The word “stainless” in appliances implies durability, quality and class. But stainless steel has its limitations. It smudges easily, for instance, and designers can only obtain a limited number of appearances out of the material.

In appliance design, cost-effective alternatives to the ubiquitous mild steel and stainless steel have been elusive. For instance, costs per-square-foot of aluminum sheet can be as little as half that of stainless steel in appliance manufacturing. But while the advantges of aluminum (environmental benefits, light weight, durability, multiple finishes) have long intrigued appliance designers, the limited performance characteristics of aluminum alloys have been a persistent frustration.

Recently, a breakthrough technology developed by rolled-aluminum manufacturer Novelis has emerged as a new solution. Novelis now is talking to appliance industry designers about using flat-rolled aluminum — in cost-effective, industrial quantities — that combines multiple physical properties in a single sheet. It’s called the Novelis Fusion™ technology, and it can produce aluminum rolling ingots with multiple layers of different alloys.

Cladded past

Two brushed aluminum samples, one brushed with a long stroke and the other a short stroke.

Rolled aluminum sheet is one of the most highly efficient, cost-effective and environmental friendly recyclable materials. Every day, it is used in the manufacture of products worldwide, including versatile packaging; advanced automotive and aerospace applications; and durable, attractive architectural panels.

Producers of aluminum sheet satisfy customer demand for specific performance characteristics by blending aluminum with small quantities of other elements such as copper, manganese, silicon, iron, or magnesium that impart the required additional properties to the metal. The final aluminum alloy is then cast into an ingot, a dense block of metal weighing several thousand pounds. Ingots are fed into a rolling mill where they are subjected to extreme pressure and rolled into sheet and foil products of various thicknesses, depending upon customer requirements.

Sometimes customers of rolled aluminum products desire product performance that is difficult or impossible to satisfy given the nature of most common alloys in the marketplace today. For instance, because magnesium adds strength to aluminum, “high-Mag” alloys are popular in a variety of applications where strength and light weight are highly valued. But magnesium is a highly reactive metal and, therefore, promotes oxidation that results in diminished surface quality. With such single-alloy sheet products, customers must always make trade-offs between strength, formability, surface appearance and corrosion-resistance.

To date, rolled aluminum suppliers have addressed such difficulties with a process called ingot-cladding, whereby a separately cast aluminum layer of one alloy is mechanically assembled (welded) to the top and/or both sides of a core ingot of another alloy. This sandwich then is fed into the rolling mill. In this way, the properties of two alloys can be combined in the finished sheet. For example, a low-corrosion layer can be clad to a high-strength, but corrosion-sensitive core, providing the customer a very strong finished sheet that maintains superior surface characteristics.

Ingot cladding of the past has made air conditioning and engine cooling systems lightweight and strong, and the outer skin on commercial airplanes tough as well as corrosion and fatigue resistant, but the complexities and economics of the cladding process have otherwise inhibited the penetration of clad products in the marketplace. Cladding is a low-tech, high-cost, labor-intensive and problem-prone process. Clad layers can slip from the core in rolling, oxidation between layers causes product imperfections, and the choices of clad-to-core alloy combinations are very limited. Still, cladding has been an industry standard for more than 60 years.

Casting together

Enlarge this picture Fig. 1. Illustration shows various steps in making and using a Fusion material.

Novelis Fusion now makes cladding obsolete because the technology simultaneously casts multiple-alloy layers directly into a single aluminum ingot, eliminating the entire cladding process (See Fig. 1.) This marks the first time that any company has achieved the commercial production of multi-alloy aluminum ingots. Engineers have tried for decades to cast multi-alloy ingots in a commercial environment. Novelis has succeeded, and has been producing Novelis Fusion ingots and delivering product to customers since March of 2006. Multi-alloy ingots are rolled into sheet products with different properties on the outside than on the inside, allowing previously unattainable performance for flat-rolled products.

Enlarge this picture Micrograph image of Fusion ingot boundary with as-cast 3003 alloy (lower portion) with 4045 alloy layer (upper portion) taken with standard light microscopy. Image illustrates the uniformity of the interface.

