Project Manager, EDAG Engineering GmbH
Hybrid Manufacturing Additive Manufacturing and Casting - a Novel Hybrid Approach
Additive manufacturing is being increasingly used for industrial applications. However, long production times currently inhibit its use in large-scale series production typical of the automotive industry. Combining conventional processes with AM can be one solution to meet this challenge.
AM and Light Metal Casting Complement Each Other
Additive manufacturing is increasingly being used in industrial applications. The geometric freedom of the processes, in particular laser beam melting (LBM) and laser metal deposition (LMD), makes it possible to implement structures in metallic materials that were previously impossible to produce. However, long production times currently inhibit its use in large-scale series production typical of the automotive industry for instance.
In order to address the challenges described above, novel hybrid process solutions have been developed that combine the advantages of additive manufacturing with those of light metal die casting. This approach was then implemented in two automotive demonstrators. In the project, LBM (Scenario 1) and LMD (Scenario 2) were each combined with light metal die casting. In Scenario 1, additively manufactured functional structures were cast-in and cast-on to reproduce different variants and additional functions in the die cast auxiliary unit holder. For customization and reinforcement purposes, geometry areas were applied to the cast part of the engine mount in scenario 2.
Implementation Scenario 1 - Laser Beam Melting
As shown in the animation, the die cast component features a component reinforcement made of stainless steel (red), a flexible adapter geometry made of aluminum (green) and a heat exchanger made of an alloy containing copper (orange). The functional LBM geometries were cast into a monolithic component by means of overmolding and cast-on. Special attention was paid to the connection between AM and die cast part by specifically developed interface structures.
To determine the mechanical properties of the connections between the different geometries and materials, special test specimens were developed, evaluated according to multiple criteria and tested during the project. In doing so, well-known standards such as DIN EN 50125:2016-12 and DIN 50099:2015-08 were used as a basis. The structures "mushroom", "arch" and "funnel" are shown in Figure 1 and were classified as particularly suitable.
This animation shows the demonstrator with the additively manufactured functional geometries:
The determination of the parameters (see Figure 2) showed that with the "Mushroom" structure the best mechanical properties of the examined connections could be achieved across all material combinations.
By using different holders for the power steering pump (see Figure 3), it will be possible in future to realize a wide range of variants of the "auxiliary unit holder". For this purpose, a special connection geometry with a "standard" adapter section was provided. On the outside, the holder can be adapted as required; the corresponding "mushroom" interface structure is located on the side facing the component. The component reinforcement and the heat exchanger did not require a special interface structure in the present case, as they are completely enclosed by material in the casting process. Due to the complexity of the die casting tool, the functional geometries were implemented step by step until the final demonstrator in Figure 4 could be successfully manufactured. Finally, a corrosion test according to PV 1210 with 15 cycles was carried out. According to the current state of knowledge, no unusual corrosion has occurred between the materials and the functionality remains unaffected.
Implementation Scenario 2 - Laser Metal Deposition
In scenario 2, a engine mount (grey) was used as the substrate body as shown in Figure 5. By means of LMD stiffeners, joining elements and other individual geometries were added. As in scenario 1, the connection with the material of the same type played a decisive role. In this case, the connection was not created by positive and frictional locking but by a welded joint. At the start of the project, gas inclusions remaining in the substrate during the die casting process were classified as problematic. The assumption was that blistering occurs during processing, i.e. expansion of the gases trapped under pressure. The consequences would be violent weld spatter, a foamed substrate material and premature process abortion.
Various strategies were tested to prevent blistering. Due to the non-uniform distribution of the pores in the processing zone, it was not possible to establish a clear method. However, it could be demonstrated in general that the build-up on die-casting surfaces with irregularly distributed gas inclusions is possible without process interruption by means of LMD.
The first additively manufactured structures were directly built on engine mounts provided by Audi AG, as shown in Figure 6 on the left. Using computed tomography, an approximate absence of pores could be demonstrated for the stiffening rib introduced by means of LMD (Figure 6, right). Only in the transition zone between die casting and the additively manufactured structure are pores enclosed due to the blistering effect. Examinations of the entire component showed that these pores have no critical effect on the application. Mechanical tests and corrosion tests carried out on samples showed consistently positive results and meet the requirements of the automotive industry.
In order to show that an adjustment of the casting strategy in combination with an additionally improved LMD build-up strategy can almost completely prevent blistering, two additional samples were prepared and analyzed by CT (Figure 7). The left sample illustrates clearly that vacuum-assisted die casting produces fewer pores in the bonding zone, since fewer gases are trapped and compressed in the die cast part, which leads to blistering during subsequent processing. A further improvement is demonstrated by the adaptation of the build-up strategy. Due to the substrate geometry of the samples shown in Figure 7, the accessibility of the processing head to the substrate was better. Therefore, it was possible to work with a more optimal processing angle of the nozzle to the substrate. This again reduced the remaining porosity in the specimen (Figure 7, right).
Summary and Outlook
In the project, two new hybrid process chains for combining AM and die casting were successfully developed. However, further/new/additional research needs to arise from the individual scenarios. For example, further research is required to improve the material-locking connections in the interface area of scenario 1 (LBM). In Scenario 2 (LMD) the LMD process parameters have to be optimized in such a way that a production of overhangs, such as bridge structures, is possible in a reliable way.
In future, it will be possible to produce several component variants based on one casting tool by combining it with additive manufacturing. Thus, the high tooling costs are distributed over several component variants and a more economical production is implemented. This offers completely new possibilities for the automotive industry. In order to save costs in production, previously components of the most powerful vehicles that are too heavy were used across all model ranges. Thanks to hybrid production routes, it will be possible in future to specifically derivatize castings by adapting or reinforcing components of small cars with high production volumes for vehicles with low production volumes by means of hybrid additive manufacturing. A rethink in automotive development can thus enable an expanded common part strategy for more efficiency in automotive industry.
Further information can be found on the project website.
Translation by Alexander Stark