The increasing demand for more fuel efficient vehicles in order to reduce Carbon Dioxide (CO2) emissions is a significant challenge for the automotive industry. Cars are responsible for approximately 12% of the total European Union (EU) emission of CO2, the main greenhouse gas. Weight reduction is the most effective route to achieving mandatory EU legislation for future emission targets. In addition, lightweighting is the key to achieving world class vehicle driving dynamics, as characterised by the high performance sports cars produced by Lotus.
Lotus Engineering has been working closely with Imperial College London, to develop an innovative aluminium forming process (solution heat treatment, forming and in-die quenching: HFQ), for the manufacture of lightweight, high strength aluminium pressings.
Pressings with complex geometry have been successfully formed whilst retaining the full mechanical strength of the AA6082 T6 alloy tested.
The Engineering team at Lotus has determined through computer aided engineering (CAE) simulation that up to 20% mass reduction is achievable for selected panels manufactured using HFQ, compared to cold pressed aluminium grades currently utilised for vehicle structures.
HFQ forming technology explained
Lotus has supported the development of the patented HFQ process with Imperial College to achieve a highly innovative technology for the production of lightweight, complex aluminium pressings.
Lotus first recognised the potential of HFQ several years ago as a process which could enable a weight reduction for automotive aluminium chassis, body and closure structures, in addition to aerospace, rail and military applications.
HFQ is a hot forming process for aluminium which enables several benefits when compared against cold stamping, the most significant of which is improved formability of the material allowing the manufacture of extremely complex, single piece deep drawn panels which would otherwise be infeasible using conventional stamping methods. In addition to the improved formability, post forming artificial ageing treatment ensures the aluminium pressing achieves full strength mechanical properties, enabling a potential reduction in panel thickness leading to vehicle mass reduction.
The first stage is to heat an aluminium sheet in a furnace to reach the solution heat treatment temperature of the material. The blank is cut from a sheet of standard heat treatable grade material, using conventional equipment and is transferred to a press and formed between a cold punch and die tool.
The tools remain closed to allow rapid cooling of the formed part until the pressing is quenched. Quenching freezes the microstructure of the aluminium in a supersaturated solid solution state. If a heat treatable aluminium alloy is used, the part can be artificially aged to increase the strength of the pressing, approximately 2.5-3.0 hours for aluminium grade AA6082 is required to achieve peak strength, compared to the 9 hours standard ageing time for this alloy.
The reduction in ageing time is due to the dislocations developed during the forming stage, as they provide nucleation sites for precipitates which is the mechanism by which the alloy achieves full strength.
The final artificial ageing time required to achieve peak strength is therefore dependent on the strain attained during the forming process. The HFQ thermo-mechanical processing cycle has been developed to ensure the final microstructure-mechanical property relationship enables full alloy strength.
The influence of forming speed and temperature
Forming speed is a critical factor with respect to achieving a successful pressing with the HFQ forming process. An appropriate forming speed is required in order to utilise the enhanced material ductility and strain rate hardening in the hot state.
If the forming speed is too low, blank cooling in the binder area will occur, which is detrimental to the material flow during the press forming operation. In addition, the strain rate hardening effect is pronounced at higher forming speeds, which is beneficial for the uniform thickness distribution of the formed part.
In general, a low forming speed is detrimental to HFQ forming since the hot blank will cool excessively in contact with the tool binder surface, which impedes the draw of the blank into the die. In addition, low forming speed leads to a lower strain rate and thus a reduced strain rate hardening effect, which may lead to localised necking.
However, if the forming speed is excessively high, formability could be reduced. It is therefore critical to the success of the forming process that the correct forming speed is used for the HFQ process.
HFQ forming technology benefits and applications
The HFQ process enables high elongation levels of up 70% at elevated temperature, allowing extremely deep drawn aluminium pressings to be manufactured, which until now would be infeasible using conventional cold forming.
The first image (bottom left page) shows a selection of successfully pressed deep drawn rear bulkhead pressings formed using the HFQ process using AA6082 T6 aluminium, the second image shows tearing of the same panel and material as a result of conventional forming at room temperature.
