Welding of aluminium die casting parts in research, development and practice

Die cast components can be welded

“Welding die cast products? Impossible!” This view is difficult to shift – even among experts. And even in recent industry publications, it’s not uncommon to read that welding and heat treating die cast products is generally unfeasible due to the formation of pores and bubbles.

The latest applications, for instance from the car industry, paint a different picture: sections and panels welded to die cast nodes have long been used as standard in the space frame bodies of the more recent Audi and Jaguar models as resilient key components. Another example of the successful series application of welded die casting is the interior door frame found in the Mercedes-Benz S-Class.

Pores pose a challenge

A quick glance at practical solutions shows that welding die cast products is now entirely possible. Nevertheless, producing weld joints that satisfy the stringent demands made by industry is still a major challenge. Not least, rapid die filling during the casting process accentuates the issue of pore formation, which even in slower processes can led to impaired weldability. A further difficulty as regards the fusion welding of die casting alloys is their susceptibility to hot cracks.

Good castings mean good weld joints

Because the cast quality is directly related to the quality of a weld joint, most approaches to welded die cast structures require an optimised casting process. In general terms, gas bubbles in pores are chiefly caused by a pronounced change in hydrogen solubility as the molten mass solidifies. The die casting process also runs the risk of air or nitrogen pores resulting from gases swept within the mass if the dies are filled quickly. This specific cause of increased gas porosity is counteracted by the latest process variants such as vacuum and vacural casting. Here air and gases are expelled from the die cavity before and, in some cases, while the die is filled.

However, even the vacuum technique can do little to about hydrogen porosity and shrinkage porosity. Hence, for many applications, even if vacuum casting is used, integral and thus costly optimisation of the entire casting process is required – from a design suitable for die casting to any subsequent heat treatment.

Gentle welding of sensitive joint structures

Depending on their seam geometry, it is now possible to join even standard-quality vacuum cast parts using an optimised welding process.
Innovative process variants such as electron beam welding using a multiple-process technique or hybrid laser welding allow, for instance, the molten baths to be configured so as to encourage degasification and minimise the undesired formation of inhomogeneous pores in the joint area.

Another option are welding techniques where the melting temperature of the material is not reached. Friction welding processes, for instance, prevent damage to temperature-sensitive material structures and result in a smaller heat-affected zone; moreover, this process can be used to bypass the issue of hot crack formation.

Friction stir welding now also in 3D

Friction stir welding (FSW) has huge potential in this context. It transfers the advantageous mechanical joining properties of conventional friction welding to butt and lap joints. Developed in the 1990s, FSW was until recently limited to two-dimensional seams due to high contact pressures. Today, however, high-performance robots can also carry out three-dimensional joins. One example of a series application for friction stir welded die cast components is an exhaust gas cooler module made by Kolbenschmidt Pierburg. This innovative combination of processes was one of the reasons why it was awarded first prize in the international Aluminium Pressure Die Casting Competition in 2008.

Die casting and welding have moved significantly closer in recent years and today are an unbeatable combination in many applications. Future developments in industrial practice are eagerly awaited.