|Designing with cast aluminum plate|
|Leighton M. Cooper, Marketing Manager, Common Alloy and Heavy Gauge Foil, Alcoa Mill Products|
|January 15, 2003|
|Aluminum plate is specified for countless uses that benefit from the inherent properties of the metal itself. Aluminum’s reduced weight, easy workability and thermal/electrical conductivity contribute to the performance of many products. With its versatility and high speed for a variety of machining operations, aluminum tooling plate has advantages that make it a popular choice for shaping intricate components.When selecting aluminum tooling plate for applications where dimensional control is critical, the design engineer must be confident of precise tolerances and dimensional stability. Fixtures and quality control equipment illustrate the need for close tolerances – in such equipment, components may require controlled thickness and flatness to serve as the base for a tool or a measuring device. This type of portable fixture is found in industries ranging from aircraft to automotive, computers to electronics, and printing equipment to food processing machinery. The second consideration, dimensional stability, is an important factor in parts that are machined by removing extensive levels of metal (as much as 85%). Dimensional stability must be considered both before and after processing, as well as in components subject to wide temperature variations.
To hold critical dimensional tolerances, important distinctions arise between cast aluminum tooling plate and its wrought counterpart (an aluminum plate product subjected to mechanical working by extruding, rolling, forging or other processes). Wrought aluminum plate is adequate for many purposes, but does not offer the tolerance control or stress relieved properties that are available in cast aluminum plate. As a result, extensively machined wrought aluminum plate products will not hold machined tolerances and tend to distort out of flat. Cast aluminum tooling plate offers much better dimensional stability.
Cast aluminum tooling plate is not new. In the 1950s, development of large commercial jet aircraft such as the Boeing 707 and the DC-8 required new levels of tooling accuracy and stability. Materials commonly used for tooling at that time included rolled steel, rolled aluminum, plastic, wood and masonite. None of these possessed the stability to assure tooling accuracy within wide temperature ranges for the assembly of giant fuselage and wing sections. Attention turned to aluminum. Its lightweight, non-pyrotechnic properties, workability and ease of machining led to initial trials using wrought aluminum plate as tooling. These failed, as there was no method for initially flattening the plate and the wrought material lacked dimensional stability.
In attempted improvements to wrought plate manufacture, several aluminum companies tried stretching wrought plate as a way of flattening and relieving thermal stress. Although such products were subjected to artificial aging treatments (typically between 250 and 350 degrees F), the procedure did not result in complete stress reduction, and dimensional stability remained inferior to that available in the cast process.
The first efforts to cast an aluminum plate product relied on the open mold method. In this process, as solidification moves through the molten metal, impurities, entrapped gas and porosity are pushed ahead of the liquid-to-solid transition zone. Heat is removed unevenly and only in one direction, so the quality of one plate surface is noticeably inferior to that of the opposite surface. The varying surface quality and non-uniform thermal stress cause distortion during machining.
Subsequent approaches to cast plate manufacture utilized vertical direct chill casting (pouring molten metal through a water-cooled collar to produce an ingot 24 or more inches thick). The ingot was stress-relieved after casting (to relieve the high stress levels induced by casting), saw cut longitudinally to required plate thickness, and then surface machined to finished dimensions. Quality improved, overcoming earlier problems of inclusions, internal soundness and grain size. However, saw cutting and machining to finished dimensions (after thermal stress relief) still resulted in a dimensional stability problem.
After considerable research, in the early 1960s Alcoa began production of a cast plate product using a DC horizontal casting method to produce a relatively thin ingot. The process was a significant improvement in equalizing the solidification process. Each thickness was individually cast and then surface machined (milled) to reach finish gauge. Currently, this product is known as Alca Plus™ and remains a good quality aluminum plate for general use.
Ultimately, it was a process pioneered by Hunter Engineering that led to today’s most advanced cast aluminum plate. The company, a forerunner of Alumax, developed a horizontal, continuous caster to produce individual plate thicknesses. Alumax refined the technology to bring dimensional control and stability to the next level. The original product (named CC-70) has evolved to become the state-of-the-art material known as Mic-6® Precision Machined Cast Aluminum Plate. Alcoa, which acquired Alumax in 1998, now produces and markets Mic-6® Precision Machined Cast Aluminum Plate at its Mill Products facility in Lancaster, Pennsylvania.
The Mic-6® manufacturing technology involves a patented continuous casting technique that allows precise casting to near net thickness – the “as-cast” thickness is only a few thousands of an inch greater than the final plate thickness. Using a continuous flow of molten aluminum, casting speed and the rate of solidification are tightly controlled. A spinning nozzle inert filtration (SNIF) unit provides additional filtration and degassing, virtually eliminating internal defects. Proprietary equipment removes heat from both plate surfaces at a carefully balanced rate so thermal gradients are stabilized. As a result, grain size and distribution are identical on both plate surfaces!
This unique casting process gives Mic-6® a granular structure that resembles a honeycomb (see Figure 1). In the solidification process, aluminum (which has a higher melting point than the alloying ingredients) forms the cell core with alloy elements concentrated between the cells. This segregation of low and high melting point compositions imparts characteristics that make Mic-6® highly machineable, producing small, uniform chips in a variety of high speed operations.
After casting, the Mic-6® plate is subjected to thermal treatment in excess of 700 degrees F for up to 10 hours. The resulting product is fully stress relieved (fully annealed), eliminating stresses that developed during casting as well as any heat-treating effects that may have occurred during or after solidification. The material is “dead soft”, comparable to the “O” temper designation in wrought aluminum.
These steps result in a significant design benefit for Mic-6®…there is no decrease in mechanical properties when the plate is exposed to elevated temperatures. This is true for both extended periods of high temperature and cyclical exposure (even an infinite number of thermal cycles). Once the Mic-6® plate is returned to room temperature, its tensile properties are the same as they were prior to exposure. There is no over-aging, which occurs in solution heat-treated plate, or the partial annealing that is characteristic of strain-hardened material. Accordingly, Mic-6® is recommended for applications where high temperature conditions would be a problem for wrought aluminum tempers.
Surface machining to final gauge and a precision surface finish is the final step in manufacturing Mic-6®. The product’s process technology minimizes the amount of surface removal required to less than one-tenth of an inch per side. In other cast plate processes (such as open mold, DC ingot slabbing and thin DC casting), it is necessary to remove as much as 50% or more of the material by surface milling. The limited surface machining of Mic-6® minimizes both friction heat and the effects of tension and compression, which contribute to metal stress and cause distortion in other plate products. In many applications, Mic-6®’s precision machined surface finish of less than 20 micro-inches eliminates the need for downsteam milling or surface grinding.
Mic-6® Precision Machined Cast Aluminum Plate undergoes 100% inspection. Laser equipment unique in the industry is used to individually check each plate to verify that it is within specification for flatness and thickness. Finally, a strippable PVC film is applied to one or both sides (whichever is specified) for protection during handling and intermediate processing, and the plate is securely carton packed for shipment.
The entire Mic-6® manufacturing process was designed and perfected to minimize internal material stress. Stability is the product’s defining characteristic, allowing for downstream processing while maintaining precise control of tolerances and dimensionality. That offers the design engineer a flat, stable aluminum plate product that is cast to near net thickness, finished on both sides, and readily machineable with precision results on a full range of high speed automatic equipment.
Uses for Mic-6® Precision Machined Cast Aluminum Plate span nearly every industry that requires flat, stable aluminum plate for general tooling and precision components. A partial list of markets where Mic-6® has been an unparalleled success includes:
Aircraft Office Machinery
Automotive Packaging Machinery
Building Materials Pharmaceuticals