The failure characteristics of the conventionally pressed cold formed part are consistent with tearing at the very initial stage of forming. The improvements in formability achieved using the HFQ process, opens many opportunities with respect to offering improved design freedom, in combination with the high levels of strength achievable.
Uniaxial tensile tests conducted on samples removed from HFQ panels gave a maximum yield strength of 304 MPa.
HFQ therefore enables the opportunity to form more complex panels with increased levels of part integration, allowing the deletion of panels which may have been previously required to either provide the final geometry of the structure or increase the strength of the assembly.
Typical examples of applications where HFQ panels will add significant benefit include primary vehicle structures such as door inner panels, bulkhead crossmembers, A– and
B–pillar reinforcements, which offer forming challenges due to their complex geometry, particularly with respect to aluminium.
In addition, these panels demand a high level of strength leading to a requirement for multiple pressings, often including advanced high strength steels or hot pressed steels in order to meet vehicle crash targets.
Potential mass reduction opportunities for HFQ exist by the deletion of pressings through improved part integration and by gauge reduction for structures where strength is a key factor.
Improving part integration has the added benefit of reducing press and assembly tool investment and associated maintenance costs, reduced assembly operations, logistics and potentially reduced bill of material cost.
In addition to improved formability and strength, HFQ offers additional benefits which include a reduction in pressing springback.
Springback or elastic recovery is an issue with aluminium and is more severe than that experienced with steel for conventional press forming.
The application of methods for reducing the severity of springback require input during the engineering and press tool design phases.
Because the forming stage of the HFQ process occurs at elevated temperature, research carried out by Imperial College with a U-shape bend test shows that springback is reduced with increasing blank temperature.
Using HFQ to reduce springback is particularly relevant for the dimensional control of channel sections such as longitudinal pressings.
HFQ press forming simulation
Numerical work has been carried out to develop a finite element (FE) model to simulate the HFQ forming process.
Validation of the simulation results against experimental tests has shown good correlation. A FE model was developed to accurately represent the anisothermal nature of the HFQ forming process.
Geometry models of the blank and press tools were imported into the software and meshed using thermally coupled shell elements to account for heat transfer between the blank and tool parts.
A comparison between the simulation and experimental results was conducted by measuring the major and minor strains across a surface section of the bulkhead panel.
Good agreement between the experimental and simulated major strain distributions was achieved since both results follow the same trend, which validated the FE model.
Further simulations have since been conducted which support the correlation.
HFQ vehicle mass reduction analysis
In order to assess the potential mass reduction enabled by HFQ pressings, Lotus carried out CAE analysis on a selection of thirty six aluminium panels from an aluminium body structure.
Analysis was carried out to determine the structural response of the vehicle during offset deformable barrier, federal side impact and federal roof crush events prior and post changing selected panels from cold pressed AA5754 H111 to HFQ AA6082 T6 grades.
Lightweighting was achieved by reducing the thickness of the selected panels, which enabled a mass reduction of up to 20% using the high strength 6082 T6 HFQ aluminium alloy, with comparable performance to the baseline aluminium structure.
Euro offset deformable barrier simulation carried out at 56 km/h with a 40% offset, resulted in similar peak footwell intrusion and door aperture reduction with the reduced mass HFQ panels.
Federal side impact simulation also showed similar peak door intrusion and fuel tank protection levels for the baseline and reduced mass HFQ model.
Occupant injury analysis showed that all dummy injury criteria where comparable or improved using the reduced gauge high strength HFQ panels versus the baseline model, and within both Lotus’ and legal requirements.
Roof crush analysis at three times the vehicle mass showed similar performance against the baseline.
Aluminium is increasingly being incorporated into vehicle structures as manufacturers strive to reduce vehicle weight in order to meet performance and emission targets.
HFQ provides a new enabling manufacturing method for the production of complex high strength aluminium pressings, with the potential to further reduce mass of both steel and conventionally pressed aluminium structures.
Lotus Engineering will continue in its development of the HFQ process to ensure the benefits this exciting technology has to offer are maximised for future exploitation.
Writer: John Sellors⎢Exec. Engineer Materials and Manufacturing