Innovative Casting Technology Offers Advantages for Aluminum Brazing Sheet Used in Heat Exchangers
One initial end use of the new technology is related to the manufacture of brazing sheet for heat exchangers. Up to now, the manufacture of brazing sheet for heat exchangers depended on the process of “cladding” aluminum – mechanically combining brazing-alloy and/or corrosion-resistant-alloy layers to a core alloy prior to rolling ingots into aluminum sheet.
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 properties to the metal. The final aluminum alloy is cast into an ingot – a dense block of metal weighing several thousand pounds. Ingots are fed first into a hot-rolling mill where they are subjected to extreme pressure at elevated temperatures then subsequently cold rolled into sheet and foil products of various thicknesses depending upon customer requirements.
In aluminum alloys, clad products such as brazing sheet used in heat exchangers have large commercial markets. Brazing sheet consists of a lower melting-point alloy clad to a brazeable alloy core with higher melting point (e.g. 4045/3003).
Traditional cladding is, however, a low-tech, labor-intensive solution. The clad layer (liner plate) must be produced via a separate route of casting, scalping, preheating, rolling and trimming to the necessary liner-plate size and thickness. The clad and core surfaces must be clean when mated, requiring a scalping step to the core and possibly requiring a surface preparation step on the mating liner-plate surface. The liner plate sometimes slips from the core in rolling, so rolling schedules for these products commence with a series of light passes focused at ensuring a frictional bond before significant reduction passes are approached. Moreover, there are limitations to the alloy combinations that can be roll bonded since some alloys form tenacious oxide films that are difficult to disrupt during the bonding process. This can result in “dirty” interfaces that degrade the useful strength of the sheet. In this latter regard, alloys with high magnesium content are particularly difficult to bond efficiently. Still, roll cladding has been an industry standard for more than 60 years.
Casting multiple alloys into the ingot itself at the molten-metal stage seems an obvious solution, and engineers have tried (and failed) for decades to cast multi-alloy ingots in a research environment. Novelis Fusion technology, therefore, marks a significant breakthrough. It allows, for the first time, the commercial production of multi-alloy ingots, eliminating the need for the cumbersome cladding and roll-bonding process.
A Perfect Metallurgical BondThe new technology produces a perfect metallurgical bond between the alloy layers. Novelis Fusion products are typified by the clean, high-strength interface with little restrictions in regard to the combination of alloys involved (Fig. 8).
Rolling Novelis Fusion IngotsAn ingot is scalped and preheated similar to a monolithic ingot of the same core alloy. In some cases where deep-draw quality Al-Mn core-alloy properties are desired, special solidification and preheating practices are used to ensure that the Al-Si layer is not remelted during preheating. The exceptional bond at the interface we mentioned earlier minimizes the light pass schedule common to current conventional roll bonding.
In addition to the liner-plate slippage we mentioned earlier, conventionally roll-bonded brazing assemblies suffer from extensive clad-layer extrusion and relative movement of the clad and core during the early rolling passes prior to bonding. After frictional bonding has occurred, the clad extrusion effect will continue but to a lesser degree of severity. This extrusion effect and clad-layer movement leads to thinning of the clad at the edges and ends of the assembled ingot and subsequent coil. Enhanced rolling practices in the reversing, tandem and cold mills cannot correct for the lateral and longitudinal spread of the clad layer encountered during the initial rolling passes. Each and every coil, therefore, has its own clad-layer “fingerprint” that follows on to subsequent downstream processing steps, adding not only waste but increased variability to the final units being produced.
Final clad-layer thickness has been measured down the entire length of an in-plant coil, at each extreme edge and down the centerline. The data in Figure 3 was generated from a rolled-to-gauge coil prior to it receiving its final edge slitting and end trimming. The specification for this radiator-tube material is 8%-12% clad thickness. Casual observance of Figure 3 shows the clad variation for this coil, which approaches a total deviation of only 1%.
In contrast, a typical conventionally clad material of the same specification would have material at the ends of the coil below 8% – requiring trimming – and a definitive deviation between the edges and centerline.
Mechanical PropertiesCustomers (manufacturers of heat exchangers) who purchase clad brazing sheet are generally interested in two sets of properties: the mechanical-core and molten-clad properties of the material they receive, which control and facilitate forming, stamping, assembly and brazing, and the strength and corrosion-resistance properties of the material after it has been brazed. Existing Novelis alloy (patented US 5,041,343 and US 6,019,939) technology provides customers with superior post-braze corrosion resistance and strength.
SummaryThus far, several hundred commercial-sized ingots with many different alloy combinations have been successfully pro-duced for brazing sheet and other novel end uses. Novelis engineers have found that increased ingot size (690 x 1750 mm), and the thermal economies that size offers, makes them easier to manage from a thermo-mechanical perspective during solidification. Similarly, while every alloy combination has its own challenges and nuances, any increase offered in the temperature differentials of the two alloys or in the sequence of solidification when approaching identical alloys will contribute to the overall probability of success in the cast/clad package. Fortunately, brazing combinations have this so-lidification differential naturally built in.
Brazing sheet manufactured with Novelis Fusion technology meets or exceeds all criteria necessary for customers to produce strong, durable and corrosion-resistant heat exchangers. The advantages to the new-technology brazing sheet include reduction in lead time (two to four weeks), cladding uniformity that allows for down gauging, and a clean and oxide-free clad/core interface. Even more encouraging, Novelis Fusion technology opens the door for greater alloy development for the next generation of products.
For more information: Contact Gary Yogan, Market Director, Novelis Corporation 6060 Parkland Boulevard, Cleveland, Ohio 44124; tel: 440-423-6889; fax: 440-423-6675; email: email@example.com. The author, Tom Davisson, is Product Development Manager and his contact information is tel: 440-423-6824; fax: 440-423-6675; email: firstname.lastname@example.org; web: www.novelis.com
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: brazing, roll cladding, roll bonding, extrusion, corrosion resistant, heat exchanger, metallurgical bond