Restoration of Aluminum Aerospace Parts and Coatings Using Cold Gas Dynamic Spraying

Title: Restoration of Aluminum Aerospace Parts and Coatings Using Cold Gas Dynamic Spraying
Authors: MacDonald, Daniel
Date: 2014
Embargo: 2019-09-17
Abstract: The majority of the structural weight of many common commercial aircrafts is composed of high strength aluminum alloys. The properties of high performance aluminum alloys such as a high strength to weight ratio (specific strength), ease of recycling, crash energy absorption capacity, and corrosion resistance make them ideal for use in the aerospace field. As a result of the high performance nature of the parts and specific properties of the materials, manufacturing requires intricate casting, precision machining, and specific heat treatments – which results in expensive components. As a result of its excellent corrosion resistance properties, pure aluminum coatings are commonly used in the aerospace field for corrosion protection of steel, aluminum alloy components, and titanium alloy components. The common method to deposit these coatings is called ion vapour deposition (IVD). These IVD aluminum coatings provide the coating adhesion, coverage, thickness, and corrosion resistance required to protect the part. The present study was motivated by the potential use of the cold gas dynamic spray (CGDS) process to repair a) damaged aluminum alloy aerospace parts and b) damaged pure aluminum IVD coatings. The primary research objective was to successfully produce these repairs using commercially available aluminum alloy feedstock powders deposited with commercially available CGDS equipment. This work was treated as prequalification work for The Boeing Company to commercialize this process and therefore the repairs aim to meet the same standards (military and industrial) required of the original aluminum alloy parts and IVD aluminum coatings. The use of CGDS was shown in this research to be a very promising as a process for the restoration of aluminum alloy aerospace components. The adhesion strength of the repaired aluminum components was found to be well above the accepted range for thermally sprayed repairs according to industrial standards. The repairs were subjected to a highly corrosive environment and showed only minor pitting. These sites could be reduced in the future with improved machining techniques and attention to surface detail prior to exposure to the salt fog. The only requirement that the repaired components did not meet was for the wear properties of the anodized layer, measured thought Taber abrasion testing. The results of this test, at times, approached the desired values, and it is believed that, in the future, the quality and consistency of the coatings could be improved and the test would meet industrial standards. The results of this research show that the use of CGDS as a process for the restoration of damaged aluminum IVD coatings is possible and is a promising alternative to conventional methods. The CGDS coatings were scrutinized to the same level as required of IVD coatings when they replaced toxic cadmium coatings in the late 1980s. The coating adhesion, demonstrated through glass bead abrasion and strip rupture testing, was shown to meet the current industrial standards. The corrosion testing of the repairs resulted in no visible red rust of the steel components, even when the steel was exposed.
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