments conducted in NaCl electrolytes.
A significant and major research push has been made to understand the effect of grain size on corrosion resistance, particularly for high purity aluminium - which adds to the work done to date on pure magnesium. Aluminium was processed using a number of different processing routes to obtain samples with refined grains, which were subsequently heat treated with the goal of obtaining a large distribution of different grain sizes for corrosion evaluation. Processing included: cold rolling, cryo-rolling, ECAP (1, 4, and 8 passes), and SMAT, with as-cast, pure aluminium, and wire samples also evaluated for comparison. Accurate grain sizes were measured with the aid of high quality optical images obtained after electro-etching with Barkers solution. Corrosion rates were determined from polarisation experiments conducted in NaCl electrolytes.
Results showed remarkable variation in corrosion rate; with more than 1-order of magnitude difference in corrosion current achieved for samples with the exact/identical chemistry and grain size by using different processing routes. Additionally, some ECAPed specimens showed an interesting (and very reproducible) behaviour in which no breakdown potential was observed. This behaviour is not currently understood and is somewhat similar to observations previously made on ECAPed Mg. Consequently, the grain size may not be the sole parameter that controls corrosion rate. Through this work and an ongoing literature review, an understanding of the interplay between grain size, processing, and corrosion resistance is beginning to emerge. The details regarding the mechanism of this phenomenon are important to establish in the ability to objectively design Al & Mg alloys.
Variation of processing route (ECAP or SMAT, for example) can be used to vary the corrosion rate of pure Al at least an order of magnitude. This variation could be used to tailor the corrosion rate for specific applications. ECAP may be a way to make alloys that don’t exhibit a “breakdown”.
A comprehensive literature review on the effect of grain size on corrosion resistance indicates that grain refinement essentially magnifies the behaviour of a coarser-grained material in a given environment. For example, if a material shows passive behaviour in a given environment, grain-refinement is likely to further slow the corrosion rate. In contrast, if a material actively corrodes in a given environment grain refinement is likely to exacerbate corrosion. If true - then grain refinement may be used in specific environments to improve corrosion resistance or perhaps tailor the corrosion rate.
Surface Modification Using Fluidized Bed Technology
A fluid bed reactor is a flexible apparatus allowing a wide range of surface modifications centred on thermo-chemical and chemical vapour deposition processes. These processes involve gas or solid/gas mass transfer reactions to chemically alter the surface of metal by interstitial or substitution diffusion. A variety of modified surface structures can be formed; previously the team at Deakin have produced surface regions consisting of solid solutions, ceramics, and intermetallics and various steel, nickel and copper alloys. Chiefly these modifications are aimed to improve, singularly or in combination, the wear, corrosion or oxidation resistance of the base materials.
As proof of concept preliminary work, an evaluation of the corrosion performance of 316, 409, and 1020 steels after chromising (chromium deposition and diffusion) was performed. The chromising treatment proved very effective in decreasing corrosion rate and increasing passivity of 316, 409, and 1020 steels. This work demonstrates the utility of using a fluidised bed to modify the surface of materials for enhanced corrosion resistance; we believe similar techniques can be used successfully to modify the surfaces of light alloys. Work is currently underway for the production and evaluation of aluminised Ti, aluminised Mg, and siliconised Al.
A fluid bed reactor is a flexible apparatus allowing a wide range of surface modifications centred on thermo-chemical and chemical vapour deposition processes. These processes involve gas or solid/gas mass transfer reactions to chemically alter the surface of metal by interstitial or substitution diffusion. A variety of modified surface structures can be formed; previously the team at Deakin have produced surface regions consisting of solid solutions, ceramics, and intermetallics and various steel, nickel and copper alloys. Chiefly these modifications are aimed to improve, singularly or in combination, the wear, corrosion or oxidation resistance of the base materials.
As proof of concept preliminary work, an evaluation of the corrosion performance of 316, 409, and 1020 steels after chromising (chromium deposition and diffusion) was performed. The chromising treatment proved very effective in decreasing corrosion rate and increasing passivity of 316, 409, and 1020 steels. This work demonstrates the utility of using a fluidised bed to modify the surface of materials for enhanced corrosion resistance; we believe similar techniques can be used successfully to modify the surfaces of light alloys. Work is currently underway for the production and evaluation of aluminised Ti, aluminised Mg, and siliconised Al.
Tests have shown that titanium aluminises very well below 700°C, with uniform and hard intermetallic layers forming on CP Ti and Ti6Al4V alloy (refer to Figure B24 a) and b). The surfaces have high aluminium content and are free from internal oxygen. The functionality of these layers is under investigation, in particular the high temperature (> 500°C) wear performance.
An experimental set up has been designed to aluminise magnesium by solid-state diffusion using a fluid bed reactor. Initial trials have shown that the set up is capable of producing layers rich in aluminium-containing intermetallics. The overarching aim is to produce a uniform intermetallic layer having very good wear and corrosion performance.
Preliminary work on the siliconising of aluminium appears quite promising. After siliconising, samples of pure aluminium showed a silicon-enriched surface region up to 10 microns. Microstructural analysis is currently underway and it appears the silicon is present as discrete distributed particles. The effect of this treatment on the wear and corrosion performance of aluminium alloys will subsequently be performed.