Using Cast Inserts
Cast-in inserts are the Botox of metalcasting.
Just as cosmetic surgeons inject the botulinum toxin into faces to make them look smoother and younger, casting designers can—with the help of cast-in inserts—put together a metal casting that has all the right things in all the right places, in exactly the right proportions.
As a designer of metal castings, you might have a base metal in mind that offers the properties that your engineered metal component requires. But you might be looking for a little extra something in one area of the part. Depending on the application, a small piece of metal cast into the structure of the larger component could be the answer to your problem.
“You have to look at application, location on the casting and cost,” said Roger Reamer, former permanent mold manager for Progress Casting, Minneapolis, and current instructor for the Cast Metals Institute, Schaumburg, Ill. “But anything you want to cast in, you can, if you put your mind to it.”
How to Spot a Patient
So how do you indentify a candidate for a cast-in insert (also known as bi-metal casting) in the first place?
According to several metalcasters that often work with cast-in inserts, the most common application for bi-metal casting is wear resistance. In many of these cases, the designer requires the light weight of a nonferrous casting with the wear resistance of iron or steel. Aluminum cylinder block designers have been drawing on this principal for years by inserting iron sleeves in the components’ cylinders. Specialty metals like copper and stainless steel offer a riff on this theme. Copper can offer corrosion resistance where fluids might be transferred; stainless steel can resist rust when in contact with water. Daniel Burnstein, chief executive officer of Burnstein von Seelen, Abbeville, S.C., even cites a customer that required magnetized steel be cast into a base metal so that the part could be operated by magnetic forces.
Wear and corrosion resistance are the most popular applications for inserts, but they are only the beginning for a designer in need of specific characteristics. If the base metal of a casting is difficult to machine, for example, an insert can be used to create a more machinable area on a critical surface. Inserts also can be used when a more pleasing appearance is desired.
Oftentimes, the goal of casting in an insert is to eliminate expensive and time-consuming post-casting operations, thus decreasing the overall cost of the part. When a tube is required for transport of liquids or gases, drilling and tapping can be eliminated by casting in a tube of a different material.
“The second most common application for inserts is tubing cast in,” said David Weiss, Eck Industries, Manitowoc, Wis. “Ferrous tubing can be used for oil lines or air lines. Typically it is steel tubing, but sometimes even aluminum tubing is used.”
Applications requiring tubing can include air cooled cylinder heads, copper tubing inserts into aluminum for heat transfer and water-carrying cooling tubes for secondary transformers.
“You can’t drill in some of the tubing configuration that you can cast in,” Burnstein said. “And oftentimes, you eliminate the assembly, that’s a cost saving venue, and you can maybe come up with a superior part.”
Designing for a Bodacious Bond
Designing an effective part with a cast-in insert is an exercise in teamwork. You will envision the part, but your metalcaster will be the surgeon in the operating room, making sure the injection of properties goes as planned. Depending on the type of bond you desire between your casting’s base metal and the insert, your metalcaster will have a set of guidelines to follow to ensure he does his job correctly.
Your cast-in insert can be formed to the base metal with one of two bonds—mechanical or metallurgical. The first is more common; with it, the insert is placed into the casting mold, and the molten metal flows around it before shrinking tight to its shape. On a microstructural level, the insert is untouched. When a metallurgical bond is achieved, however, the two metals actually fuse together. This can be an advantage depending on the additional properties required out of the insert, but it is a practiced art.
“The re-melting of an insert’s alloy is harder to control,” Burnstein said.
Assuming the metalcaster does his part in achieving the proper mechanical or metallurgical bond in the metalcasting facility (see sidebars), all you have to worry about is how to design your part for optimum insert efficiency.
A metallurgical bond might be more difficult for your metalcaster to achieve, but more of the onus is on the designer when a solid mechanical bond is desired. “You have to design the insert properly,” Reamer said.
Specifically, the shape of the insert and its location should be considered. Intricately shaped inserts operate most efficiently. Ridges, sharp corners or undercuts give the base metal more to grab onto.
Burnstein offered an example to support the point. His company casts a part in copper that requires the high conductivity of the base metal. The part also requires a wear resistant attachment point so it can be bolted to a larger assembly. So, Burnstein von Seelen casts a ferrous bolt into the copper base metal. Because the insert is designed with a hex-shaped head and is threaded up to its neck, the mechanical solidification around the bolt maintains sufficient strength for the application.
