ALUMINUM STRIP CASTING TECHNOLOGY
Each new Hazelett twin-belt aluminum strip casting machine incorporates the latest mechanical, electrical, and process improvements and delivers ever-increasing production volumes and product quality.
The technology incorporated into the Hazelett aluminum strip casting machine has evolved over decades of innovative in-house engineering and design as well as at the locations of our partners where casting trials are conducted.
closed pool metal feeding
When casting lower temperature metals such as aluminum, a fully closed pool "injection" metal feeding system can be employed. The metal is introduced under slight pressure into the closed mold cavity, and the metal flow is automatically controlled by maintaining a preset level in the tundish.
metal feeding tips (snouts)
The feed tip is machined from a special fired ceramic material that is thermally stable and permeable to any gas that may be released from the flowing metal as it passes through.
To ensure an even velocity across the full width of the mold and to maintain consistent localized cooling rates at the mold surface, Hazelett has developed a full-width slotted snout, the Strip-Stream™ snout. To provide the needed structural strength, a special vacuum-formed aluminosilica fiber material is machined in Hazelett’s snout manufacturing facility.
The belts are made of low carbon steel, stretcher leveled in Hazelett's shops, and held under tension on the casting machine to ensure flatness and accuracy. However, as a cold belt enters the mold region at widths less than the entire belt width, it is subjected to powerful "cold framing" forces that must be controlled to eliminate buckling and reduce thermal distortion of the belt at the mold entrance. These forces are augmented by thermal bending ("oil canning") forces resulting from the temperature differential through the belt thickness. These forces are controlled by adding induction belt preheating and magnetic back-up rolls.
For wide strip casting, a powerful induction belt preheating system with heating coils that span the full width of each belt, is capable of bringing the belt up to 150°C (302°F) or higher immediately prior to entering the casting mold. The system heats the edges of the belts where metal will not be applied, reducing cold framing. In addition to preventing thermal distortion, the high preheat temperature serves to eliminate any moisture present on the surface of the belts.
magnetic back-up rolls
When casting wide strip, any tendency of localized thermal distortion is resisted by the use of an array of high-strength, magnetic back-up support rolls. The moving belt is held against the back-up support rolls by powerfully magnetized rotating fins that maintain the belt in a precise, flat plane. The magnetic forces, when combined with the inherent beam strength of the 1.2 mm (.048") thick steel belt, effectively resist any distortional forces.
The ability to maintain belt flatness is only possible with use of neodymium magnetic material on the roll shaft which, unlike conventional magnets, creates a powerful field that maintains its strength over a reasonable distance. This unique "reach out" magnetic field magnetizes the fins as shown above and extends the belt stabilization area beyond just the fin-to-belt contact points.
Molten metal solidifies adjacent to the belts, forming a shell that encloses a liquid sump as the slab proceeds down the mold. Solidification is normally complete approximately 70% to 80% down the mold length. The critical area of the mold is the interface between the solidifying metal and the moving, water-cooled belts.
The key components of the mold interface include the permanent Matrix™ belt coating insulation and topography, and the consistency of thickness and quality of any additional overcoat or parting agent applied before or during casting. The atmosphere is controlled in the mold interface with the use of a gas layer.
The Matrix™ or textured coating is thermally or plasma sprayed onto the belt surface, which has been grit blasted to a specific topography. Various powders are blended and applied at the appropriate thickness to achieve the desired thermal transfer characteristics according to the alloy to be cast. By controlling the bond strength, an extremely wear-resistant coating is produced.
Inert gases are injected via small ports in the casting tip (snout) support apparatus directly onto the incoming belt surfaces immediately prior to contact with the molten metal. The gases infiltrate the mold interface to control oxidation and, more importantly, heat-transfer rates. A low-pressure application of the gases ensures even distribution. An important advantage of the system is the ability to easily, quickly, and selectively change the gas makeup and/or volume across the belt width to accommodate changes in belt cooling rates, as indicated on the exit slab temperature scanning image.
Hazelett aluminum strip casters may feature an ESP™ coating system, which is a unique patented ElectroStatic Powder deposition system. This system allows a continuously controlled application of very fine powders, including fumed silica. The application rate can be adjusted for alloy and plant atmospheric conditions.
exit temperature image profiling
To provide the machine operator with real-time feedback of the as-cast strip conditions, the Hazelett aluminum strip casting machine includes an infrared thermal imaging line scanner mounted at the exit of the casting machine. The thermal scanner image is displayed in the control pulpit. Adjustments to various process control parameters, including mold interface gas levels, drive motor torque relationships, and tundish head level, are based on this image.