Up with the bonnet

Intricate locking disc for passive pedestrian protection manufactured using powder metallurgy requires reliable impregnation and coating

The locking disc is part of a passive pedestrian protection system, i.e. a system that serves to minimise the negative consequences of an accident such as injuries. In case of a head-on or similar collision with a pedestrian, the locking disc actuates a mechanism that causes the bonnet of the vehicle to spring open just below the windscreen. The bonnet thus cushions the impact and reduces the risk of the pedestrian's head, for example, colliding with hard parts of the engine. 

The amazingly intricate disc thus fulfils an essential unlocking function, making it all the more important that this component remains fully functional throughout the life cycle of the vehicle. Here, corrosion resistance plays a key role, particularly in the exposed engine compartment area. Although zinc-nickel platings are commonly used to protect components from corrosion in engine compartments, intricate parts such as this locking disc can hardly be manufactured using cutting or milling techniques. For that reason, the locking disc and the cams that interlock and connect in the event of a collision are manufactured using powder metallurgy. However, despite their structural advantages, components made using powder metallurgy (sintered materials) have the disadvantage that they cannot simply be electroplated, making it difficult to improve their resistance to corrosion.

Intricate structure guarantees functionality

GKN Sinter Metals of Bonn manufactures the locking disc for Edscha, a well-known automotive supplier based in Bavaria, which developed and produces the complete component for unlocking vehicle bonnets. In order to manufacture the locking disc with varying heights (cams) without compromising its stability, the pressing technology had to be improved. Moreover, the manufacturing process is continually monitored, which means the parts do not need additional testing. Another significant point was the stringent corrosion- and temperature-resistance requirements for use in the engine compartment. The required contour precision made a zinc flake coating system unsuitable, leaving electroplating as the only other possible solution. The locking disc was therefore given a zinc-nickel plating in order to meet all the manufacturer's specifications.

However, structural components produced using powder metallurgy generally tend to absorb the liquids used during the plating process and gradually release them in the course of time. The resulting defect is known as "bleed-out". With electroplating processes such as zinc or zinc alloy, this phenomenon frequently leads to salts being deposited on the surface. In most cases, these salts also attack the plating, causing localised surface corrosion. Other plating problems also occur more frequently when components manufactured using powder metallurgy are plated with zinc-nickel. For example, it can happen that only a nickel layer is deposited or that the component is not plated at all. The technological advantages of using sintered materials for making components are thus offset by disadvantages in surface finishing.

Solutions for impregnating and plating

The cause of the "bleed-out" defect is the porous structure of the sintered metals. In order to counteract the process of liquid absorption and its subsequent release, the porous structure is closed using synthetic resin or similar materials prior to surface treatment. This process is known as impregnation and has its origins in the casting industry. In order to cope with growing demand, these processes were also used to impregnate metal parts manufactured using powder metallurgy. However, the success in impregnating sintered components prior to plating was quite limited and inconsistent. With other components too, the specialists at GKN had previously experienced sporadic problems with conventional impregnation processes followed by zinc-nickel plating.

Conventional impregnation processes unsuitable for sintered metals

Tests conducted by the Holzapfel Group showed that when using conventional methods for impregnating sintered metals, around 3-5% of the pores were not filled, most of which were located on the rim of the component (see picture). This unfilled rim is approximately 200-400 μm thick and explains the poor results achieved when applying surface finishes to conventionally impregnated sintered materials, particularly when alkaline alloying techniques such as zinc-iron or zinc-nickel are used, which the experts at GKN also experienced.

Special Sinter Surface Solutions

It was therefore not possible to reliably produce the locking disc using a conventional impregnation process and so a Sinter Surface Solution was selected using a special-purpose impregnation and a plating adapted to suit it.

For Sinter Surface Solutions, the impregnation process was specifically optimised by adapting both the process itself and the resins used, in order to meet requirements compatible with the plating process. The key innovation of the Sinter Surface Solutions process is the adaptability of the hardening process, which ensures that the pores are reliably impregnated to cover the entire surface of the component.

Adapting the hardening process to the requirements of the best possible surface makes it possible to ensure reliable and reproducible almost 100% pore filling right up to the edge, without leaving any disturbing resin residues on the surface. This is achieved by a hardening process in which the resin inserted in the pores begins to harden from the interface surface at the entrance of the pores towards the inside. The droplet formed closes the pores on the outside and prevents the resin from bleeding out, similar to a cork in a bottle. The underlying reaction mechanism of this innovation differs from the currently used conventional systems in that all of the factors that could lead to the resin bleeding out of the pores have been eliminated.

The width of the non-impregnated rim has been reduced by 70-80% and is now approximately 60-80 μm on average. The entire filling volume of the pores has been increased by up to 99.8%, which means a 100% test is not required (see rim table).

Optimised plating process

This evolutionary leap in the quality of impregnated components is reflected in the plating results. At the same time, the plating method was changed to a special zinc-nickel process ideally adapted to meet the specifications of the locking disc. The combination proved to be highly successful, as both the impregnation and the plating are specially designed for the component and guarantee a zinc-nickel plating with flawless layer growth.  The benefit for the customer is clear-cut, as the impregnation and plating made possible by the use of Sinter Surface Solutions enabled the component to be manufactured using the selected base material and the required surface.

Other possible applications for Sinter Surface Solutions

The specially developed Sinter Surface Solutions process makes it possible to impregnate and subsequently plate all widely used types of sintered materials for structural applications. Following impregnation, anti-corrosion platings such as zinc or zinc-alloy systems can be applied, including decorative surfaces such as copper-nickel-chrome and other high-quality surface finishes. 

A sintered product treated with Sinter Surface Solutions has a far thinner rim than a similar component impregnated and coated using conventional methods. This shows that the process makes it possible to produce components with greater reliability, proving the success of the coordinated strategy.

Comparative assessment of five different impregnation systems. All components were impregnated and plated using production equipment under series conditions. The assessment was conducted after plating, based on plating imperfections (test scope: approx. 3,000 parts).

The intricate locking disc designed by GKN Sinter Metals actuates an unlocking mechanism for vehicle bonnets in a passive pedestrian protection application and is manufactured using a powder metallurgy process.