SELECTING ALLOYS FOR SPECIAL CASTING APPLICATIONS

by Jon P. McGraw

Alloy Casting Co., Inc.

This article originally appeared in Fabricator magazine, Vol. XXXVIII, No. 5, September-October, 1997

The aluminum alloys chosen to make ornamental castings and other special aluminum castings are chosen by Alloy Casting Co. from a family of alloys noted for providing good detail and good weldability, while at the same time having good foundry characteristics. These alloys contain silicon for good detail retention and flowability, but do not contain elements detrimental to weldability, i.e. Nickel. Nickel is detrimental because of its high melting temperature.

A job requiring an enhanced set of properties like more strength, more corrosion resistance, or more bendability can be obtained by selecting alloys to provide those properties. Alloy Casting can provide the aluminum castings necessary for those special requirements. Below are listed some of the projects we have successfully completed using specific alloys.

BENDABLE CASTINGS

Fabricators in the trade desire for jobs to look good. It is also sometimes helpful if they look unusual. One way to achieve this combination is for the castings to be bent into unusual shapes. Most castings have metal combinations that are stiff and difficult to bend however Alloy Casting Co. can provide alloys that have proven to be malleable.

FIGURE 1 is of a rose panel casting. It is 8" x 20" and has stems 3/8" in diameter with a varied cross section of stems, leaves, and flowers. We were able to achieve a bend of 3" in a 20" length. A bend of more than 3" would cause cracking because of the difference in bending between the varied cross sections.

Casting #790, shown in FIGURE 2, shows a more consistent cross section of 3/8" x 1 1/4" therefore we were able to achieve a much larger bend of 10" in a 27" length.

FIGURE 3 shows the most dramatic bends. These have been obtained in a leaf design, 4 1/2" X 40", with a consistent 3/16" thickness. These leaves were specially designed and the alloy chosen to provide maximum ability to bend.

Good bending results can be obtained at room temperatures. When bending you can feel the casting stiffen as work hardening takes place. The twisting and compound bends of the leaves were obtained by beating the casting, bending, and reheating as necessary. It is a trial and error process but very dramatic shapes can be fabricated in this way.

FIGURE 1

FIGURE 2

FIGURE 3

WELDABILITY

Most aluminum castings provided by foundries to the ornamental trade are provided in the "as cast" condition (temper F). These castings are readily weldable and no special cautions are necessary. The casting alloy is normally in the 300 Series having good castability for the foundry and good detail retention and weldability for the fabricator.

In those cases where there is a special application for high strength and/or corrosion resistance, extra care is necessary in the selection of welding rod. It must be remembered that a casting that has been designated with a special beat treating requirement cannot be welded without destroying the heat treatment in the area affected by the weld. The fusion of the weld and base metal of the casting changes the microstructure of the metal and transforms the casting in that area back to an "as cast" condition.

CORROSION RESISTANCE

The alloys that have good corrosion resistance show poor casting characteristics for the foundry and poor weldability to the fabricator. FIGURE 4 is a handrail panel that was made from alloy 514 (4% magnesium) and was used in a sea coast application. Alloy 514 exhibits good corrosion resistant properties. However, alloy 535 (7% magnesium) because of its higher magnesium and lower and tighter specifications on trace metals, shows a higher strength and a superior corrosion resistance. 514 was specified for the job but for the same cost, alloy 535 could have been used. A situation where the specifier did not take full advantage of all available options. These two alloys are from the fame family as the sheet and plate used for marine applications. Those metals that strengthen aluminum the most, such as copper and zinc, are also the most detrimental to its corrosion resistance.

FIGURE 4

STRENGTH/HEAT TREATMENT

Those alloys exhibiting high strength are normally poor in other characteristics. Alloy 713 (7% zinc, 1% copper) was chosen for the chair leg piece shown in FIGURE 5. Alloy 713 has high strength in the "as cast" temper and needs no heat treatment for maximum properties. However, the alloy pours like molasses, has a high shrinkage rate, and tends to lose detail. Fortunately, the rectangular leg cross section of 5/8" x 1" is plain and simple and showed shrinkage only in the hidden back side of the legs.

Another alloy commonly used in the "as cast" temper is alloy 319 (7% silicon and 3% copper). In this alloy, silicon has replaced the zinc and imparted more flowability and detail retention. The strength loss through the elimination of the zinc has been partially compensated for by an increase in copper. FIGURE 6 shows some castings that have been cast in alloy 319 for god machinability. The holed clutch housing is designed to break under certain conditions and Alloy 319 imparts a consistent microstructure for that predictable breaking.
 

Alloy 356 (7 % silicon, 1/4 % magnesium) is the aluminum foundryman's all purpose workhorse. It has good detail retention and pours well. It is the most commonly used alloy and is specified in either the "as cast" or heat treated condition. The "as cast" condition strength levels of 19,000 psi (pounds per square inch) can be raised to 34,000 psi in the T-6 temper. T-6 temper is soaking at 1000 degrees Fahrenheit for 12 hours followed by an aging treatment of 310 degrees Fahrenheit for 3-5 HOURS. This strengthening occurs because of a hardening process caused by the 1/4% magnesium. The heat treatment has a powerful influence on properties but can increase the casting cost by as much as 20 to 40 per cent.

