Edited by David S. Hotter
Most engineers, even materials engineers, have little knowledge or experience with clad metals. The materials rarely receive more than superficial coverage in college classes and no design manuals exist.
What may be a surprise is that you don’t have to look far to find clad metals, no farther than your pocket or purse. Take a look at the edge of a quarter and you’ll see copper clad with cupronickel. In this case, stock material is formed by rolling three strips of material together under high pressure to form a multilayer composite. It’s then heated to temperatures that diffuse atoms of each layer into the other, forming a permanent bond.
Another example comes from designer pens with a gold finish. Though they may have the look and feel of solid gold, many pen caps and barrels are made from gold-clad brass because the composite is more durable than electroplated gold coatings. Other applications for clad materials include cookware, coaxial-cable shielding, and catalytic-converter substrates.
Historically, metals cladding was a process of bonding two or more layers of metal together by hot forging, hot rolling, or hammering hot metals. More recent cladding developments include a cold-bonding method called pressure/ temperature (P/T) bonding, developed during the 1950s. This precisely controlled thermomechanical process is so effective it requires no intermediate brazing alloys or adhesives.
In the early stages of P/T development, engineers tried various alloy combinations, using different cold-rolling and heat-treating parameters, until they produced the best balance of properties.
Over time, materials produced by the process found increasing application as engineers gained experience with varying clad combinations and processing routines. Today, the sophistication of P/T bonding has led to substantial families of clad combinations, accepted as standards in numerous industries. The process enables high-volume production of clad metals at reasonable cost.
Engineers typically use clad metals to solve one of two design challenges: To deliver a combination of properties not possible with conventional metal alloys, or provide the most economical solution for meeting a design requirement such as corrosion resistance. The most common combinations of materials include aluminum and steel alloys, copper and steel alloys, and nickelbearing alloys.
P/T bonding produces large volumes of material at lower costs than hotbonding methods because it is a continuous coil-to-coil process, with coils weighing up to six tons resulting from a single pass through the bonding mill. By comparison, hot bonding is generally a piece-by-piece process. Cold bonding also holds tighter tolerances than hot bonding.
P/T bonding begins by cleaning the individual strips of material to remove contaminants. Material then passes through a rolling mill to form a composite material by creating bonds between surface electrons. Once bonded, workers heat treat materials to promote atomic diffusion, increase the bond strength, and relieve stresses for subsequent cold-working operations. The resulting clad metal fuses permanently and can’t be pulled apart. The ratio of metals remains constant throughout subsequent forming processes, giving engineers predictable performance.
Finishing operations include rolling to intermediate and final gauge, annealing to temper, cleaning, buffing, edge trimming, and slitting. Finished material can be up to two-feet wide, with thickness ranging between 0.001 and 0.185 in. Almost any combination of ductile metals can be clad, with each material making up 2 to 98% of the total composite thickness.
Clad-metal composites combine the specific qualities associated with each layer or clad component, properties that can’t be obtained with a single monolithic alloy. For example, engineers combine the strength and corrosion resistance of stainless steel with the light weight and electrical properties of aluminum. As a rule of thumb, the properties of composite clad materials such as tensile strength and conductivity can be estimated as the arithmetic sum of the volume percent of each component.
Cold-bonded clad metals, however, are more than a random combination of two alloys. The challenge for engineers is to select the right alloys and appropriate processing — particularly the cold-rolling parameters and heat-treat cycles. While some materials bond well together, they may be difficult to cold work and heat treat. For example, aluminum-clad stainless steel presents a particular challenge because aluminum melts at temperatures normally used to heat treat steel.
CLAD TO BE HERE
Coinage was the first high-volume clad-metal application using P/T bonding. It was developed because the government wanted to eliminate costly silver from coins. For a replacement material to be compatible with vending machines and coin-operated telephones, it had to have the same electrical and mechanical qualities as silver alloys. Engineers met these requirements using a cupronickel-clad copper composite, the standard for coins today.
After coinage, stainless-steel and aluminum composites were developed for applications in automotive and cookware. In the automotive industry, stainless-clad aluminum trim replaced chrome-plated steel or aluminum. The outer layer of steel provides a bright finish, while an aluminum underside provides a barrier to galvanic corrosion not only between the stainless steel, but also adjacent body panels.
Since automotive styling trends were moving away from the bright trim of previous years, developers found applications in new areas such as the trucking industry for tractor-trailer rig bumpers. Bright-finished bumpers are a standard option for Class-8 truck manufacturers. While bumpers have much larger dimensions than trim, engineers choose clad metals as bumper materials for the same quality that is critical for trim — corrosion resistance.
