The main factors for material selection, especially for the body include thermal, chemical, or mechanical, environmental resistance, durability, ease of assembly and manufacturing.
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There are several materials used to make our cars which we can see, touch and feel. But there are some which we do not see yet contribute significantly for the vehicle to function as intended by the manufacturer. The main materials used for making cars and their parts are steel, aluminum, magnesium, copper, plastics, composites, rubber, glass, fabric/leather and a few more. These materials have been around for quite some time but have evolved greatly and diversely. For example, we call the shell of the vehicle as BIW (body in white) and everyone would assume steel or aluminum in the past, but now the naming BIW might imply the shell (as per automotive jargon) be made using Carbon fiber.
The main factors for material selection, especially for the body include thermal, chemical, or mechanical, environmental resistance, durability, ease of assembly and manufacturing. Affordability is an important consideration in vehicle design, which includes factoring in the costs associated with a car’s complete life cycle, including producing, running and scrapping costs which are mostly controlled via regulations at least in Europe. The automotive sector is constantly pushed by the governmental policies to reduce carbon emissions, improve the fuel efficiency simultaneously increasing passenger and road user safety. One solution for the manufacturer is to achieve this is via material selection and optimization. There are separate departments under advanced research in every OEM, testing and suggesting new materials to be used on the cars.
Less known to the outer world, there is a significant effort that goes into maintaining a database of materials used by the manufacturer on each model and variant. This is because the ELV (End of life) requirements govern the amount of recyclability and reuse of materials used. There is a list maintained called “IMDS”, which almost all global OEMs, their suppliers use to consolidate the materials used. This list is used to verify the compliance to local and international regulations.
Let me put the materials into certain clusters for ease of explaining. It is important to understand that there are differences of materials selected in different applications and segments, so I will be referring to the cars that most of us use every day in this article.
Metals and Alloys
This cluster of materials form the major chunk of the overall vehicle weight and raw material cost. Metals are time tested materials but have had major shift when it comes to improvement of properties thanks to the alloys (adding a combination of different materials to a base metal). To make it easy for the readers, I will start with a summary table before going to the details of each material.
It is the most common, go-to material for body structure and outer panel design due to its strength, formability, well known process of welding and maintainability. It has a good life with proper treatment and coatings. There are different types of steel in modern cars.
Sheet steel –
The most used in previous generation for the most part of the body. Even here there are classifications such as cold rolled and hot rolled low carbon steel with different thickness. Typically, the thickness ranges between (0.6 to 2mm) for different parts ranging from roof, bonnets, doors with lower thickness and load bearing structures like flanges and mounts with higher thickness.
RRST 1405: strength 270–350 MPa, relative elongation @ 36%, thickness 0.6–0.9 mm
UST 1203, 1303: strength 270–410 MPa, relative elongation @ 28–32 %, thickness 0.6-0.9 mm
ST 4: strength 280–380 MPa, relative elongation @ 38 % thickness 1.5-2.5 mm
High strength steels–
With the increased focus on safety, the application now requires more impact absorbing but strong enough materials to withstand vehicle dynamic stresses. This makes the HSS/UHSS options so viable since achieving similar performance using traditional materials will result in increase in weight. This is due to their property of higher tensile/yield strength. But this comes with a downside of increase in cost (up to 50% more expensive), not only because of the material but also the process involved in manufacturing (typically HSS/UHSS requires special forming process like hydroforming).
HSS: strength between 210 MPa and 550 MPa
UHSS: strength higher than 550MPa
Stainless Steel: –
Yes, SS was used on door handles, even bumpers, exhaust tips, before the plastic parts with chrome coating took away their usage. Currently only part which I could think off is the seatbelt buckle.
I would say this is father of all automotive materials that was used to make almost every part in older days. Its heavy, brittle and not so good for making lighter cars. Gradually got replaced with steel and aluminum, but still used in some rare cases.
It is lighter (by almost 40% to steel), corrosion resistant, malleable, easily machinable and has good thermal/electrical conductivity. Also, the material can be used as load bearing when extruded. Manufacturers started using aluminum for making lightweight yet stronger shells quite recently for mainstream cars (I remember Audi explaining its space frame chassis during Iron man movie release). The problem with aluminum other than cost is the joining process, it required special welding process as it has very good electrical conductivity. Also, the NVH engineers needed to revisit blackboard as the absorption properties were completely different to that of steel.
Used in sheet and cast form for other applications than the BIW are:
Cast – suspension components, engine & transmission parts, alternator casings, flanges, starter motor housing, heat sinks for electronics and a lot more.
Sheet – Heat exchangers (radiators), door panels and bonnets (specially for pedestrian safety).
In some rare cases, few OEMs are trying to use them to replace copper wiring on non-critical components due to cost reasons – copper cladded aluminum wires. There are many more aluminum alloys with even more applications which are not mentioned here.
I would say the least looked upon material in automotive industry, but the most important material when it comes to electrification. So, I am increasing the scope of my article for copper and added certain interesting trends and details.
Before we start, did you know?
1. The body of the 1921 Rolls Royce Silver Ghost is completely copper.
2. The Tesla Roadster is also the first commercially available automobile powered by an electric motor powered by a copper rotor.
3. In 1948, the average family car contained only about 55 wires amounting to an average total length of 150 feet. Today’s luxury cars, on average, contain some 2000 copper wires adding to few kilometers in length.
4. Exxon added copper in their lubricants to make engines run smoother and last longer.
5. There are copper-carbon bushes and for dry lubrication applications.
Copper is in my view a superman of the material world; over the years its properties have made copper to have a vital role in many of the applications. Some of the properties include, thermal and electrical conductivity, corrosion resistance, malleable, lubricative, solderability, creep resistance. There are many alloys of copper that is used in auto industry especially brass.
Mostly copper is seen as just an electrical application material where the main component is wiring harness and connectors. Typically, in 2020 an ICE vehicle uses anywhere between 3 Kgs to 6 Kgs of copper for wiring harness and adding more features means more wires and more copper. But the use of copper can be optimized be selecting different grades. Electrolytic-tough pitch is the most common type used in wiring. Oxygen free copper is also used in special applications to reduce the resistance. Apart from wiring, it is also used in PCBs (Printed circuit boards) and electronics, components like alternators and motors, brake lines (Copper-nickel), alloys in gearbox components as coatings, new generation of radiators replacing aluminum.
With the increase in EV penetration the copper content used in vehicles will only be going up and more research will be on making more cost effective and less resistance copper alloys.
It is the lightest of all the metals that are currently used in automotive industry with density of just 1.74 g/cm3, which translates to being 30% lighter than aluminum and 75% than the steel. There are some complications in using magnesium and special design considerations are required due to its drawbacks compared to aluminum when it comes to fatigue, ultimate tensile strength but structural properties generally are comparable. It is used mainly on steering wheel core, instrument panel structure, wheel rims and transfer case.
Major chunk goes to battery and in older vehicles solders on PCBs and electronics. It is one of the six most hazardous materials on which restrictions are imposed. Slowly this material will be phased out due to advancement of the LI and other chemistry batteries and also the lead free solder(already in use).
Yes, there is gold in your car and it is not the same as you think where rose gold or white gold panels are used on ostentatious cars. There are plating requirements for the connector terminals and relays of safety critical ECUs and also some elements on PCBs where processors are assembled.
Read about usage of rubber and glass and BHPian comments
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