Casting answers & advice.

Q We have a customer that is asking for a finer surface finish from our sand castings than we can provide. Is there an industry standard we can use to tell the customer the typical cast surface finish that can be expected?

While there is no single industry standard for surface finish, tools do exist that can help you illustrate standard capabilities to your customer. But before you turn to those tools, consider changing your processes to improve surface finish.

The phrase "as-cast surface finish" typically means the roughness or smoothness of the final casting on unfinished surfaces. Those surfaces remain in the condition produced by the interface of the metal with the mold material.

Different casting processes produce different as-cast surfaces. Traditionally, metal mold processes (diecasting, permanent mold, etc.) and investment casting provide finer surface finishes than sand casting due to the roughness of the sand grains and metal penetration between them. Sand cast surface finish also can be affected by conditions unrelated to the molding media, such as metal temperature, mold coatings, shrinkage effects, reactions at the mold-metal interface and turbulence during pouring. But, even if these factors are controlled, sand cast surface finishes can vary greatly.

Because the surface finish reflects the properties of the sand used to make the mold, the first place to look to improve as-cast surface finish is the sand system. Excessive surface roughness can be an indication of a poorly maintained system. Properly testing your sand should identify trends in the system that lead to rougher surface finishes. Finer sands can improve typical sand cast surface finish but may reduce other vital sand properties such as permeability. Some metalcasters have successfully improved cast finish by using specialty sands for the surface of the mold directly facing the cast surface.

There is no single definitive industry specification on casting surface finish. Some of the ASTM specifications may contain references to casting surface finish, and metal-specific specifications exist, such as the Aluminum Association handbook for castings.

Many metalcasters use a surface comparator gauge as a reference. Surface comparators provide customers a comparison of typical surface conditions for castings produced from different processes. Additionally, metalcasters can use a comparator to provide the customer with an indication of the expected as-cast finish for their process.

Q We are scheduled to make an aluminum casting that will include a cast-in-place iron insert. What can we do to ensure a strong bond between the metals?

Cast metal components often demand isolated features that can provide different properties than the surrounding base metal. This process (sometimes called bi-metal casting) is fairly common in aluminum casting, where a customer may want the weight advantage of aluminum for the base but require properties in a particular area of the casting that exceed those of the aluminum alloy.

Ferrous inserts can be inserted into the casting after production or set in the mold as a cast-in feature. When a cast-in feature is required, adequate bonding between the outer matrix (aluminum) and the ferrous insert ensures the insert will stay in place during the rigors of final component use.

Due to the difference in melting temperatures, when molten aluminum contacts the iron insert, it is not hot enough to melt and penetrate the surface of the iron and create a metallurgical bond between the two metals. The only bond that holds the iron insert into the final casting is the mechanical bond created as the aluminum solidifies and shrinks tight onto it. If the iron insert repels the aluminum, creating a gap between the two metals, this mechanical (shrink fit) bond is weakened.

The most critical key to successful use of iron inserts in aluminum castings is the surface condition of the iron component. For maximum bond, iron inserts should be:

* Clean. The surface of the iron insert must be free of rust, dirt and grime, sand, cutting fluids or oils, moisture and other surface conditions. If the iron insert has substantial surface imperfections, sand blasting may be required to restore the surface cleanliness.

* Pre-Heated. Preheating the insert will help improve cleanliness by burning off surface oils and moisture, and preheating also reduces the temperature differential between the iron insert and the molten aluminum. If the iron is room temperature, it can cause a chili effect, solidifying the aluminum on contact and creating a gap between the two metals.

* Rough. A very smooth surface on the iron can make it more difficult for the aluminum to shrink tightly to the surface on solidification. A rough or coarse surface finish can aid the mechanical bond. Geometric features on the insert also can help improve the bond. If there are undercuts, grooves on the surface of the iron insert or features such as protruding ears, the aluminum will form around these features.

In addition to mechanical bonding, some aluminum metalcasters dip their iron inserts into molten aluminum in an attempt to develop a metallurgical bond. If the iron sits in an aluminum bath for a period of time, the molten aluminum will begin to dissolve the iron and erode the surface. The metalcaster then will remove the iron from the aluminum, place it directly into the mold and pour immediately. The aluminum metal front in the mold then will etch into the surface for a metallurgical bond with the iron.

Recommendations are the opinion of the AFS Technical Dept. based on referenced literature and experience, If you need assistance with a technical issue, fax or email your question to: Casting Answers & Advice, c/o MODERN CASTING, at 847/824-7848 or moderncastingOafsinc.org.