FIGURE 7 shows some industrial tire spreaders that have been cast in Alloy 356 and tempered to T6. The extra strength was a requirement because the heat treatment improves the metal's "as cast" microstructure and any subsequent welding will destroy the benefits of the heat treatment.

Because of its reasonable cost and high strength, alloy 713 would always be chosen if strength and cost were the only considerations but because of other needs like corrosion resistance, weldability, detail retention, and castability, Alloy 356 and other alloys are more frequently specified.

POLISHING

Some alloys polish better than other. The high strength Alloy 713 has good luster, but for my personal preference and use, I have chosen Alloy 514 (4% magnesium) to impart a beautiful polish and high luster to the sea shell cocktail table shown in FIGURE 8. The sea shell and legs of this table are now four years old and have the same shine as they did when they were first professionally polished.

SUMMARY

The job requirements for most ornamental work can be handled with decorative castings made from the common, everyday alloys. But for that special job requiring a unique set of physical properties, it is good to know that other alloys are available that can provide those special characteristics. Strength, corrosion resistance, weldability, luster, and bendability can all be modified with an alloy that can give that blend of unique properties.

CUSTOM CASTINGS

by Jon P. McGraw

This article originally appeared in Fabricator magazine, Vol. XXXVIII, No. 3, May-June 1987

UNDERSTANDING CUSTOM CASTINGS

by Jon P. McGraw

This article originally appeared in Fabricator magazine, Vol. XXXIIII, No. 6, November-December 1992
 

CUSTOM-DESIGNED ALUMINUM CASTING PATTERNS

by Jon P. McGraw

This article originally appeared in Fabricator magazine, Vol. XXXV, No. 6, November-December 1994

Once a fabricator provides a dimensional sketch for a custom casting much of the work is already done. From that point, the foundry can provide a quote for a turnkey program that includes creating the master pattern, fabricating the tooling, and manufacturing the final product. The only part that should concern the fabricator is the creative concept and creating the sketch.

Like most custom projects, creating the sketch is the easy part. Selling, fabricating, and installing the job are always more involved. One of the most encompassing projects that Alloy Casting has completed was a custom project for a locksmith shop in West Palm Beach, Florida The program included five custom castings of various sizes.

The owner of a Miami Beach, Florida home was renovating a mansion to its original 1920s design. The source document for the design was a 1920s newspaper article containing a photograph of the house's original exterior. The newspaper photograph was studied and the -ornamental iron work was redrawn to a full-scale drawing, which included three of the five custom castings. The castings were to be used on the exterior of the house as a window dressing and security bards, and in the interior of the house as a room divider.

The sketches were submitted to Alloy Casting for a quote. The quote is for a turnkey project that included five original wood patterns and carvings, four pattern boards, eight sets of castings, one 5/8 inch square core box, and a match plate. A match plate is a piece of production tooling that has the design of the casting imbedded into it. When the foundry's sand is pressed against the design an imprint is left in the sand. It is this imprint that gives the aluminum casting its artistic shape. The core box enables the foundry to make and bake hard sand pieces. These hard sand pieces (called "cores") are then placed in the mold to block out areas of the casting where horizontal holes are needed.

The wood-carver began the project by carving the plaque face and the carved dragons. The full-sized drawing enabled the woodcarver to carve with confidence knowing that there would be no pattern modifications. The master patterns for the round collar (5/8 inch square hole) and tall baluster were turned on a lathe. The square collar (5/8 inch square hole) was created by a pattern maker and given to a match plate fabricator to make the production tooling.

The first phase of the project included only eight sets of castings. With such a small number of castings, four of the five original master patterns were mounted directly on wooden boards to save tooling expenses. Because the dragon faced both left and right it was tooled to be double-faced. This allowed the same dragon to be used for both left and right applications.

The collars and balusters were also double-faced. The face plaque was constructed on a single face pattern board in order to save carving duplication and expense. During the second phase of the project, two single face plaques were placed back-to-back to create a two sided design.

The dragon is double faced and the sun faced plaque is flat back. The tall baluster and the round collars were lathe turned and mounted to wood Pattern boards to provide a double face casting. The square collar, its match plate, and the 5/8 inch core box. The square core print on the tooling enabled a 5/8 inch sand core to be cast into the casting. When the core was removed the result was a 5/8 inch square hole. The collar could then be slipped on to a 5/8 inch square bar.

A custom project for one item typically takes from start of master pattern work to prototype casting about four to six weeks. The exact time depends on the intricacy of the master pattern carving. In the Miami Beach project, it took seven weeks to complete the five original casting designs.

A foundry is organized to handle custom projects. Wood carvers and patten makers are trained to provide sufficient drafts to eliminate undercuts and to compensate for metal shrinkage. Foundries that handle all stages of custom casting production in-house, such as Alloy Casting, insure high quality tooling and prompt deliveries.

Most projects received by Alloy Casting are initiated by a fabricator with a sketch. When a sketch is used, the foundry quotes from this drawing after acceptance. The foundry then handles the project on a turnkey basis from master pattern through tooling fabrications, to final castings.