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Application of 3D printing in FSAE Upright Optimization

We are suspension engineers of the E-formula Racing Team from the Hong Kong Polytechnic University. This blog records our journey of designing, manufacturing, and inspecting using metal 3D printing technology for our suspension uprights. 

PolyU e-formula racing team, 03e & 04e

Our previous vehicles, 03E (left) and 04E (right)

Contents:

-- What is Upright

-- CAE optimization

-- SLM printing & CNC machining

-- Validation and tests

-- Further improvement

 

What is Upright?

Suspension upright is the key component connecting the suspension link control arm, tie rod and axle in formula SAE. (J.LI, J.TAN, et al. 2020). It transfers all forces and moments between the wheel and control arms. Weight reduction, as well as performance optimization, is regarded as a key for FSAE suspension optimization in order to reduce weight on structure and improve the energy efficiency of the vehicle.

Traditional manufacturing method:

CNC is the traditional way of upright manufacture.  The advantage is that it provides satisfactory strength, a relatively low cost. On the other hand, it has some limitations for further FEA optimization on the shape of the parts (curves are hard to manufacture).

Our previous upright designs were mainly based on CNC machining methods, and the below image shows the original upright design for the 04E. As shown in the image, both components had undergone a lot of weight-reducing processes. However, due to the limitations of CNC machining, weight reduction can only be done here.

CAD Design of suspension

Therefore, we decided to try metal 3D printing to improve our design. The following form shows the machine and material we used.

3D Printing:

£D Printer and material

Technical details of the machine and material:

Technical data - Materials Data sheet

Technical data - Analysis Certificate

 

CAE optimization

Before optimization

In order to obtain the load needed in the CAE, the suspension geometry was simulated in braking and acceleration scenarios first. Then, we will use the loads in the scenarios with the highest g force.

Suspension with breaking data

 Software: Solidthinking inspire (link)

Software

Preparation of optimization

We enlarged the original design in Solidworks to provide a larger volume for the later topology optimization.

Preparation of optimization

Optimization

The topology was executed in Solidthinking Inspire so the model was imported from Solidworks to Inspire. To run the topology. we applied the connection and defined constraints and loads to the enlarged model.

Model loaded into software

We ran the topology optimization and obtain the result from the optimization.

topology optimization

Further adjustment from the optimization

We smoothed the surface of the topology result to obtain the printable structure.

smoothed ready for 3D Printing

We analyzed the smoothed structure to validate whether the structure can stain the load. Additionally, the thickness was further reduced in the region with sufficient strength.

                                            Analysed structureAnalysed structure - Data

Finally, the optimized upright had lightweight, sufficient strength, and minor deformation.

             Optimized for lightweight and strength - image 1Optimized for lightweight and strength - image 2

 

PBF Printing & Post-processing

Power bed fusion (PBF) is applied for the printing of the upright, while Selective laser melting is applied for the additive manufacture of the upright.

Pre-processing

Pre-processing

After freezing the CAD model after simulation, the part’s orientation, support and slicing parameters can be generated. Judging by the weight of the model and the heat dissipation, the support diameter, and density can be generated. As the major force on the upright is from the A-arm of the suspension linkage, the layers are built in the direction shown in the picture above to increase their strength in one direction. This orientation setting can also be beneficial to minimize the material loss during support removal while keeping the strength of the support. At the same time, to balance the cost and manufacturing accuracy, with the help of Magics, slicing is done with an integrated STL editor for manipulation of STL parts for directly transforming the part into a readable format for the SLM printer.

Processing

After slicing, the path is generated, which is reflected on the exposure strategies, path offset and laser path on the powder bed surface. Proper exposure strategies can be beneficial for the heat input distribution. With this, a strong binding between particles can be established while preventing possible material shrinkage caused by overheating in some particular layers. 

Processing

Powder bed fusion is executed after the pre-processing of the printing parameters. With the preset parameters, the volume energy can be calculated with the following formula.

