Permanent molders share successes, production techniques.
Nearly 150 aluminum permanent mold foundrymen shared design and production techniques on unique aluminum castings and learned new technologies when they gathered in Anaheim, California on November 9-11 for the AFS International Conference on Permanent Mold Casting of Aluminum.
In addition to the meeting" 20 technical presentations, plant tour of aluminum wheel manufacturer Ultra Wheel Co. in nearby La Mirada was featured. The tour featured its entire permanent mold operation (gravity-pour and counter-pressure), from casting through heat treating, machining, plating and packaging.
As described below, the conference also included a unique operators' session where the design and production of four unique and technically challenging castings were reviewed.
Eck Industries, Inc., Manitowoc, Wisconsin
Foundryman: David Weiss, vice president-engineering.
Casting: Corvette brake pedal, low pressure cast in 356-aluminum alloy.
Unique Feature: Engineering and production issues.
The auto industry is rapidly moving to reduce vehicle weight, and aluminum and magnesium castings are enjoying more opportunities to replace iron and steel. The new C7 Corvette is a showcase example of innovative uses for aluminum and magnesium castings, including an aluminum engine block, motor mounts and cross-members. This cast brake pedal was converted from a steel fabrication. The cast pedal is a structural, safety critical item that also requires good cosmetics.
The part requirements created interesting foundry engineering issues. The mechanical properties (35 ksi tensile, 25 ksi yield and 7% elongation) were only part of the requirements. The casting also had to meet 0.5 in. deflection before fracture, ASTM E-155 Frame 2 requirements for shrinkage in critical areas and, because the pedal face is visible (no rubber boot is attached), exceptional appearance (including good feature definition) was also mandatory. Additionally, gating access was limited by the mold design that included two moving slides.
Low-pressure casting was chosen for its gating access, improved properties, better process control and lower costs. To solve the structural and mechanical property concerns, tightly controlled modified alloy chemistries (B-356 with magnesium and strontium additions) and a modified heat-treat cycle were used. With very low variations, parts regularly exceed all mechanical property requirements.
The mold was also modified by removing metal above the feed zone so that the die ran hotter to improve feeding to critical areas. Test bars are machined out of critical areas to ensure quality. The appearance issue was handled by mold temperature, injection pressure and control of mold wash.
In the production of structural components "management of alloy chemistry and control of mold temperatures are critical and nonstandard heat treats may be required to attain superior properties," Weiss concluded.
CMI-Precision Mold, Inc., Bristol, Indiana
Foundryman: Greg Guilliams, plant manager.
Casting: 1-meter crossmember (cradle) for Chrysler NS Minivan, tilt-pour cast in B-356 aluminum alloy; 26 lb machined part weight.
Unique Feature: Size, requirements of automotive structural parts and volume (plant improvement issues).
Another example of automotive "lightweighting" is the conversion of structural components like this aluminum cast crossmember. When CMI committed to producing these parts, it also committed to a full plant improvement and expansion process that eventually enabled its Bristol operation to produce 750,000 fully heat treated, machined, X-rayed and assembled crossmembers each year.
Aluminum is charged in 120,000 lb reverberatory furnaces and transferred in electrically heated, covered launders and passed through ceramic filters before pouring. Production process control includes full documentation and precision of all processing parameters, such as precision in chemistry control, master alloy additions for grain refining and silicon modification, heat treat response and mechanical properties.
Guilliams contends that even the best casting, heat-treat and machining facilities "can't consistently make the properties and reliability that today's vehicle manufacturers demand," without meticulous and precise production controls. A molten metal process control that might seem overly elaborate - even overkill - "is an absolutely essential investment when making high-integrity structural castings."
Production is handled via three casting cells, each consisting of a 10 meter turntable with 6 tilt-pour machines complete with automatic pouring, robotic casting extractors and an initial casting inspection station. Three cells can produce 270 castings per hr. A total of 32 molds used in rotation are needed to service the three casting cells. The molds are also equipped with a large number of thermal control circuits to control mold temperature and provide directional solidification, chilling and cycle time control.
Heat-treat is customized and racking is precisely configured to minimize dimensional change. Between solution heat-treat and aging, castings are sent to a dimensional gauging operation that measures six dimensional tolerances and applies force as necessary to straighten. All parts are fully machined on a dedicated machining line. Each casting is placed into a real-time X-ray unit and viewed at 17 critical areas. The unit has artificial intelligence that recognizes defects and accepts/rejects parts.
