High Pressure Aluminum Casting High Strength Al-Al Compound Casting
A novel surface functionalization by means of laser technology was developed, which enables the reproducible fabrication of high strength compound castings with the HPDC process via a micro form fit between cast aluminum and the laser structured insert.
Modern assembly processes are subject to growing requirements in terms of material combinations, component complexity and optimization of manufacturing processes. A prominent example is the mobility sector, in which legal requirements to reduce CO2 emissions lead to an increasing demand to save fuel, e.g. via new lightweight design concepts. Compound casting offers an opportunity to meet these requirements in component fabrication while curtailing the process chain. Combining the joining process with the original forming process opens up great technological and economic potential by integrating two different materials or alloys in one component and simultaneously greatly enhancing the strength of this metalmetal-joint compared to what the state of the art offers. Furthermore, compound casting offers the possibility of an increase in the component complexity of cast parts as well as an integration of functions like embedding of sensors or media conveying channels. In addition, the complexity of the geometry can be increased, thus broadening the range of possible applications.
A major challenge in Al-Al compound casting is the natural oxide layer, which forms instantaneously and protects the material from corrosion. It melts only at temperatures above 2050° C and therefore acts as a separating layer preventing direct material joints. Current investigations in Al-Al compound casting concentrate on pickling and metal based coatings (e.g. Zn) to substitute the oxide with a metallic buffer layer. But to this date these methods are associated with technological disadvantages and only achieve lap shear strengths of up to 67 MPa1. The alternative is an innovative approach using ns-pulsed laser pre-treatment developed at Fraunhofer IFAM. This treatment of the metallic insert leads to a surface structure which enables a stable connection with the cast aluminum via a microscopic form fit, independent of the existing oxide layer. The result is more than twice the interface strength of competing solutions.
Laser Structuring of Inserts
The laser pre-treatment of the inserts is conducted with a commercial ns-pulsed Nd:YAG-laser with a wavelength of 1064 nm. The individual laser pulses are applied in such a way that the surface is locally molten and evaporated, with the liquid aluminum being reshaped by the expanding material front of subsequent laser pulses. The results are undercut surface structures ideal for establishing strong inter-material links when infiltrated by the molten aluminum. By arranging the laser pulses in different patterns, regular structures with a defined depth and width can be produced. In contrast to laser structures for compound casting previously reported in the literature, the Fraunhofer IFAM process leads to three-dimensional trench- or grid-like geometries. This facilitates the fast venting mandatory during the casting process.
HPDC Experiments with Laser Structured Inserts
The new approach has been thoroughly investigated in several casting campaigns: the influence of insert surface structuring was studied and the structure type optimized in terms of orientation, number of interlocks and infiltration with aluminum melt. While significant differences in compound strength were achieved by varying the structure pattern, the change in casting parameters, such as pressure and piston speed, had a negligible influence. With an undercut and overlying grid-structure it is possible to achieve a lap shear strength of 138 MPa solely based on the mechanical interlock of insert and casting. This does not only represent a doubling compared to state of the art coating-based pre-treatments (zincate process 1), but also outperforms these by being highly reproducible.
Due to the significant increase in joint strength and its reproducibility, pre-treatment by means of laser technology not only appears to be a suitable alternative to material-locking joints and has a high potential for different application fields - it also constitutes a paradigm shift in Al-Al compound casting.
(1) Wedler, J.; Schwanckl, M.; Körner, C.: Al-Al-Verbundguss mit höherfesten Knetlegierungen. Giesserei 7/2015