Volume energy equation

h: hatch spacing

vscan: Exposure velocity

tpowder: Powder diameter

Based on this, the process window can be calculated to prevent a lack of fusion, balling or keyhole formation which may harm its quality, as shown in the picture below. 

Graph of exposure to beam power

Before the start of the printing process, preheating is done to lower the temperature gradients by heating the building platform between 80 and 1000 ℃. control the heat balance to reduce residue stresses and avoid crack formation.

After all these settings, the PBF printing process can be executed with the SLM printer. During the process, although pulsed exposure strategies may lower the consumption of powder with higher resolution, continuous strategies are still adopted due to their low cost and short production cycle, at the same time, relatively higher strength when high load is applied.

After 24h of printing, to turn a rough printed piece of aluminium into a car component, post-processing is done with the flow shown in the flow chart.

Post-processing

Post Processing Block Diagram

As for PBF process, the part is buried in the metal powders. By vibration, rotation and usage of a fan (AlSi10Mg powder is relatively easy to be removed from the surface), the part is separated from the metal powders without any damage. As the diameter of the particles are so small that they might be inhaled by human, a proper safety measurement like a gas mask is necessary to ensure the health of the operator and other people nearby.

        Heat treatedClose up of suspension

After removing the powders on the surface, the part is removed from the printing surface in a mechanical way and goes through heat treatment. Heat treatment, which is stress-relief hardening in the manufacture of uprights, can reduce the residue stress by applying 535℃ temperature on the surface to increase ductility. The temperature is held for 1h for the part to reach homogeneous temperature. 

after heat treatment

Because the upright is fitted into the hub of the car for assembly, CNC machining is done to smooth the surface for fitting. This step also makes sure the diameter of the hole in the middle of the upright is appropriate, which may cause safety concerns for the car. 

Part being CNC machined

After CNC machining, the hub is successfully pressed into the upright using hydraulic. The upright and the bearing on the hub have an interference fit. The tolerance of the 3D printed part satisfies is within the safety range. 

hub pressed into upright

Validation and tests

After completing the manufacturing and processing, we came the exciting test session.

Fitting test:

Fitting test - suspension

Fitting test - chassis

On-ground test: Pushing:

On-ground test: Pushing

Non-destructive testing:

Non-destructive testing

After spraying the high sensitive immersion fluorescence hydrophilic emulsifier on the part and exposing it to UV light, it could detect surface pitting and hair cracks. As shown in the images below, the cracks showed up and can be marked.

Cracks identified with UV light

 

Further improvement

Existing problem

There are several cracks on the printed upright, which may result in the printed upright having less strength than the design strength. We thought the cracks is caused by insufficient support. 

Supports can transmit heat to the platform to prevent overheating. The overheated liquified powder may flow to an unwanted region and cause curling. Additionally, the supports can also hold the part in position, which also prevents curling. In our printed upright, there are insufficient supports so they cannot prevent overheating and location shift properly. Finally, the cracks are encountered due to the curling.

Existing problem

Possible solution

In the future, we will also simulate the support to ensure that we have sufficient supports to prevent curling and avoid excess supports, which will be difficult to be removed.

Simulation Software

Software from Materialise

Additionally, we will try to have better temperature control to avoid overheating

Other improvements

In the future, we will try to use Ti alloy which will have higher strength but still maintain lightweight. The smaller powder diameter will be used in the future to have a smaller step effect and have finer structure.

The 3D print upright gives us the possibility to assemble hub motors on the upright at the same time the wheel weight will not be extremely high.

The upright design for hub motors

The upright design for hub motors from TU Munich

 

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PolyU E-formula Racing Team

First student e-formula racing team in Hong Kong

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About authors

Valen Qiu, Sam Yao, Tom Zeng

Suspension engineers of the PolyU E-formula Racing Team

 

A student major in Mechanical engineering, minor in Electronic and Information Engineering. Interest in robotics and vehicle.
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