The car industry faces a crucial weight problem resulting from increasing customer demands in terms of safety and performance. This trend leads to fully equipped cars in all classes getting more luxurious and comfortable. To escape from this vicious circle car manufacturers are forced to take action in the form of lightweight concepts. Light metals are seen as a promising opportunity to decrease the weight of a car. An increasing use of metals such as aluminium in the automotive industry shows that there is still large scope for improvements. Using aluminium car body panel is just a representative example of weight-saving effort did by car manufacturers.
Aluminium has some properties that make it ideal for car bodies. The strength of aluminium frame and extruded sections is approximately the same as that of steel. However, the rigidity of aluminium is lower than that of steel. That is partly due to the modulus of elasticity of aluminium that is just one-third that of steel. (1) The effect is that aluminium has a higher elastic distortion when exposed to the same force as steel. Therefore the simple replacement of steel by aluminium engine is not an optimal solution as it does not exploit the whole scope of the advantages of aluminium. New ways need to be pursued to make full use of aluminium’s advantages as a light weight material, which means it can be supplied in various aluminium alloys.
Although the requirement for improved corrosion resistance and light-weighting are significant factors for use low-carbon steels in the manufacture of cars, (1) this traditional material still has some advantage such as higher strength and lower price.
Many kinds of steel and aluminium alloys can be found commercially using in producing car body panels. Each material has its own characteristic.
Steel is known as an alloy consisting mostly of iron, with carbon content between 0.2% and 2.1% by weight. (3) One of the main factors contributing to the utility of steels is the broad range of mechanical properties which can be obtained by heat treatment. For example, easy formability and good ductility may be necessary during fabrication of a part. Once formed very high strength part may be needed in service. Both of these material properties are achievable from the same material.
All steels can be softened to some degree by annealing. The degree of softening depends on the chemical composition of the particular steel. (4) Annealing is achieved by heating to and holding at a suitable temperature followed by cooling at a suitable rate.
Similarly, steels can be hardened or strengthened. This can be accomplished by cold working, heat treating, or an appropriate combination of these.
Cold working is the technique used to strengthen both low carbon low alloyed steels and highly alloyed austenitic stainless steels. Only reasonably high strength levels can be attained in the carbon low alloyed steels, but the highly alloyed austenitic stainless steels can be cold worked to rather high strength levels. (5) Most steels are commonly supplied to specified minimum strength levels.
Heat treating is the primary technique for strengthening the remainder of the steels. Some common heat treatment of steels is listed below:
- Martensitic hardening
- Pearlitic transformation
- Age hardening
The recyclability of steel is brilliant and it is economically advantageous to do so. It is cheaper to recycle steel than to mine iron ore and manipulate it through the production process to form new steel. (6)
1.2 Aluminium-based alloys
In the Earth’s crust, aluminium is the most abundant (8.3% by weight) metallic element and the third most abundant of all elements (after oxygen and silicon). Because of its strong affinity to oxygen, however, it is almost never found in the elemental state; instead it is found in oxides or silicates. Aluminium was isolated in small quantities early in the 19th century and it remained an expensive curiosity until 1886 when discovery of an economic method for its electrolyric extraction. Since then, the emergence of aluminium as a practical, commercial metal has relied primarily on the availability of electricity at economic prices.
Aluminium is obtained from bauxite which is the name given to ores containing 40-60% hydrated alumina together with impurities such as iron oxides, silica and titania. Two steps are needed in the production of aluminium. First, alumina is extracted from bauxite. Second, use electrolysis to dissolve the alumina in molten cryolite and a typical electrolyte contains 80-90% of this compound and 2-8 % alumina together with additives such as aluminium and calcium fluorides. (7)
Because the cryolite has a melting point as high as 1010?, the electrolysis of it needs a temperature of about 960? which wastes lots of energy and makes the cost of producing aluminium relatively higher. However, a new method—Cambridge FFC which comes from the production of titanium, was introduced to China. This method use a NaCl-CaCl2 electrolyte which has a lower melting point of 520?, even lower than the melting point 660? of aluminium. By using alumina and liquid aluminium as negative electrode, the expanded reacting area on negative electrode allows the electrolysis of alumina to happen, which can theoretically reduce the electrolysis temperature to the melting point 660?. This research opens a new chapter of production of aluminium and will dramatically reduce the cost of producing aluminium some day.
