Bugatti Body Development Contruction Engineer Jens Wenge [JW] and APWORKS’ Head ofProject Engineering AM Sebastian Lepa [SL] talk to TCT about the design and additive manufacture of exhaust components for the automotive company’s Chiron models. In 2019, an exhaust finisher was developed for the record-breaking Chiron derivative and, earlier this year, a lightweight exhaust tailpipe was produced for the Chiron Pur Sport.
Bugatti has a renowned relationship with SLM Solutions, so why has the company teamed with APWORKS’ to 3D print Chiron exhaust parts?
JW: APWORKS came into play when I attended a workshop with EOS and CSI group, another important engineering supplier for Bugatti. Within this workshop we identified the taillight trim cover as a potential part for 3D printing. We realised the chance to implement not only an incredibly stylish object but also to reduce weight in one step. Because of the good experiences CSI had with APWORKS and the relationship between APWORKS and EOS, we decided to do this project with them.
And what made APWORKS want to work with Bugatti and believe it had the right technical know-how and capacity for this project?
SL: Bugatti is well known for its impressive hyper sports cars and for being one of the early adopters of Additive manufacturing. Additive manufacturing allows innovative geometric shapes of components that contribute to the extraordinary performance of the hyper sports cars. Furthermore, Bugatti sets a high level of quality in every process step. APWORKS is the perfect fit to produce the Bugatti Pur Sport exhaust series, being able to produce big titanium parts on a Quad-Laser System and being used to highest quality standards as an Airbus subsidiary and tier one supplier for Airbus.
We first heard of the Bugatti and APWORKS partnership after the Chiron derivative broke the 300mph barrier in 2019, while equipped with metal 3D printed exhaust finishers. How were these parts designed and why was 3D printing suitable?
JW: The advantage of 3D printed parts is that we are able to focus on the part’s function. The shape is optimised for the exhaust mass flow and we can ignore conventional restrictions that lie in the manufacturing process. The second important fact is that we only needed one prototype for the Super Sport 300+ world record car.
SL: AM allows for innovative geometric shapes that contribute to the extraordinary performance of hyper sports cars.
JW: The team around the world record did a great job, working with members from each engineering department of Bugatti. The aerodynamic team, for example, spent hours simulating the airstream and tested it many more hours in the wind tunnel at Dallara. They performed various CFD analysis to investigate aerodynamic phenomena occurring in the wake zone behind the vehicle and they calculated in their simulations exactly how much of the exhaust flow has to be directed into the rear diffuser, depending on the height of the vehicle and speed, so that the optimum downforce was achieved on the rear axle and that the exhaust fumes support the air roller at the rear.
Soon after, the 3D printed exhaust tailpipe for the Chiron Pur Sport was unveiled – what were the design considerations for this part and what changed from the traditionally engineered exhaust components?
JW: Really important in the first place was the realisation of an outstanding exhaust tailpipe: an ‘objet d’art.’ But bringing the exhaust tailpipe wasn’t a one-shot solution.
SL: The design of the trim brings AM to its limits. A lot of optimisations have been applied on the part design to reduce the part’s weight, but also enable a perfect and reliable manufacturing process. The numbers of the part speak for itself: Down to 0.4mm wall thickness; approximately 22cm long, 48cm wide and 13cm high; weighing just 1.85kg including grille and bracket. Due to the requirements and the dimensions of the part, the material of choice is titanium and the system is an EOS M400-4. Printing huge titanium parts including walls down to 0.4mm means overcoming thermal stress in order to avoid component distortions.
JW: Our engineers therefore worked in close collaboration with APWORKS’ applications team. Regular exchange, design iterations and printing tests finally led to the impressive part design and low weight.
SL: We integrated lattice structures to stabilise walls while keeping the part’s weight low and added honeycomb structures to support walls with only 0.4mm thickness. In addition, we developed a completely new way of support geometries for thin walled parts from a geometry and laser parameter perspective.
JW: There is no way to transfer a stylish design to the real part in a conventional production process. The degrees of freedom of AM allow you to realise the vision from our designers, to optimise mass flow, to implement a thermal isolating function and, in this case, reduce the part’s weight by 45%. The exhaust tailpipe is only what it is thanks to this new and innovative technology.
In the design of this component, you managed to include very thin walls down to 0.5mm, saving around 50kg in weight and enhancing the aerodynamics of the car – how difficult is it to do that and what were the significant factors that allowed you to achieve those results?
JW: The intention was to reduce the overall weight by 50 kilograms for the Pur Sport – so our job was to reduce the weight of single parts and still to optimise them, even so they have been optimised in the Chiron already. You can imagine, that wasn’t an easy task to do. When you look at the exhaust tailpipe for example: Switching the manufacturing process to AM we had the chance to use titanium instead of ferrous alloy. A lightweight material is a great base to start with. We also needed to develop strategies to realise the thin walls. So we decided to combine latticed and bionic structures to stabilise the walls while printing. The availability of AM production capacities for such large titanium parts was quite a challenge, because in the end the ready to use part would still need to meet Bugatti quality requirements.
Since the cars are built to be fast and light, are you encompassing topological optimisation and latticed structures within these exhaust components, in addition to having thin walls?
JW: Topological optimisation is used to define the shape of the mounting brackets and fixation points. Also, the junction of load application is optimised by using FEM methods. The latticed structures are essential to be able to produce parts with thin walls having the exact size and clean surfaces.
Were there any challenges in the printing phase of the project with such a streamlined design? If so, how did you overcome them?
SL: The requirements of such a part are high. Due to the requirements and the dimension of the part size, the material of choice is titanium and the system of choice an EOS M400 quad-laser system with maximised productivity. As noted before, printing titanium parts [with] single walls down to 0.4 mm means overcoming thermal stresses in order to avoid distortions. We could overcome this risk, as we were involved by Bugatti already in the early design and application development phase. We integrated lattice structures to stabilise walls while keeping the parts weight low and added honeycomb structures to support walls with only 0.4mm thickness. In addition, we developed a complete new way of support geometries especially fitting for thin walled parts from a geometry and laser parameter perspective.
For both of these exhaust applications, titanium has been the chosen material. Why is that?
JW: High temperature exhaust mass flow requires temperature resistant materials. Inconel or stainless steel was fine in the past, but didn’t work with the weight reduction target for the Pur Sport. So the simple reason is the density of titanium in combination with heat resistance and strength.
What temperatures do these parts have to endure? And what other demanding requirements do you have of exhaust components like these ones?
JW: The temperature in general exceeds 600°C – peak values can be much higher. Important requirements for the Pur Sport exhaust tailpipe was to isolate exhaust fumes and keep it away from the rear end of the car and it’s carbon fibre parts. The surfaces are designed to modify the mass flow to avoid a backflow.
In terms of scales, how many exhaust tailpipes need to be manufactured for a sports car like the Chiron?
JW: The advantage of 3D printing is that production tools are not required. This means that a higher price for materials such as titanium can be compensated by a lower investment in equipment. As a rule, the benefit is greater with a small number and we only produce 60 exhaust tailpipes for the Pur Sport.