The prior art has proposed various methods and apparatus to produce composite materials. U.S. Pat. No. 2,931,082 to Brennan discloses a casting method and apparatus wherein a composite metal article is formed by continuously casting molten metal against a longitudinally moving base such as a metal strip or the like. In Brennan, a strip is disposed between the material being cast and a rotating casting wheel.
U.S. Pat. No. 5,077,094 to McCall et al. discloses a process for applying a metal coating to a metal strip substrate. In this process, a melt pool of a metal coating material is deposited on a casting surface of the substrate material and rapidly cooled to form the coated metal strip.
U.S. Pat. No. 4,224,978 to Klein discloses a twin roll casting method and apparatus for forming a composite material. In this method, a material having a mechanical strength and melting point substantially higher than that of aluminum is plated on at least one face of a continuously cast aluminum core material. Referenced in this patent is French Patent No. 1,364,758 which describes in principle a continuous casting method in which still liquid metal is introduced between two cooled work rolls and in which a metal plating strip is interposed between the liquid metal and the work rolls. The metal plating strip is thus plated onto the continuously cast material. This French patent discloses plating an aluminum blank with a strip of aluminum.
In the prior art, it is also known to provide a brazing sheet comprising a core of an aluminum alloy and a brazing material, i.e. a coating of a lower melting point filler metal. Typically, the coatings are roll bonded to one or both sides of the core sheet during fabrication. Brazing sheet can then be formed without removing the coating, assembled, fluxed and brazed without placing additional filler metal at a joint site.
In one type of roll bonding, the brazing material is bonded to a core material at an ingot stage. The bonded ingot must then be hot rolled to brazing sheet thicknesses, typically 0.125". This hot rolling step is conducive to the formation of surface oxides which impair the quality of the brazing sheet and can adversely affect brazing performance.
Alternatively, the filler metal can be produced by casting into an ingot form and rolled to a thin gauge liner stock. After rolling, the wrought filler metal can be roll bonded to the aluminum core material using conventional techniques. This method requires numerous annealing and surface preparation steps to prepare the thin gauge liner stock for bonding. The core material may vary depending on the application. AA3003 or AA6951 aluminum alloys are typical examples of core materials. The brazing filler metals can also vary depending on the desired use, usually comprising an AA4XXX-type aluminum alloy.
Besides the drawbacks noted above concerning excessive surface oxides in hot rolled brazing sheet and the additional processing steps of annealing and surface cleaning for wrought liner stock, prior art methods of making brazing sheet lack the ability to vary the cladding or filler metal composition for a given core material.
In response to the drawbacks and disadvantages of the prior art discussed above, a need has developed to provide an improved method for making twin roll cast composite materials offering flexibility in choice of composition, cost effectiveness and energy efficiency.
In response to this need, the present invention provides a method for making a twin roll cast clad material having an acceptable structure and quality in combination with low operating and capital costs and the ability to utilize different brazing filler materials with a single core material.