The total investment to produce this specialized facility was nearly $40 million. "The proof that it was the right commitment - and the right level of investment - lies in the fact that customer return rate is less than 0.5 ppm," Guilliams said.
Foundryman: George Mako, vice president/technical director.
Casting: 48 in. three-blade fan, gravity-pour permanent mold cast in 380 aluminum. Finished casting weight is 22.5 lb.
Difficult Feature: Size and distance from center pour to blade tips.
The mold requires heating to ensure filling to the fan tips and is cooled to control mold overheating. Metal is poured in the riser through a ceramic foam filter set on a core locater pin on the iron core that forms the center hub. The core is plasma spray-coated to minimize erosion (see sidebar).
GRAC uses a manufacturing cell concept for the fan blade production. During the 8-min casting cycle, the molder mounts the fan castings into a fixture and saws off the riser. The parting line flash is removed, the casting is rough balanced and packed for shipping at the casting station work cell.
Progress Casting Group (PCG), Plymouth, Minnesota
Foundryman: Randy Oehrlein, vice president-manufacturing engineering
Casting: Eddy current clutch, low-pressure permanent mold cast in 355-aluminum alloy.
Unique Feature: Cast-in iron quill and machined steel ring.
The real difficulty was casting-in the machined steel ring, which is completely encased in aluminum and then machined to expose the steel. PCG chose the "Alfin" process to assure a bond between the steel ring and the molten aluminum. Patented in 1941 by Fairchild Aircraft, the process forms an intermetallic iron/aluminum bond, but requires precise control to achieve maximum bonding - primarily good steel surface preparation, "tinning" the steel with 99.9% pure aluminum, and consistent, precise timing and temperatures.
First, the cleaned ring is dipped into 99.9% pure aluminum (dip and agitation time 3-5 min). The mold opens and the previous casting is ejected. The mold is blown out and filter screens are placed. The ring is then removed from the bath, shaken to remove excess aluminum and placed in the mold. A pre-glued sand core and the preheated iron quill are set and the mold is closed. Low pressure aluminum injection time is 3 min. The full cycle time is 5.5 min.
Keys to success are good surface preparation, even agitation to form the intermetallic bond, minimum time in the air for the ring (quick setting in the mold), ring and quill placement accuracy and casting ejection without stretching, which can break the intermetallic bond.
Ultrasonic testing is performed to check for discontinuities at the interface of the ring and aluminum.
RELATED ARTICLE: Thermally-Sprayed Coatings
GRAC faced a particularly difficult challenge with the large three-blade fan casting. Although the iron core onto which aluminum was directly poured regularly received a mold coating, it eroded so significantly that it was regularly replaced with new cores.
This "sacrificial" core became a prime candidate for a specially engineered thermal sprayed mold coating. As explained by Bruce Dulin, White Engineered Surfaces (WES), Newtown, Pennsylvania, thermal spraying is a generic term for different de* vices that can melt a variety of materials in either a powder, wire or rod form. The material is atomized and accelerated (particle speeds range 300-500 m/sec) in a molten or semi-molten state, and applied onto a prepared surface such as the iron core.
Thermal spray devices - such as plasma, low and high velocity combustion and electric arc - provide different levels of energy and heat. The diversity of thermal energy, heat, application velocities and materials allows custom-designed coating properties. Although used in many industries, plasma coating's use in permanent mold casting is still relatively new.
For the GRAC casting, a plasma-sprayed zirconia material was used. The iron core was coated with the zircon thermal coating and put into the same production conditions as the "sacrificial" core. The plasma-coated core was used for 8000 castings before the job ran out and the mold was retired. "The total life of the coating is not known because further testing was stopped with the completion of the job," said Dulin.
While Dulin has not conducted a full-mold thermal coating, he speculated that the technology and engineering is available and would only take the right mold application and trials to test it. While there are limitations to thermal sprays for special applications and needs, these engineered coatings have already shown their value to the permanent mold foundry industry, he said.
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|Title Annotation:||4th International Conference on Permanent Mold Casting of Aluminum|
|Author:||Robison, Stephen T.|
|Date:||Jan 1, 1998|
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