Reamer also recommends that the designer consider what portions of the casting will be load bearing before dictating the location of the cast-in insert. While Burnstein said the bond can be as strong as a cohesive metal component, repeated loading and unloading could cause an insert to loosen in its seat.
A designer might desire a metallurgical bond if energy transfer from the insert to the base casting is required. In most instances, the energy transfer desired between a cast-in insert and the base casting is either heat dissipation or electrical conductivity. In high heat applications where a casting is required to cool quickly, the insert must transfer the heat efficiently to the base metal. Only a metallurgical bond can pull it off.
A metallurgical bond can be achieved through a special process performed by the metalcaster, or it can be designed for with the use of an alloy with a similar or lower melting point than the base metal. This is often done with aluminum metal matrix composites (MMCs). Because they have the same melting point as aluminum, MMCs can be specified as a high-strength insert that will re-melt when the base metal is poured, thus forming a metallurgical bond.
“You absolutely achieve a more efficient heat transfer [with a metallurgical bond],” Weiss said. “We designed [a high-heat part] so that we got partial re-melting of an MMC insert and had significant improvement in heat transfer.”
Reamer described a similar situation in which a customer required a current-motor capable of transferring electricity between a coil and the base casting. A metallurgical bond solved that problem, as well.
The heat and electrical transfer desired by Weiss and Reamer were necessary attributes when designing an insert for their specific applications. However, energy conductivity is a secondary need. The difficulty only arose because the castings already required an insert. METAL
This article was originally published in the March/April 2009 issue of Engineered Casting Solutions.
SIDEBAR: Types of Bonding for Inserts
The metalcaster can achieve one of two types of bond when casting an insert into a larger cast component—mechanical or metallurgical. For the former, the insert is placed into the casting mold, and the molten metal flows around it before shrinking tight to its shape. On a microstructural level, the insert is untouched. When a metallurgical bond is achieved, however, the two metals actually fuse together.
For a traditional cast-in insert, where the melting point of the inserted metal piece is higher than the base metal, the metalcaster must ensure that the surface of the insert is prepared for bonding. For the best results, the surface should be:
Clean. Any foreign material on the surface of the insert, including rust, dirt, grime, sand, cutting fluids, oils or other moisture, can cause a reaction when the base molten metal contacts the insert, resulting in a gap between the metal pieces. The metalcaster should sand blast any potentially dirty inserts.
Preheated. Preheating an insert serves two purposes. First, it cleans the insert by burning off foreign materials. Second, it reduces the difference in temperature between the molten metal and the insert. If the insert is too cold, the chill effect can create a gap between the metal pieces.
Rough. A coarse surface aids in the mechanical bond, as the molten metal is better able to form around features. Metalcasters can place inserts in a shot blast for short periods of time to achieve a rough surface.
According to Roger Reamer, former permanent mold manager for Progress Casting, Minneapolis, and current instructor for the Cast Metals Institute, Schaumburg, Ill., additional steps must be taken if the metal insert has a lower melting point than the base metal. By spraying such pieces with the material used to coat molds in permanent mold casting, they can be kept from melting.
The process used by the metalcaster also can affect the quality of the mechanical bond. Low pressure permanent molding, for example, can produce an improved bond due to the pressure on the metal front as it solidifieas in the mold cavity. High pressure diecasting, though, can cause problems, as inserts can shift due to the more-quickly advancing metal front.
A metallurgical bond can be achieved by the metalcaster in one of two ways. If the melting point of the insert is at or below the base metal, remelting of the insert can occur naturally, and under carefully controlled conditions, a microstructural bond occurs. If the melting point of the insert is higher than the base metal, the process is more complex.
To form an intermetallic iron and aluminum bond, for example, a process known as the Alfin Process, patented by Fairchild Aircraft in 1941, can be used. In this method, an iron insert is held in an aluminum bath for three to five minutes, during which the aluminum etches into the iron. The iron then forms a metallurgical bond with aluminum when the base metal is poured around it in the mold. METAL
--By Shea Gibbs, Metal Casting Design & Purchasing