Generally speaking, about 85% of aluminium is used for wrought products which are produced from cast ingots. (8) The structures of alloys are greatly changed by the various working operations and thermal treatments. Each individual class of alloys behaves differently, with composition and structure dictation the working characteristics and subsequent properties that are developed.
Casting aluminium alloys are also important parts of Al. Apart from light weight, the special advantages of aluminium alloys for castings are the relatively low melting temperatures, negligible solubility for all gases except hydrogen, and the good surface finish that is usually achieved with final products. Most of the cast alloys also show good fluidity and composition can be selected with solidification range appropriate to particular applications. (1) Casting alloys normally have mechanical properties that are inferior to wrought products and these properties also tend to be much more variable throughout a given component.
2 Study of possible alloy using for producing car body
At present, low carbon steel sheet is most commonly used in car body production. Low-carbon steel has good machinability. Its strength and rigidity can fully meet the car body strength requirements, as well to meet the body welding requirements.
2.1.1 Low carbon steel
Steel with low carbon content has properties similar to iron. Low carbon steel is the steel containing approximately 0.05-0.15% carbon. Content influences the yield strength of steel because it is a BCC structure and carbon atoms fit into the interstitial crystal lattice sites. (3) These carbon atoms reduce the mobility of dislocations. But it has a hardening effect. Because the interstitial carbon atoms cause some of the iron BCC lattice cells to distort, a high enough stress level must be applied in order for the dislocations to move.
Low carbon is one of the most common steel and it is relatively cheap than most other steels. However, the strength is relatively lower, which can be seen in appendix.
2.1.2 Higher strength steel
In order to reduce the weight of an automobile, it is absolutely essential to reduce the thickness of the sheet steel and compensate for loss of strength by using higher strength steel sheets. For inner panels, though a rather substantial amount of high strength steel is used, the percentage use is a little lower than in the outer panel due to stiffness limitation and insufficient formability. (9) However, the overall use of high strength steel is estimated to increase further more. The problems in press performance of high strength steel are wrinkles, surface deflection and springback, fracture especially in stretch flanging and galling on the forming tool. Until now, the average level of strength used widely is at most 400 MPa but in the near future the high strength steel with over 400 MPa may be used for the reduction of weight and in this case the forming difficulty will increase further more. (5) The thickness of a panel sheet decreases as a result of application of higher strength steel. Many kinds of microstructural hardened high strength steel sheet are applied by automobile producers.
Ø Dual phase steel
Dual phase steel show lower yield strength compared with other types of steel having the same tensile strength because of mobile dislocations existing in the vicinity of the boundary of ferritic and martensitic phases. So presents low yield ratios. And that’s why its elastic recovery after forming work is small and shape fixability is good. As it also shows larger elongation than precipitation hardened steel sheets and fatigue resistance. (3)
Ø Trip steel sheet
Transformation induced plasticity steel is also becoming an ideal material for car body panels. It has meta-stable austenite transformable into martensite contained up to 30% in bainite or ferrite and bainite matrix. Commercial Trip sheets have simple C-Si-Mn series chemical compositions. The simpler chemical compositions are made viable by stabilizing the austenite phase through distribution of alloy elements in the two phase region and concentration of carbon into austenite during bainite transformation. (4)
2.2 Aluminium-based alloy
Pure aluminium cannot be applied to normal passenger car due to softness. Moreover, surface damages during stamping and handling are often subjected to subsequent surface hand finishing. This additional operation on the panel is usually inevitable. In contrast to pure aluminium, aluminium alloys contain solute additions which can markedly affect grain structures and particularly the microstructures within the grains. This in turn strongly influences the responses of alloys to working and heat treatment. Both crystal structure and microstructure influence mechanical properties. Slip is inhibited by grain boundaries, which are disordered regions, (and so small grains improve strength). Slip can also be made difficult by dispersing particles of another phase throughout the matrix. This indicates the typical hardening mechanism of aluminium. (9) There are some kinds of wrought aluminium alloys typically used for car body.
2.2.1 Aluminium – Manganese Alloys 3xxx
This series of alloys is non heat-treatable. The addition of approximately 1% manganese increases the strength by approximately 10 – 15% compared with 1200, without any major loss in ductility. This non-heat treatable alloy generally finds a wide application where greater strength is required without any major loss in corrosion. The addition of manganese to the chemical composition creates phases like (Mn,Fe)Al6 or (Mn,Fe)3SiAl12 that can be revealed by a solution of 10% of phosphoric acid. The grain structure obtained by work hardening or by annealing is usually revealed by anodisation. (8) 3103 is typically used for car body in the automobile industry. The composition of 3103 alluminium is 0.7% Fe, 1.5% Mn and some others.
