Die casting Life and Failure
Proper selection of the die material and of the die manufacturing technique determines, to a large extent, the useful life of forming dies. Die may have to be replaced for a number of reasons, such as changes in dimensions due to wear or plastic deformation, deterioration of the surface finish, breakdown of lubrication, and cracking or breakage. In hot impression die forging, the principal modes of the failure are erosion, thermal fatigue, mechanical fatigue and permanent (plastic) deformation.
In erosion, also commonly called die wear, material is actually removed from the die surface by pressure and siding of the deforming material, wera resistance of the die material, die surface temperature, relative sliding speed at the die/material interface and the nature of the interface layer are the most significant factors influencing abrasive die wear. Thermal fatigue occurs on the surface of the die impression in hot forming and results in “heat checking”. Thermal fatigue results form cyclic yielding of the die surface due to contact with the hot deforming material. This contact causes the surface layers to expand, and, because of the very steep temperature gradients, the surface layers are subject to compressive stresses. At sufficiently high temperatures, these compressive stresses may cause the surface layers to deform. When the die surface cool, a stress reversal may occur and the surface layers will then be in tension. After repeated cycling in this manner, fatigue will cause formation of a crack pattern that is recognized as heat checking. Die breakage or cracking is due to mechanical fatigue and occurs in cases where the dies are overloaded and local stresses are high. The dies are subjected to alternating stresses due to load and unloading during the deformation process, and this causes crack initiation and eventual failure.
Die life and die failure are greatly affected by the materials properties of the die materials under the conditions that exist in a given deformation process. Generally, the properties that are most significant depend on the process temperature. Thus, die materials used in cold forming processes are quite different from those used in hot forming.
The design and manufacture of dies and the selection of die materials are very important in the production of discrete parts by use of metal forming processes. The dies must be made by modern manufacturing methods from appropriate die materials in order to provide acceptable die life at a reasonable cost .Often the economic success of a forming process depends on die life and costs per piece produced. For a given application, selection of the appropriate die material depends on three types of variables:
(a)Variables related to the process itself ,including factors such as size of the die cavity ,type of machine used and deformation speed ,initial stock size and temperature, die temperature to be used ,lubrication ,production rate and number of parts to be produced.
(b)Variables related to the type of die loading, initial speed of loading, i.e., impact or gradual contact time between dies and deforming metal (this contact time is especially important in hot forming), maximum load and pressure on the dies, maximum and min. die temperatures, and number of loading cycles to which the dies will be subjected.
(c)Mechanical properties of the die material, including harden ability, impact strength, hot strength (if hot forming is considered) and resistance to thermal to thermal and mechanical fatigue.