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Technical Paper

The Significance of Environment for Performance of Structural Adhesive Bonding

The development of a durable adhesive bonding technology for joining of aluminium automotive structures requires a full understanding of the importance of the environment on the chemistry of the adhesively bonded system. This paper describes the accelerated testing procedures used by Alcan to provide information on the significance of environmental factors on adherend surface, the bonding interface and adhesive and so establish the best combination of adhesive and surface pretreatment for good long term durability. The stress/humidity test provides information on adhesive and interface performance, while the neutral salt spray test illustrates durability and corrosion resistance of the pretreatment. Outdoor exposure testing provides the means of comparing the accelerated tests with real life durability.
Technical Paper

The Recycling and Reclamation of Metal-Matrix Composites

The recycling and reclamation of metal-matrix composites (MMC's) are critical aspects of the commercialization process. By recycling, we mean the economic processing of MMC scrap for reuse as composite. Reclamation refers to the separation and recovery of the individual components of the composite, i.e., the various aluminum alloys and ceramic particles. Three forms of MMC wrought alloy scrap have been considered; i.e., D. C. (direct chill) cast log ends, extrusion butts, and cut extrusion scrap. Recycling each of these forms of scrap back into D. C. cast extrusion billet has been demonstrated. This has been accomplished by recycling the scrap back through the basic mixing process. Various ratios of scrap to virgin composite have been explored and optimum blends are being studied. Similarly, for MMC foundry alloy (high silicon) gates and risers produced in shape-casting, fluxing and degassing techniques have been developed so these may be recycled back into useful castings.
Technical Paper

The Lincoln Mark VIII Cast Aluminium Suspension Control Arm (Parallel Development)

An A356T61 cast aluminum lower suspension control arm has been put into production for the Lincoln Mark VIII. The mechanical requirements which drive the design for a critical part like this are discussed, together with some of the background knowledge needed to address the issues surrounding alloy and process selection. Particularly as it must be realized that the process impacts the degree to which the potential of the alloy can be realized. With this in mind, some of the research activities which have been spawned in parallel with the production activities are briefly covered. The sequence of events involved in the design and prototyping of the part itself are outlined, as is the implementation of a specialized low pressure casting line to produce the part. Part performance to date has been excellent and the quality controls and test methods which have been put in place to see that this remains so are also covered.
Technical Paper

Reduced Finishing Costs for Aluminum Intensive Vehicles

Due to the inherently superior corrosion resistance of aluminum compared to automotive steels, phosphating and electrocoating are not necessarily required to provide good corrosion protection to aluminum intensive vehicles. This allows the potential for significant cost savings in the overall finishing process by eliminating these steps. Advantage can also be taken of the movement towards the use of powder primer surfacers to reduce solvent emissions in that the powder coating can be applied directly to a suitably pretreated aluminum surface. Pretreatments which are optimized for aluminum and much simpler to control than phosphating were chosen for trials based upon discussions with chemical suppliers. In this paper, the adhesion and corrosion characteristics of these selected pretreatment/powder primer systems were compared to standard phosphated and electrocoated AA6111 automotive closure sheet.
Technical Paper

Methods for Assessing the Corrosion Behaviour of Heat Exchanger Materials and Components

To determine the true galvanic compatibility of radiator components a test has been developed using a zero resistance ammeter (ZRA) technique, which measures the magnitude of the galvanic current between different materials, thus allowing specific corrosion rates to be calculated. It is believed that the use of the ZRA technique will help provide a better balance between sacrificial behaviour and thermal performance of fin alloys. In particular, it will be demonstrated that it is not necessary to make additions of zinc to the fin alloys to attain a sacrificial effect, which in the longer term may compromise the recyclability of radiator units.
Technical Paper

Galvanic Corrosion Prevention of Steel-Aluminum Couples

Efforts towards weight reduction are leading towards increasing use of aluminum components on automobiles. Although aluminum on its own has inherently superior corrosion resistance to steel, galvanic action between the aluminum and steel or galvanized parts can lead to severe corrosion. Straightforward and effective methods of preventing galvanic corrosion from the subject of this paper. Since many aluminum components are connected to steel structures by mechanical fasteners, protective coatings on fasteners were evaluated as well. Galvanic test couples were prepared in a manner simulating typical automotive assembly conditions while incorporating features which would lead to enhanced corrosion. A variety of chemical treatments and coatings on the fasteners as well as barriers between the dissimilar metals were evaluated for corrosion prevention between the aluminum and cold rolled or galvanized steel. Comparison between neutral salt spray and cyclic corrosion tests is provided.
Technical Paper

Effects of Surface Texture on the Appearance and Corrosion of Painted Aluminum Body Sheet

Studies have indicated that enhanced corrosion resistance and appearance after painting can be obtained on automotive steels by texturing the sheet surface. To determine whether these same improvements could be obtained on aluminum, paint performance and appearance were evaluated on heat treatable alloys that had been given shot dulled and laser textured finishes. Corrosion performance was measured by filiform and SCAB tests and paint appearance by distinctness of image (DOI). Profilometry was used to characterize the roughness of the sheet surface at various stages of paint application. Experiments demonstrated that corrosion performance was not significantly affected by surface texture. However, the DOI of painted textured surfaces varied with the extent of reduction of the sheet when rolled and, in particular, with the paint system. Mill finish aluminum was shown to have DOI equivalent to that achieved on cold rolled or electrogalvanized steels.
Technical Paper