2.2.2 Aluminium – Magnesium Alloys 5xxx
This series of alloys is non heat-treatable and exhibits the best combination of high strength with resistance to corrosion. This series also exhibits good weldability but when the Mg level exceed 3% there is a tendency for stress corrosion resistance to be reduced, dependent on the temper used and temperature of operation. (1) Magnesium is largely soluble in aluminum compared to the other elements, but the content in excess can appear as eutectic Mg2Al3. After cold rolling and annealing they can be found at the grain boundaries or after cold working they can precipitate on deformation bands. In both cases the structure can be revealed by an etching of 10% H3PO4. At the same time as Cr is a frequent additive in this series, Cr2Mg3Al18 may appear as a fine dispersoid. (8) 5182 can be used as car body panel. The composition of 5182 alluminium is 4.1% Mg, 0.4% Mn and some others.
2.2.3 Aluminium – Magnesium – Silicon Alloys 6xxx
This group of heat-treatable alloys uses a combination of magnesium and silicon (magnesium Silicide) to render it heat-treatable. These alloys find their greatest strength, combined with good corrosion resistance, ease of formability and excellent ability to be anodised. This family takes the main advantages from the strengthening due to the precipitation of Mg2Si. So the etching will reveal the iron rich phases like Fe3SiAl12, Fe2Si2Al9 that are insoluble and the coarse precipitates or the excess soluble precipitates (0.5%HF). (8) Typical alloys 6009 in this group are used for car bodies. The composition of 6009 aluminium is Si 1.1%, Mg 0.6 %, Mn 0.4% and some others.
3 Study of competition between steel and Al
Automobiles today are over 63% iron and steel by weight. (10) With rising energy and environmental concerns, as well as increases the performance of the car, vehicle light-weighting continues to be a prominent concern for vehicle manufacturers. At the present, more and more aluminium alloys are introduced to automobile industry because the aluminium alloys can normally meet the requirement but is only half weight of steel. However, both the traditional steel car and aluminium have their advantages or disadvantages on aspects of manufacturing, safety, repairability, cost, recyclability, and environmental protection.
Aluminium parts can be more complicated in their design because the high number of design solutions likes the castings available in nearly any shape. One casting can replace a complex part consisting of several steel panels. Consequently a reduction of parts up to 50% is feasible. (1) This again makes design, construction and production easier as fewer parts need to be dealt with in any stage of the design and manufacturing process.
Aluminium car body has fewer parts lead to fewer tools and fixtures, which makes manufacturing easily and therefore saving working spaces and expenses.
Because the sheets can tear easily, low elongation and yield limits of aluminium alloys make it difficult for stamping. This has to be taken into account when designing and building car bodies. Also a higher resilience of the sheets causes difficulties in keeping the tolerances during stamping for individual parts and therefore for the whole car body. (11)
Furthermore, it is difficult to keep the body tolerances after welding processes since aluminium extrusions and cast sections behave unpredictably when subjected to uneven heat influence. The heat expansion of aluminium is higher than steel, so all fixtures need to be built more solidly and stable to cope with the higher forces.
Extensive pickle treatment is necessary to create a weldable surface for aluminium car body. Also the finish of the surface for the paint shop is costly and requires a lot of time and knowledge since the sheets tend to get scratched more easily than equivalent steel panels and the surface of the sheets is liable to form waves. (12)
The 6xxx series of aluminium alloys which has been used a lot for the car body requires heat treatment to achieve the demanded strength. The drying process in the paint shop is not sufficient as the conventional temperatures are not high enough. Hence, an additional heat treatment line needs to be installed to heat the car bodies up to 210°C for 30 min to harden the aluminium alloys. (13) This cannot be done in advance of manufacturing as it would influence the welding processes negatively.
