Process Overview Automotive Structural Parts made in HPDC
Various structural components can be produced using the HPDC process. This article gives an overview of how these parts are defined for automotive applications and how their physical properties can be achieved.
Definition: Automotive Structural Components from HPDC
Structural parts are components from the body and chassis of passenger cars, trucks and motorcycles that are manufactured by die casting with thin wall thicknesses and high mechanical properties. Typical structural parts are shock towers, longitudinal beams, A-pillars, B-pillars, door frames, cross beams, suspension devices and, in recent years, battery housings. The components are classified according to crash relevance with high elongation ( > 10 %) and tensile strength ( > 180 MPa) and according to strength relevance with elongation > 7 % and tensile strength > 215 MPa.
High Surface Requirements
To achieve these mechanical properties, usually an AlSi10MnMg alloy in combination with a single or a two-stage heat treatment is used. The magnesium concentration (Mg) inside the alloy influences the mechanical properties. This allows the production of crash- or strength-relevant parts with the same alloy but different chemical compositions with regard to Mg. Manganese (Mn) is also important to compensate for the low iron content and to avoid soldering.
Structural parts are connected to other body parts by welding, bonding, riveting, clinching or screwing. Therefore the contact areas have high surface requirements and there is usually a straightening and deburring process after the heat treatment. To avoid blister problems during heat treatment, a casting process is required that can produce these parts with a minimum of air or gas porosity. This means that a vacuum is required to evacuate air / gas out of the die before injection is required.
The correct choice of lubricants is also important to avoid gas inclusions during the casting process. The spraying procedure has a high influence on the component quality - especially when the part has areas with welding. To achieve a good vacuum inside the die, the foundry needs a sealed die casting tool and a plunger / shoot sleeve system that ensures a good seal between the plunger and shoot sleeve. The sealings in the casting tool is not only to archive a good vacuum. The sealing must also avoid, that humidity can be getting into the cavity. Every drop of water, lubricant or oil will create blister during heat treatment. That means also maintenance of the diecasting tools is important, to avoid leakages inside the die during production.
Optimized Cooling Concept Required
Areas with accumulations of material need a optimized cooling concept like Jet Cooling or Squeeze pins to compensate shrinkage during the solidification process. Especially when machining takes place in these areas. The third phase from the die casting machine is usually unable to compensate the shrinkage during solidification because the thin wall thickness areas solidify earlier.
To produce structural parts, it takes an increased effort to check the quality. All parts require a DMC code to ensure traceability to all process steps. Also, the effort in the melting area is higher. After melting a degassing process with nitrogen or argon is standard. Mostly also the dosing furnaces are prepared with porous plugs with continue flow of argon or nitrogen. Density Index (DI) should be under 2%.
The trend in the die casting structural market is towards machines with higher clamping forces of up to 5,600 tons, as the battery housing in particular has a large projection surface. Since the wall thickness is higher, different opinions exist as to whether a battery housing should be classified as a structural part or a drive train part. Based on the mechanical properties and the fact that the die casting process is the same for both products, Knorre believes that this is a structural part.