MAJOR DIE CAST BODY PARTS – HURDLES AND OPPORTUNITIES
There is a familiar tension between various teams who create vehicles – the industrial designers present the vision; the engineering community are tempted to knock that back into a shape they know already works in existing products and the production engineers want maximum simplicity with minimum parts. Of course, rarely does a product fit all demands easily, so the art – the engineering – is to work together.
An outstanding example…
Long ago I was on the Ford Ka B146 programme, and cooling was a ‘go/no go’ issue. We had fought and fought to get a viable concept, and needed maximum space to get the cooling pack in. It was already the smallest pack ever fitted to a Ford vehicle. Proof of concept had been done, but we needed to get the bonnet latch moved. So, we approached the engineer in charge, who had a carry-over latch in a carry-over position and component (i.e., no tooling or investment cost), who understandably bluntly refused.
The engineer concerned did not realise his refusal was a programme cancellation decision, and the organisation knew this was a solvable problem. It was solved. The latch moved – not by much, but it moved. Everyone got to share some pain in order to deliver. Why should we do this? The vision was a vehicle which looked nothing like Fiesta, and yet certain elements of the engineering team wanted to turn it into… a version of Fiesta. This, of course, would be a mistake.
Large scale castings
More than two decades ago the first thin wall heat treated pressure die cast aluminium alloy inserts appeared in otherwise steel bodyshells. The art of making such castings and then ensuring manufacturing consistency as well as meeting the performance requirements is not an easy task. One item which offered significant weight saving (circa 1.3 kg per component) compared to the functional equivalent made from steel alloy was the front strut tower.
The casting is attached to the surrounding sheet metal panels by a bonding agent, and a mix of rivets/flow drill screws which mainly serve to pin the assembly together during the curing process. In the event of impact, the casting will yield, but not in the same way as the surrounding steel alloy panels – the latter provide the main impact energy load paths.
We have seen aluminium fabrications inserted into the area above the rear subframe, again, a significant weight saving, as well as cast aluminium alloy rear chassis legs. In each case this is calculation based on investment, production cycle time and piece cost. As a rule, the more ‘exotic’ the component, the higher the piece cost.
However, if we consider the front strut tower other benefits come into play:
1. A single part.
2. All required holes for everything from harness location clips to suspension strut mounts are integrated – although each feature not in the casting plane requires a moveable pin in the die casting tool which adds investment cost.
3. The cumulative assembly tolerance during manufacturing is easier to control.
In short, there are benefits within the component, but also beyond. What if that idea was made even bigger?
Tesla Model Y
The first phase of large-scale aluminium alloy pressure die casting for the body structure at Tesla was a rear module which integrated the rear chassis legs, two cross members and the inner rear wheel arches. Note the forward tabs which connect to the steel sill panels, and the sacrificial aluminium alloy members between the rear bumper beam and the ‘mega casting’.
This replaced a mix of pressings and castings premiered on Model 3, to reduce assembly time as well as to provide a small weight saving. The component was made as a single- or two-piece assembly, with appropriate collision repair instructions for each solution.
In effect the material flow during the casting process had to produce the correct crystalline structure – hence the immense pressure to ensure molten metal flowed throughout the mould before cooling, but did not require progressive yield beyond the basic mechanical structural demands.
The next step was piloted at the Tesla assembly plant in Austin, Texas. Learning from the engineering to create the first rear module, the team worked with the supplier to develop castings with variable heat treatment which allowed integration of the rear chassis leg sacrificial elements as well as the forward cross member.
But we’re not done yet. The upper face of the battery pack was fitted with the interior carpet, centre console and front seats, to be fitted as an assembled module into the body structure.
The dry run and Cybertruck
The front module technology was developed for Model S II and Model X II, which initially were only sold in the U.S. The objective was to reduce the number of components, reduce weight and to push technology boundaries.
Tesla were not alone in pursuing this goal. BMW for example had worked on this technology nearly three decades ago, but while components could be engineered to produce variable yield throughout the part – progressive deformation from the extremities to the front wheel centreline – process consistency was a real issue. Some castings would not behave as intended.
Further, Porsche on the 981 Boxster/Cayman and 991 ‘911’ had significant front and rear aluminium alloy sub-modules joined to a steel and aluminium alloy intensive central structure, allowing the entire removal of each module or localised repair depending on the extent of the damage.
Tesla do not permit use of pulling on the body structures due to the number of bonded joints.
Tesla Cybertruck – not destined for Europe at the moment since it does not conform to elements of EU25 plus UK whole vehicle type approval – takes the large ideas we have seen so far and, well, expands them. Aside from the stainless-steel skin panels, door and tailgate structures, there are two simply enormous die castings. The front module has the next generation of ‘freeform’ webs, allowing the aluminium alloy to flow into position more easily before crystalline formation. The same design process was used for the rear module as well.
Ever bigger die casting machines and selective zone cooled tools mean it is not possible to deviate from the manufacturer’s repair process.
The Model Y structural battery front end repair process
The allowable repair process is quite complex. Please refer to the two images of the front and rear modules with this story.
Blue: No heat can be used, cold straightening of webs is possible, as is welding.
Green: No heat can be used to straighten, welding up to 50 mm is allowable, cold straightening of webs is possible.
Yellow: No heat can be used to straighten, welding up to 30 mm is allowable – or 50 mm with the use of a Tesla reinforcement plate, cold straightening of webs is possible.
Orange: No heat can be used to straighten, welding up to 30 mm with the use of a Tesla reinforcement plate is allowable, cold straightening of webs is possible.
Red: The part is scrap.
Note the number of statements where heat must not be used to re-form or straighten elements of the castings, and the very strict repair limits by zone throughout the component.
Story by Andrew Marsh