When using aluminium alloy as car body material, contact corrosion plays a much more crucial role as steel accelerates the corrosion of aluminium than using steel body. So that screws or other parts made of steel need to be coated. Otherwise holes in the car body are unavoidable when exposed to moisture. (7)
3.2 Repairability and safety aspects
Nowadays, Crash tests have aroused the public’s attention since occupant safety is a feature that is receiving considerable public attention. Automobile companies even promote car safety as a leading aspect in their advertising campaigns. Additionally, insurance companies set great store by repairability of cars as they cause a significant part of their costs. In some countries the expenditure per car for repairing certain types of damage is assessed and taken into account when the car is being classified for insurance. (15)
In approximately 5% of all accidents the frame structure of a car is damaged. (2) This can be crucial if casting parts or extrusions are concerned as they cannot be reformed properly. They need to be replaced in sections or as a whole dependent on the severity of the accident. Cast aluminium sections especially are liable to develop cracks and consequently need to be examined carefully.
Additionally, a completely different approach to repairing damage forces dealers to train their mechanics in the appropriate techniques. In particular aluminium shielded arc welding requires accuracy, practice and the appropriate equipment but not steel. This becomes a problem especially for small dealers as they cannot afford the necessary equipment or staff. (12)
Also, tools must not be mixed up with steel tools to avoid corrosion caused by steel particles in the aluminium panels. A separate set of tools for aluminium is mandatory to fully avoid these effects. (14)
However, the properties of aluminium offer several advantages to steel. Tubular aluminium sections crumple in the ideal way when subjected to impacts. They develop a crumple pattern that can absorb more energy than equivalent steel elements. Consequently with half the weight of steel an aluminium structural member provides the same safety. (15)
It is obvious that aluminium is more expensive to manufacture than steel. The costs of a sophisticated aluminium car body are many times larger than of an ordinary steel body. This can be especially crucial when being applied to small cars as the margin there is lower than at luxury cars. Higher material costs, a more sophisticated handling of aluminium parts in comparison to steel and some alloys need heat treatment to achieve the required strength, are the main drawbacks. (9)
However, the use of aluminium causes lower investments for tooling due to the fact that stamped steel parts are replaced by extrusions which cause costs only a fraction of the costs required to manufacture stamping tools. This is especially interesting for low volume series as the tooling costs here have a relatively high influence on the unit costs.
Additionally, punch riveting and clinching consume less energy than spot welding and will cause lower energy costs. But the development of high volume technology for welding, riveting and bonding is a crucial issue that has kept car manufacturers from producing all-aluminium car bodies as it is accompanied by high initial investment for research, development and design. (15)
Recycling is likely to become more important in the future as governments force industry to design fully recyclable products just as is an increasing customer awareness in terms of environmental protection and hence a growing demand for eco-friendly products.
To preserve the quality of the recycled material the different alloys must not be mixed up. This is especially crucial for the production of extrusions and sheets out of recycling aluminium alloys as they have lower tolerance levels of impurities than steel. Additionally, they can only be recovered if they are not mixed with cast parts. (6)
Therefore, joined aluminium alloys panels with different alloys cause problems. Also panels that are joined with steel parts like rivets make recycling problematic since steel parts need to be removed to keep the number of foreign substances low. (6) This becomes more difficult as aluminium cannot be magnetically separated from other wastes.
In order to recycle aluminium only a fraction of the original energy consumption is needed, even less than for recycling steel.
3.5 Environmental protection
The environmental issues of different materials can be assessed by a comparison of the energy household of an aluminium and steel-made car during production and over their whole lives. Only having regard to all processes that cause energy consumption during production and operating life can an accurate result be given of the eco-friendliness of a car. It includes material cycles and the amounts of energy needed to produce and maintain a car during operating life time, including fuel. (15)
If primary material is used, aluminium starts with a drawback of an additional energy consumption during production. Compared with steel, far higher amounts of energy are needed to produce lightweight metals like aluminium.
For illustration, a car could achieve a weight reduction of 20 kg by the intensive use of aluminium alloys body other than the steel one. The additional energy consumed could be compensated after 3500 miles. After having covered this distance, the energy comparison works in favour of aluminium.
A further comparison can be made in terms of carbon dioxide emissions. If primary aluminium is used it takes 60000 miles until the lighter aluminium auto has compensated the higher CO2 emission during production. However, when the proportion of recycled, secondary aluminium exceeds 75% the CO2 emission household is positive for aluminium from the first instance. (1)
Considering the energy and environmental issues, using light weight metal for car bodies will be the trends in the near future. However, the traditional steel products still have the advantage of price and easy for manufacturing. Apart from luxuries cars already using light metal bodies, steel car bodies still take a large part of auto body market for it cheaper price and relatively better mechanical properties.