The development of a wear resistance aluminium bronzes (Cu-Al-Fe) coating

Abstract

Aluminium bronze alloys (Cu-Al-Fe) with Al > 14 wt. % are known to have high wear resistance and low friction coefficient against ferrous metals thus making it an ideal material for forming dies application. However, the use of these alloys has been restricted by the high cooling rate required to prevent embrittlement of the alloy during production. The plasma transferred arc technique (PTA) is an attractive production technique that offers the required high cooling rate, however the resulting microstructure is strongly dependent on the composition change induced during deposition. Therefore to optimise the microstructure for application such as forming dies, thorough understanding of the effects of PTA induced composition change on the microstructure, wear and corrosion resistance properties are required. The composition change induced by PTA involves primarily an increase in Fe. Therefore, in the present research four aluminium bronze coatings with 9, 20, 27 and 35 wt. % Fe were produced from a gas atomized Cu-Al-Fe powder by deposition on to an E.N. 10503 steel substrate by PTA. Microstructure characterisation was carried out using complementary techniques involving SEM, EDS, XRD, EBSD and depth-sensing nano-indentation on etched and electro-polished specimen. The results show that Fe content above 9 wt. % leads to a phase change from the Cu3Al martensitic ?1' to solid solution (Cu) phase. This is also accompanied by an increase in size of the Fe3Al intermetallic ?1 phase. The redistribution of Al solute during cooling was identified as the main factor for the observed phase change. These microstructure changes lead to a hardness increase from 4.9 GPa in the coating with 9 wt. % Fe to 5.6 GPa in the coating with 35 wt. % Fe, however hardness mapping using depth-sensing nano-indentation shows that in the high Fe content coating, the hardness distribution is not uniform. This is due to the large volume fraction of the intermetallic ?1 phase which has high hardness of ~7 GPa. The wear resistance of the coating was found to be strongly influenced by the Cu-rich matrix phase. In the coatings with 20, 27 and 35 wt.% Fe, delamination and abrasive wear are the dominant wear mechanisms.SEM observations show that pile-up of slip at the hard intermetallic phase leads to the formation of surface cracks. Coalescence of these cracks coupled with the adhesion between the coating and the ferrous counter material were found to be responsible for promoting delamination wear, which results in high wear rate. The coating with 9 wt.% Fe has the lowest specific wear rates of 2.11-2.87 x 10-4 mm3/Nm against AISI 316, 420 and 440 stainless steel. This is significantly lower than the specific wear rates of 5.95-15.36 x 10-4 mm3/Nm measured for the currently used AISI D2 tool steels at the same condition. This is due to the uniform hardness and retention of the martensitic ?1' phase. The effects of PTA induced microstructure change on the corrosion resistance were investigated by electrochemical and immersion corrosion tests in an aerated 3.5 % NaCI solution. The results show that the corrosion resistance of the coating is strongly dependent on complete formation of Al2O3 protective layer. The Al content in the coating is a critical factor in the formation of the protective layer. In the coating with high Fe content where limited Al solutes are available, high corrosion rates of 300-400 x 10-3 mm per year were observed. The 9 wt.% coating which contains the highest Al solute, the lowest corrosion rate of 22.5 x 10-3 mm per year was measured. This corrosion rate is comparable to the more expensive and highly alloyed nickel aluminium bronze. Based on the results obtained in the present research, the coating with a martensitic ?1' phase and submicron size intermetallic ?1 phase has the highest wear and corrosion resistance. Such a structure can be achieved by controlling the PTA parameters to minimize the composition change promoted by melting of the steel substrate during deposition. The results from the present research also highlight the importance of interface properties, which have been shown to have a significant influence on properties such as adhesion, wear and corrosion. As more composite materials are utilised, further understanding of the microstructure and properties near the interfaces between materials becomes ever more important. It is hoped that the methodology and results presented in this thesis will provide the initial groundwork for future experimental and modelling work on multiphase material.

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