Basic Knowledge These are the Most Common Post-Treatment Methods
Castings are most widely used in the automotive and aerospace industries, but also in mechanical and plant engineering and medical technology. In order to match the properties of the metal specifically to the application, there is no getting around post-treatment of the cast parts.
By post-treating cast components, various mechanical and chemical material properties can be obtained that the untreated metal does not exhibit. The processes differ between those that merely influence the surface structure of the components and those that optimize their internal structure. For example, depending on the industry and application area of the final product, electrical conductivity, hardness or optics may play a special role. But what are the most commonly used post-treatment methods for castings and how do they work?
Heat treatment exposes castings to a series of extreme temperatures and fluctuations (or lack off), which can improve metallurgical properties such as ductility, abrasion resistance and rigidity. Depending on the desired properties and composition of the metal, different methods with different temperatures are used. One of the most common procedures is:
- Quenching: The hot material is cooled very quickly by forced air, water or oil. In this way the metal becomes very hard and abrasion resistant.
- Age hardening: The molecule's ability to dislocate freely is blocked by heating it up to 20 hours at relatively low temperatures. This increases the yield strength and reduces ductility.
- Annealing: To soften the material, it is heated almost to its melting point. Then it is cooled down naturally so that the structure relaxes and becomes less brittle. This also improves formability and machinability, while the hardness is maintained by prior quenching.
- Normalizing: The material is heated similarly to annealing, but then cools down faster. This reduces internal and external stresses, but also refines the grain structure.
- Solution heat treatment: The elements are brought into a solid solution by heating and solidified by rapid cooling. This softens the alloy and makes it suitable for further manufacturing applications.
- Stress relieving: By heating at low temperatures and slow cooling, stresses in the structure are reduced. This minimizes the risk of dimensional deviations during further processing.
Since heat treatment is a costly process, Alcoa has developed new NHT alloys that can compete with conventional foundry alloys in terms of elongation, tensile strength and yield strength. This makes them suitable for casting complex and thin-walled cast parts for body structures in both conventional ICE and electric vehicles.
Due to its low weight, aluminum is a popular material for lightweight design and is therefore an essential material in the automotive and aviation industries. However, aluminum components may have a porous internal structure which endangers the proper functionality of the components. Porosity can cause liquids or gases to escape from the part under pressure, and the part must be scrapped, increasing costs and causing production delays. This problem is countered by vacuum impregnation, which effectively seals the porosity of metal without having an impact on any other feature of the manufactured part. In this process, the part is placed in an impregnation chamber. There, the air is evacuated from the leak path by means of a deep vacuum. The evacuated leak path is then filled under pressure with a sealant. In the next step, the remaining seal is removed from internal passages, taps, pockets and other features where sealant is undesirable. Then the part needs to harden, whereby the sealing polymerizes in the leak path with hot water.
A case study shows that modern vacuum impregnation systems can increase the efficiency of manufacturing companies. For instance, a manufacturer of air brake and train control systems achieved positive results with the High Value Low Volume (HVLV) system from Godfrey & Wing in just one month. The system operates according to the Dry Vacuum and Pressure (DVP) method and resulted in virtually eliminating scrap from porosity by means of HVLV delivering a FTT rate of more than 99 %. Moreover, the system's centrifuge conserves excess sealant, and prevents it from being dragged into the wash, reducing water consumption by 40%. Through the integration of the system with leak testing, the fix on fail parts are delivered from leak testing via a conveyor belt. Then they are forwarded to the next processing step via an outbound conveyor belt.
With regard to the surface quality of cast parts, resistance to external influences such as corrosion and wear is a particular challenge. Depending on the type of metal, various methods can be used to improve the mechanical properties and refine the appearance of the surface. These include deburring processes in which sharp edges, fraying or splinters are removed. The trend is towards special brushes that optimize the procedures. For instance, the manufacturer Kullen-Koti offers turned internal brushes and alpha honing brushes for deburring, rounding and smoothing hard-to-reach areas during the internal machining of components. The former enables the machining of bores and internal threads, the deburring and smoothing of edges on O-ring grooves, cross bores and the cleaning, derusting and descaling of pipes. The second brush variant is also suitable for many deburring tasks and offers an elastic tool whose abrasive filling with grinding balls embedded in synthetic resin at the bristle tips can deliver good results.
In order to achieve a smooth surface for cast parts, a process called slight grinding is frequently used. Parts are shaken in a vibrating tank filled with abrasive grinding chips. In this way, the parts grind, deburr and round each other. This produces a high surface quality in a short time.
Other techniques offering optimum corrosion protection and create color and gloss effects include anodizing, electroplating or powder coating of metal. Read more about coating and surface technologies here.