The art and science of designing appliances and electronics involve merging form, function, and aesthetic appeal. The selection of the right material is a crucial step for designers to meet these demands, as well as support the product’s performance requirements. Consumers have demonstrated their purchasing preferences based on aesthetic appeal. Manufacturers have addressed this trend some years ago through the use of material finishes in the shift from traditional solid-color panels to brushed stainless-steel surfaces.

Although stainless steel offers many important design and performance attributes, there are certain constraints that appliance manufacturers must address with this choice, including the high cost of the material, level of fingerprint resistance, and susceptibility to corrosion for certain grades. Alternatively, clear-coated aluminum sheet offers designers a number of distinct advantages and benefits with few tradeoffs. In combination with a wide variety of surface textures, coatings or anodizing, aluminum delivers non-corrosive, stainless, and fingerprint resistance benefits for end products. The high strength-to-weight ratio of aluminum means that the cost of aluminum per unit area can be correspondingly lower than many grades of stainless steels.

Enlarge this picture Fig. 1. Samples of textured finishes available on Novelis aluminum sheet products. Left: A short-line brushed pattern used on freezer panels. Middle: A long-line brushed pattern. Right: A modified long-line brushed pattern used for appliance control panels.

Presently, there is a heightened environmental awareness in industrial design. This movement towards more sustainable solutions requires materials that can be easily recycled and materials that also contain a high degree of post-consumer recycled content. Novelis, a global supplier of aluminum sheet products, has responded to the growing demands for stringent technical specifications, aesthetic properties, and sustainable material solutions.

When specifying aluminum sheet materials, it’s important to understand all its different characteristics, as well as the options available within each characteristic.

Textures: A wide variety of surface textures can be applied onto aluminum sheet through a roll-on finish. This allows the appliance maker to adopt a unique pattern for their product lines. An aluminum brushed finish has a pattern that can be either a uniform directional or non-directional surface finish. Some of the more commonly textures produced at Novelis are shown in Fig. 1. Since the rolling process is very well understood and controlled, the reproducibility and quality of the textured finish is optimal for each and every coil.

Enlarge this picture Fig. 2. The chart demonstrates the dent depth as a function of applied load for Z854 aluminum alloy compared to stainless steel 304 alloy.

Mechanical Properties: It is essential that the aluminum sheet material has the appropriate mechanical properties to meet all the forming requirements for the part in question. Novelis has developed specific alloy packages to meet these widely varying specifications. For example, a control panel part requiring deep drawing would have an elongation of approximately 30 percent, but correspondingly low ultimate and yield strength. By contrast, many panel applications for refrigerators, freezers and other applications require relatively little forming, but high strength to meet dent-resistance requirements. In Fig. 2, the deformation under applied load of Novelis Z854 appliance sheet is shown in comparison with 304 stainless steel alloy. Testing was carried out according to ASTM D2794. The Z854 aluminum alloy had a gauge of 0.8 mm while the stainless steel 304 alloy was at 0.6 mm.

The data shows equivalent denting for the two materials. Apart from actual dent depth, it is important to realize that since the dent depths are all very low, it is not possible to visually distinguish between materials with somewhat different dent resistance even at the same gauge. In practice, a typical up-gauge of about 20 percent is required for appropriately selected aluminum sheet materials to match the dent performance of stainless steel. Typical mechanical properties of the Z854 aluminum material are shown in Table 1. The exact properties of the alloy package are fine-tuned for the specific application.

Apart from dent resistance, the material for front and sidewall panels requires bending at 90 and 180 degrees. The Z854 aluminum material can be formed in a brake press at 90 degrees to a radius of about 0.04 in. and at 180 degrees, provided an allowance of 2T to 3T is left at the bend.

Enlarge this picture Table 1. Mechanical properties of aluminum sheet requiring high dent resistance.

Coating Properties and Corrosion Resistance: Appliance panel applications require a high degree of resistance to food stains, cleaner stains, fingerprints, and corrosion. This is accomplished with aluminum by judicious selection of a coating system that meets all of these customer requirements. Typical coating properties used for Novelis Z854 aluminum alloy are shown in Table 2.