This technology produces an oxide-free metallurgical bond (see micrograph photo) between the alloy layers at the molten metal stage with little restrictions in the range of alloys that can be combined. Proper selection of alloy hardness, temper and gauge can produce a product that is comparable to the thickness and functional benefits of stainless steel, especially when compared to single-alloy aluminum products.

The technology was originally introduced by Novelis to serve its brazing sheet market for heat exchangers and has now converted virtually its entire North American production of this product to the Fusion material, thereby proving the viability of the new manufacturing process. But, there are many other opportunities for using the new technology.

Aluminum alternative

Enlarge this picture Fig. 2. Stress/Strain curves show how Fusion material can combine strength and formability in one product. A high-magnesium alloy by itself exhibits high strength (top curve), providing more than 400 Mega Pascals on the vertical axis. But the formability of the high-magnesium alloy only goes to 6 percent elongation, as seen on the horizontal axis. By contrast, a lower magnesium alloy, in this case 5005 (lower curve), delivers a better degree of formability that approaches 8 percent, but the strength, shown on vertical axis, falls short. The middle curve shows the benefits of a Fusion solution, a 5456 high-magnesium alloy core with a highly formable 5005 external layer, solving the tradeoff problem.

Fusion technology opens the door for new generations of premium applications for aluminum. Fusion enables a myriad of aesthetic combinations, and an expanded range of surfaces on high-strength core alloys in the automotive, architectural, building and construction, appliance, and consumer electronics markets.

In appliance design, one can expect to see new, exclusive looks that go well beyond those possible with stainless steel. Possibilities include long-line and short-line brushed finishes, or non-directional, micro-matte finishes. Thin-film finishes common to the anodizing of aluminum can generate attractive translucent finishes, a weathered bronze look, or an eye-catching patina. The Fusion products will be able to add colors and effects in applications where traditional anodized surfaces cannot meet the strength requirements of some critical components. The possibilities for combining aesthetics and performance are only limited by the imagination of the designer.

The process

The Novelis Fusion process utilizes a conventional direct-chill mold and starting head that serve as the primary-heat removing apparatus. The process also includes a secondary heat removal apparatus internal to the mold opening. Multiple coolant flows and liquid metal streams, controlled by a series of flow-control valves and metal-level sensors, maintain the necessary thermal, structural and mechanical boundaries during continuous solidification. The secondary heat removal apparatus developed by Novelis is typically engineered and manufactured with a curvilinear shape to compensate for normal contraction during the cast and to minimize the amount of mixed alloy scrap produced at the edge of the sheet.

The process begins when liquid metal enters the cavity corresponding to the alloy with the highest melting temperature. Metal is allowed to flow for a period of time prior to lowering the starting head. After the starting head is lowered and before it exits the lower end opening of the mold, the level of a second stream of metal is raised in the cavity to contact the semisolid interface of the first alloy immediately below the secondary heat removal apparatus. The Novelis Fusion process is a semi-continuous process, producing ingots weighing 12 tons to 15 tons that are removed from the casting machine and processed in much the same way as a traditional single-alloy ingot. Fusion ingots are introduced to the rolling mill completely bonded in a manner previously unrealized.

Mix and match

Going forward, Novelis will work with partners to develop the right mix of alloys to achieve maximum benefits in terms of surface finish and mechanical properties. Novelis Fusion can match the yield strength of stainless steel while offering a wider range of surface options. Fusion also minimizes the up-gauging needed to achieve the dent resistance of stainless steel and can be used in appliance applications that have flat panels or aesthetically pleasing gradual bends or contours. In the past, to achieve the dent resistance of stainless steel with a single-alloy aluminum sheet, it was necessary to up-gauge the thickness of the sheet. A Fusion panel constructed with a structurally strong core with an attractive, unique outer layer dramatically reduces the up-gauging requirement.

The new technology doesn’t just supplant existing cladding technologies, it grants designers the flexibility to create alloy combinations that were never before possible using traditional methods. Designers no longer have to choose between alloy attributes. With the ability to create alloy combinations, they can combine strength, formability, and surface finishes all in one sheet.

For more information email: nando.tedeschi@novelis.com