Some designs impose conditions that exceed the capabilities of the selected alloy. These conditions include:
- Excessive wear on tapped threads caused by frequent removal and reinstallation of fasteners
- Concentrated loads requiring external or internal threads with very high strength
- Abrasion or wear, typically at bearing points
- Continuous long-term loading or cyclic loads that make it necessary to limit casting stresses to relatively low levels
Threaded inserts made from a material with properties superior to those of the die casting are used under these conditions.
Types of inserts
Inserts may be cast-in-place or post-installed. This is determined first by function, then by economics.
Inserts may be knurled, grooved, splined or eccentrically shaped to develop the necessary anchorage for the anticipated loading. They should be configured to avoid sharp corners, projections surrounded by thin sections of die cast alloy, or other factors that lead to stress concentrations in the casting.
Insert materials: galvanic considerations
Steel, brass, and bronze inserts are commonly used with all die casting alloys. Zinc alloy inserts are sometimes used with magnesium castings. Although ferrous and copper alloys are not galvanically compatible with die cast alloys, most of the contact surface is within the casting, in which water is effectively sealed out. If galvanic corrosion is a potential problem at the external interface, the dissimilar metals may be surface treated, plated, or insulated.
Controlling stresses induced by inserts
Die cast inserts, whether cast in place or post-installed, induce residual stresses in the casting. Residual stresses must be analyzed to determine whether there will be problems in long-term retention.