Imprecise molding
Preforming is the first step in the manufacture of metal products. Everything that has to do with casting in the broadest sense counts as primary forming. In this process, previously liquid and hot metal is transformed into a cold and solid form.
It is irrelevant whether the metal is poured into a defined mold or initially comes out of the furnace as a fairly raw strand. In either case, the result is never what you are used to from metal processing.
Primary molded metal products are not dimensionally accurate and usually have a rough surface. This is partly due to the casting process. For the most part, however, a characteristic feature of metals is responsible for this: the volumes shrink during solidification.
Barely maintainable final dimensions
The shrinkage of metals can only be factored into the final shape with a large tolerance. Technically resilient components that are installed in a composite of other mechanical components are therefore rarely produced by primary forming.
At best, retaining pieces or housings that have no function in the transmission of dynamic forces can be produced by primary forming. Forming and machining processes are required for all other tolerance requirements.
However, there is another circumstance that often goes hand in hand with the casting of metal: the formation of pores.
Classification of pores
Pores are formed when metal solidifies. These are bubbles that form in the metal and are enclosed by the solidifying material. This is more or less unavoidable. However, they occur more frequently on the surface and decrease more and more towards the inside of the material.
It is therefore all the more important to classify the pores as precisely as possible in order to design the material accordingly for the components. A basic distinction is made between microporous and macroporous surfaces.
For the exact classification, however, it first depends on the type of stress to which the component is to be subjected.
Load type “S” refers to all components subjected to static, i.e. static, loads. The forces prevailing in the component after installation no longer change until it is removed. This is the usual load type for supports, cantilevers, fixed bearings, etc.
The stress type “D” refers to all dynamically stressed, moving components. The forces acting on them are constantly changing and so are the stresses that pass through the component. This type of stress is considerably more conducive to wear than stress type “S”.
The stress type “F” is components with stresses on the contact surfaces to other components, the so-called “functional surfaces”
The last type of load bears the letter “G”. It is intended for indeterminate or undefined load cases.
The porosity is specified as a percentage for stress classes G, S and D. For stress class F, the number of pores on a defined reference surface is assumed.
In addition, the diameter or the mean value of the diameter of all pores on a reference surface is also a quality feature of porosity. This can be used to determine the pore classes in advance.
Addition to the pore classes
The pore classes must be estimated based on the load cases. The more porous a component is, the less it can be dynamically loaded. The additional conditions were introduced to narrow down the pore classes.
Addition AN: This abbreviation defines the distance between the pores
Addition M: The “center of the component wall” addition M is important if there are pore nests in the core material.
Addition C: C refers to accumulations of material.
Addition R: This addition refers to the core area of the component wall
Addition Pn: The pore size is considered here.
The additions can be added up as required. However, more than five additions are not usual for determining pore classes.
Pore classes in practice
Typical pore classes are combinations of letters and numbers that can be interpreted based on the parameterization.
Example: Pore class F5/3.5/a0.6/P0.7
This means the following classification according to the pore classes:
- A component has special requirements for the functional surface
- The component has a maximum of five defined pores on the defined reference surface
- The maximum permissible size of an individual pore is 3.5 millimeters in diameter
- The minimum distance between two pores is 0.6 times the distance of the smallest pore
- Pores up to a diameter of 0.7 millimeters are not taken into account
With the help of these pore classes, the characteristic properties of the material, the geometry of the component and finally its calculated stresses, it is possible to define very precisely how it will behave in use.
The classification of a component into pore classes is therefore an important part of quality measurement.
The methods available today can determine these statements very precisely. Common tools for determining the pore classes are the microscope and the ultrasound device.
