The transportation industry increasingly uses Aluminum for big and complex parts. They allow large and very complex shapes and therefore integration of different parts and functions into a single casting, avoiding complicated and costly assemblies. The rapidly increasing electrification of vehicles is now shifting the market for castings even more, away from traditional powertrain castings to applications in vehicle (sub)structures, chassis parts, cross-members, subframes, closures, and battery trays, among others.
Precisely those new parts – which require tight tolerances and high mechanical properties to improve crashworthiness – are now bringing new challenges for casting suppliers: Ejection of such large castings as well as heat treatments and especially rapid quenching are often unavoidable to achieve the required properties, which can easily distort the castings. Even when heat treatments are completely avoided by using self-hardening alloys or a simple T5 is sufficient, large structural castings usually can’t meet tight tolerance requirements and therefore require straightening.
Conventional processes like manual straightening are no longer sufficient to master those distortions and simply not feasible in very large castings. Manual or semi-automatic straightening processes of such large and complex castings are also very time consuming and costly; they can easily create other problems in the parts and contribute to higher scrap rates. Automatic, intelligent straightening systems that are adapting through self-learning algorithms (Artificial Intelligence - AI) are therefore more and more the solution for casters and enable the economical production of large and complex castings with high mechanical properties and tight tolerances.
Distortion in castings
In aluminum casting, distortion occurs typically when the part is cast, when it is heated up, when it is hot and when the part is (rapidly) quenched from high temperatures.
It can therefore occur during the casting process, in the die casting die or press, through
- sticking and soldering of the part
- die surface defects,
- inadequate gating systems,
- unbalanced solidification, and
- residual stresses.
During ejection and extraction, due to
- uneven ejection forces,
- insufficient draft angles.
It also occurs in downstream processes like
- sawing,
- machining, and
- welding of assemblies.
Most often, castings distort due to heat treatment. The risk is particularly high if they are hot and therefore soft or have to be heated quickly to high temperatures. Quenching is no less problematic. The faster quenching takes place, the more the casting warps. Efficient quenching after ejection from the mould or after solution heat treatment can be very important to achieve high mechanical properties.
Rapid and efficient quenching after ejection from the die or after solution heat treatment can be very critical for achieving high mechanical properties. This is usually the largest contributing factor of distortion, especially when the quench rate is inhomogeneous. We are talking about an optimum “quench rate” that will give us the best compromise between desired properties and allowable distortion. In many cases, however, it is very difficult to find this compromise – and customers are unlikely willing to compromise on properties or tolerances, a solution has to be found to manage the distortions.
In order to control and minimize distortions technical advancements and improvements have been made with respect to simulation of the process and the part, improved heat treatment furnaces, proper support during heat treatment, new quenching techniques, as well as its temperature and agitation. As often, even all of these are still not sufficient to meet all properties and tolerance requirements, and therefore innovations in straightening of castings are now being increasingly implemented in casting and assembly plants.
Basics of straightening
Straightening means the correction of distortion with plastic deformation of a part to bring it back into its intended shape and tolerances. It means that a section of a part is pushed beyond elastic deformation (which would simply spring back) but clearly below its tensile strength. It is important to compensate for the elastic spring-back through basically “over bending” of the casting, which means that it is bent more than would be necessary to get it into the right shape and tolerance, and it will only get there once it “jumped back”. As this spring-back is not always identical (and neither is the distortion of the part), this bending (straightening) process sometimes has to be repeated several times – especially in manual or semi-automatic straightening processes. This makes it almost impossible to simply straighten a large casting with straightening systems using fixed parameters, like in the trimming press with one single pressing step.