Effects of Alloy Composition and Condition on Filiform Corrosion Performance of Cast Aluminum Wheels

The movement towards extended warranties in the automobile industry has focussed attention on corrosion performance of many components, particularly cast aluminum wheels. Filiform corrosion is of particular concern since it can severely affect the appearance of the wheel. The appearance and the choice of wheel design are the most attractive features to customers. In order to enhance the filiform corrosion resistance of cast aluminum wheels, cleaning, pretreatment, coating and alloy parameters are critical and need to be optimized. In this paper, the effects of alloy composition and condition on filiform corrosion are reviewed. A series of cast discs were prepared with variations in iron, zinc and copper levels around the standard A356.2 alloy composition. Apart from composition, certain specimens were subjected to different heat treatment and ageing conditions. The effects of porosity and different machining procedures were also evaluated.
Technical Paper

Development of an Understanding of the Critical Factors Influencing Waterside Corrosion Behaviour of Brazed Aluminium Radiators

The application of aluminium alloy materials for automotive heat exchangers, including engine cooling and air conditioning systems, is now widespread. To meet the industry demands of both extended service life and improved reliability for heat exchanger components, it is important that the critical factors influencing corrosion behaviour are properly understood, particularly with the trend towards downgauging of materials. To maximise resistance to waterside corrosion, manufacturers have adopted the approach of using an internal cladding, commonly a high purity or zinc - containing alloy, to provide sacrificial protection of the core material. Recent studies have shown that the presence of an internal cladding can, under certain conditions, promote rapid localised attack of the core alloy.
Technical Paper

Development of an Improved Cosmetic Corrosion Test for Finished Aluminum Autobody Panels

A co-operative program initiated by the Automotive Aluminum Alliance and supported by USAMP continues to pursue the goal of establishing an in-laboratory cosmetic corrosion test for finished aluminum auto body panels that provides a good correlation with in-service performance. The program is organized as a task group within the SAE Automotive Corrosion and Protection (ACAP) Committee. Initially a large reservoir of test materials was established to provide a well-defined and consistent specimen supply for comparing test results. A series of laboratory procedures have been conducted on triplicate samples at separate labs in order to evaluate the reproducibility of the various lab tests. Exposures at OEM test tracks have also been conducted and results of the proving ground tests have been compared to the results in the laboratory tests. Outdoor tests and on-vehicle tests are also in progress. An optical imaging technique is being utilized for evaluation of the corrosion.
Technical Paper

Development of a Long Life Aluminium Brazing Sheet Alloy with Enhanced Mechanical Performance

The use of aluminium alloys for automotive heat exchangers has increased considerably in the last 15-20 years and, in parallel, new alloys have been developed to meet the increased demand for strength and improved corrosion resistance. A non-heat treatable Al-Mn alloy, X800, has been developed by Alcan to significantly increase the corrosion resistance of radiator tubes when subjected to typical service environments. The alloy development employed considerable microstructural understanding to provide heat exchanger manufacturers with an improved product that readily attained enhanced performance during any brazing cycle. A similar philosophy has been adopted to address the issue of increased mechanical performance, higher intrinsic sheet strength, both during and after brazing, provides the opportunity for sheet downgauging and thus lightweighting of components.
Technical Paper

Aluminum Wheel Alloy Variants: Effects on Corrosion Performance

Previous work has shown that variations in wheel alloy chemistry, particularly with respect to copper and iron levels, can have a pronounced effect on filiform corrosion performance. In this study, an examination of A356.2 alloy chemistry variants and their effects on corrosion was carried out in greater detail. The emphasis was on copper and iron variants, both alone and in combination. Copper levels ranged from 0.005 to 0.22% and iron from 0.04 to 0.23%. The effect of manganese additions was also examined, with levels ranging from 0.002 to 0.07%. In addition to the alloying variants, the level of dispersed oxides in the castings was varied to determine any effects on corrosion performance. Although filiform corrosion performance of painted samples was the primary focus of this study, the corrosion behaviour of unpainted samples was also evaluated for comparison purposes.
Technical Paper

Alternatives to Electrocoating Aluminum in Automotive Applications

The inherent corrosion resistance of aluminum is much greater than automotive steels. To demonstrate this principle in a fashion acceptable to the automotive industry, a test program was run which incorporated lab, test track and real life trials on both unpainted and painted aluminum and painted steel. The lab program consisted of neutral salt and cyclic corrosion tests. Having demonstrated that aluminum does not need electrocoating for good corrosion integrity, alternatives to electrocoating which would allow primers to be applied only where necessary for esthetic purposes were sought. Several primers were selected for study based upon current automotive usage. Factors such as the degree of pretreatment prior to primer application and the presence of residual lubricant on the metal were evaluated.