The data in Table 2 show that the coating properties meet or exceed the specifications commonly used by the major appliance manufacturers. The coating also provides excellent protection against fingerprint marking. The coating system is applied onto the aluminum by a coil-coating process that allows for optimal process control and, therefore, tight quality standards on every panel. The coating systems also results in a lower cost than typical paint spray application of panels.

Some additional aspects regarding corrosion resistance of the coated aluminum should be considered. Many appliance manufacturers have attempted to use lower grade stainless steels in their products as a result of the increased cost of alloys with high nickel content. The use of lower grade stainless steels has generally resulted in corrosion issues and unacceptable performance. Fig. 3 shows panels of coated Novelis Z854 aluminum and 304 grade stainless steel after 1,000 hours of neutral salt-spray exposure. The coated Z854 aluminum had no corrosion, while the 304 stainless steel clearly exhibits a significant level of corrosion. In this testing, lower grades of stainless such as 201 exhibited even higher levels of corrosion and only high grades such as 316 exhibited the corrosion resistance of the coated aluminum.

Enlarge this picture Table 2. Coating properties.

The coating can also be tinted or varied in gloss to provide a wide variety of aesthetic options to the appliance maker. Fig. 4 shows a prototype of a freezer door (now in production) that is made from coated, short-line brushed Z854 aluminum material. The formability of the coating is such that it conforms to the metal during the forming process without loss of adhesion. In addition, any minor damage that occurs to the coated surface can be remedied by application of touch up paints and/or simple grinding techniques.

Anodizing: Anodizing is an electrochemical process that results in the formation of an aluminum-oxide film on aluminum sheet that is both protective and decorative. There are many aspects to anodizing, the most important ones being how the process increases corrosion resistance, wear resistance, and provides for excellent color uniformity, surface appearance, and reflectivity.

Novelis recently developed anodizing-quality materials that contain a minimum of 70-percent post-consumer recycled-content material. This is a significant advancement, given that traditional anodized quality aluminum has virtually no recycled content. The product, Novelis Z833R recycled aluminum for anodized quality, meets industry standards for anodizable surface (AA 5005). This unique feature is achieved through the use of aluminum-clad alloy packages that have a thin, anodizing-quality skin made from smelter-grade material to achieve high color and gloss consistency, while possessing a core of 3000 series alloy made principally from recycled aluminum. The ability to provide appliance panels with a very high level of recycled material can help appliance makers serve a market with growing environmental awareness reflected by consumer demand for green products.

Fig. 4. Prototype freezer door made from short line brushed and coated Z854 aluminum alloy.

The Novelis Z833R recycled aluminum meets the ISO 14021 standard for post-consumer recycled content. In addition to environmental issues, anodizing offers some unique aesthetic finishes that can be difficult to meet with conventional paint systems.

Recycling: Aluminum is 100 percent recyclable, making it a uniquely sustainable material that can be recycled without losing any of its fundamental properties. Aluminum is a highly efficient, cost-effective, and environmentally friendly material that offers numerous possibilities for a variety of end-use applications. As an example, panels made from aluminum in any alloy form can be recycled into new aluminum products once the service life of the appliance is complete. Producing aluminum from recycled materials provides the advantages of significantly lower energy consumption and reduced emissions of greenhouse gases compared to production of primary aluminum.

These different characteristics give the appliance industry reasons to consider specifying aluminum. Specialty finishes that merge color and texture on aluminum provide appliance designers with flexibility. The formability and dent resistance of aluminum, along with its protective coatings delivers utility to appliance engineers. Aluminum’s recyclability and the ability to make appliance panels with 70 percent recycled content helps appliance marketers meet the demand for greener products. Those features, combined with lower cost per unit area than many stainless steel alternatives, help make aluminum an attractive material choice for appliance manufacturers. For more information, visit: www.ipg.novelis.com