Unlike decorative trim, bumpers are structural components and therefore must have greater mechanical strength and durability over the long haul. Engineers achieve this by increasing thickness and modifying alloys and alloy ratios. Many stainless-clad aluminum bumpers have traveled over a million miles and retained a durable, shiny finish that looks brand new. Development is underway to bring this technology to the lightweight truck industry as well.
In the cookware industry, P/T bonding slashes material costs and gives manufacturers more control over thickness compared to traditional methods such as hot rolling and hot stamping. Coils of stainless-clad aluminum replace individual plates that workers must hand feed into forming equipment. What’s more, cold-bonded materials form the bottom and sides of cookware which gives more consistent heating compared to products with only clad bottoms. In addition, controlled thermomechanical processing increases the thermal stability to prevent pots and pans from deforming during use.
Other cookware using clad metals includes those used for induction cooking — a method popular in Japan and Europe, and now gaining acceptance in the U.S. Induction cookware relies on the uniform thickness produced by cold bonding to avoid hot spots in the ferritic (magnetic) steel/aluminum clad system. Besides cookware, clad metals are also used in household appliances such as sole plates of irons, where stainless-clad aluminum provides a bright, tough finish on the surface and lightweight aluminum that heats more evenly underneath.
The cans for button-cell batteries used in watches, hearing aids, cameras, toys, and calculators, are another application for P/T-bonded clad metals. The material systems in this case consist of austenitic stainless steels combined with nickel and copper alloys. These alloys resist corrosive battery chemicals, form easily to tight tolerances and thin gauges, and have the strength and durability to last the life of the batteries. In addition to meeting more stringent material requirements, engineers also use clad metals to meet the demands for mercury-free batteries, by using their experience with P/T bonding to develop new material combinations.
Other applications for clad materials include copper-clad stainless steel used for electrically shielding underground cables as well as protecting them from corrosion, attack by gophers, and damage from farm and construction equipment. Instead of using thicker copper cables, manufacturers use copper-clad stainless to boost strength while maintaining electrical properties and corrosion resistance at reasonable cost.
ON THE ROAD
Besides auto trim, clad metals have moved into several new applications in the automotive industry. Catalytic-converter substrates are the latest development for clad metals. Aluminum-clad steel composites, called DuraFoil, help engineers meet government emissions standards by producing catalytic converters that reach catalyzing temperatures more quickly, thereby reducing emissions during engine warmup.
Metals light off (catalyze) quickly, are stronger than ceramics, withstand thermal and mechanical shocks better, and can be located close to the exhaust manifold where they will heat up more quickly.
Metal substrates made from aluminum- bearing chromium stainless steels have been used in Europe for several years. The materials are typically made by casting ingots of the metal, forging, and rolling into thin foils. However, fabrication costs are high because the aluminum content makes steels hard and brittle, and therefore difficult to form.
DuraFoil clad metals are easier to manufacture and form into shapes, and resist high-temperature corrosion and thermal cycling better. The material starts as high-chromium stainless steel with aluminum clad to both sides. Workers cold roll the clad material into foil and then form it into catalyst substrates.
During the final brazing step, the clad metal undergoes a transformation where some of the aluminum remains on the surface forming a dense aluminum oxide coating. The remaining aluminum diffuses into the stainless-steel core forming an alloy identical to the chemistry and structure of the materials currently used. The transformed clad material provides superior high-temperature oxidation resistance and cost advantages compared to conventionally produced materials.
A family of steel-clad aluminum transition materials shows promise in cars as engineers continue to push for increased use of aluminum to reduce vehicle weight. Aluminum can’t be welded to steel, but by using shims or gaskets made from clad transition metals, car manufacturers can produce strong, durable spot welds. The materials also avoid crevices between steel and aluminum, eliminating potential corrosion sites.
Another application uses clad metals as self-brazing materials to slash manufacturing costs for brazed automotive oil coolers. Oil coolers often have up to a hundred fine-detailed parts. Using traditional manufacturing methods, workers place individual fine-gauge shims and flux between all of the assembly components. It is difficult to produce high volumes of such thingauge materials and, therefore, shims are typically oversized and can produce uneven braze joints that fail during pressure tests. By fabricating all braze parts from copper-clad stainless steel, manufacturers consolidate the number of parts, simplify manufacturing, and maintain consistent braze thickness.