In this report I am going to discuss the modern sheet metal forming processes being used in the current automotive industry. I am going to compare various types of sheet metal forming processes on how they are being used and also how the traditional method have modernised to suit the current demand in the automotive industry. I am also going to discuss which materials are most suited for which forming process.
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Sheet Metal is simply thin metal which have a thickness between 0.4mm to 6mm. any thickness above that is known as plate and below the range is known as a leaf. Sheet metal is one main materials used in metalworking due to its ability to be cut and formed. Sheet metal parts are generally characterised by high strength, good dimensional accuracy, and good surface finish and relatively low cost. Parts created from Sheet Metal can be found in cars, offices, airplanes, construction equipment and more. Historical uses of sheet metal used to be for armour plates and horses.
Metalworking is the process of a metal being made into a part. Metalworking has existed since the ancient times, with evidence that even pharaoh in Egypt did some sort of it. General metalworking processes are sand casting, die casting, extrusion and etc.
Automotive Industry
Despite the arrival of carbon fibre, Sheet metal forming is still of great importance to the automotive industry. Most high volume production cars are made from sheet metal. Metals are still the main materials used in the automotive industry. This is due to Metals being able to undergo plastic deformation. This means the metals can be made into most shapes.
Plastic deformation of metal act as the fundamental to all the sheet metal forming processes, without this property, metal will not be allowed to be shaped into all types of objects because they will break.
In the automotive industry the process of deformation starts with a blank. Blank is a metal in its basic form that will undergo the process. In the automotive industry the blank will most likely be made of a sheet of metal which then will cut, stamped or bent into shape.
Processes
Sheet metal forming has existed for many centuries. Nowadays both traditional and more modern methods of sheet metal forming are being used in the automotive industry. Most metal forming processes which are now done by machines which are controlled by Computer. This enable vehicle parts to be create in a fast and reliable way. There are various ways of sheet metal being deformed.
Forging
This process is one of the oldest forms of Sheet Metalworking. The traditional method of forging is just a metal being hammered against a surface usually done by the blacksmith. Nowadays most of the stuff is done by machines like Drop-Hammer which works in the exact same way like blacksmith did in the past. The process was usually done by pre-heating to certain temperature before being worked on. In the current times forging can be done in warm or cold condition to achieve right balance of force, ductility and the properties of final product. Manufacturers can nowadays decide the temperature they require, due to superior material knowledge and more control in temperature condition
Forging parts are normally used as component parts inside an assembly.Parts made from forging are commonly found in the automotive and aerospace industry. They are particular interest to those industries due to great strength, reliability and relatively low cost. Moreover parts made from forging have around 20% better strength to weight ratio compared to machined parts. Objects which are exposed to shock and stress are normally from forging like, torsion bars, king pins or wheel spindles. Normal materials for forging are carbon and alloy steel, in recent years there has been greater use of aluminum and micro alloyed steel. Forging makes good use of material and requires less machining
There are two main types of forging used in the Automotive:
Open die forging- the simplest example of the open die forging is a cylindrical shape metal being shaped by two die on the opposite side to compress the metal in to desired shape. Open die forging is very good in creating shaft, disc or saddle rings. The advantages of this method are good component strength and wide range of size. However this type of forging requires high degree of skills and it is difficult to hold close tolerances. Furthermore it is not the best option for mass production due to slow production rate.
Impression/closed die forging- two or more dies are brought together as metal sheet undergoes plastic deformation. The metal flow is restricted by the die contours, this allow more difficult shapes to be created. The advantage of impression forging is that makes good use of material and has better strength compared to open die forging. It is useful in the automotive industry due to good production rates and good accuracy. However the startup cost is relatively high and machining might be required for the end product.
Table 1 . Process Comparisons
Criteria
Mode
Hot
Cold
Warm
Ductility
Good
Poor to Good
Moderate
Forming Loads
Moderate
High
Moderate
Forming Rate
Fast
Fast
Fast
Dimensional Precision
Poor
Good
Moderate to Good
Surface Finish
Poor
Good
Moderate
Material Conservation
Poor
Moderate
Good
Die Cost
Moderate
Moderate
High
Die Life
Poor
Good
Moderate
Cold forging tend to have better surface finish compared hot forging. However objects made from hot forging have better ductility. The decision to use which forging will depend on the materials, the desired properties and economical benefits.
Stamping
Stamping is a variety of sheet metal forming process which are done mechanical and hydraulic presses depending on the end product. Punching, blanking, bending, coining and drawing are types of stamping. Despite high equipment and tooling cost, stamping is still very popular In the automotive industry due to large volume production and low labour cost. The most common type stamping used in the automotive is deep-drawing
Deep drawing is one of the most popular metal forming methods in the automotive industry. Deep drawing is mainly used to create cup-shaped or box shaped parts. Usually the blank be held down flat against the die by a blank holder. This form of metal forming is popular in the automotive industry due good strength against weight ratio. It also has good production time and cost. When appropriate, deep drawing has been used with metal spinning process to enhance the quality of the end product.
Figure 2
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Compared to forging or sand casting object made from drawing have better roughness value. This is one many benefits the drawing and that is why it is so commonly used in the automotive industry.
One of the problems of deep drawing is obtaining high drawing limit ratio without the occurrence of wrinkling or fracture.
Extrusion
Extrusion can be used to produce long metal objects, such as rods and tubes. The metal sheet workpiece is forced into a die with a hole in the opposite end. The metal is extruded through the hole to form the shape.
Cold extrusion is more popular than hot extrusion due to less oxidisation, higher strength and better surface finish. More materials are suited for cold working like aluminium, lead or tin than for hot working. Extrusion can be used to manufacture important parts of a car’s drive train or the anchors that hold seatbelts in place or automotive pistons.
In recent years due to aluminium replacing steel as the main metal, aluminium extrusion has become more popular in the automotive industry. Chassis structure of the Lotus Elise has been made of extruded aluminium. Extruded aluminium has found more in the structural applications due to its lean weight. Extrusion is expected to find more use in the coming years, due to its good use of aluminium which is been replacing the use of steel in the automotive industry. Furthermore it is expected to assist and be implanted into a hydroforming process.
Spinning
This process metal forming process is done normally on a lathe. In this process an axially symmetric part is gradually shaped over a mandrel or form by means of a rounded tool or roller. Spinning process normally shapes like cups, cones and tubes. Automotive parts included wheel discs, rims, hubcaps, hydraulic cylinders and engine inlet parts.
Spinning has a lot of benefits which include low energy costs, short setup time and relatively quick process. Additionally Most sheet metal material can be used for spinning which include, aluminium, steel and titanium.
Spinning has moved from a manual with lot of skill needed to an easier and better and more automated form. Due to greater technological advances, spinning has gained greater accuracy due to automation and are mainly performed on CNC lathe.
Shearing
Shearing is a sheet metal cutting operation along a straight line between two cutting edges. Shearing is typically used to cut large sheets into smaller sections, those sections will then be used pressworking operation. It is normally performed on machines like power shears.
It is a very good process to make straight line cuts on flat sheet metal. It is also very capable to cut very small pieces of sheet metal. Aluminium, brass and stainless steel are commonly used with this process.
Laser Cutting
There has been greater use of lasers in the recent years. Despite lasers being in use since the 1960, in those times the laser cutting method was complex and difficult to achieve the desired result as a result mechanical cutting was more preferred in the past.
Nowadays Lasers cutting works by directing the output of a high-power laser at the materials, the laser will be controlled by computer. The material then will melt or burn currently they are used to cut, form holes. The advantages of using laser cutting are accuracy and great consistency in the product being made. It is also seen to be faster and safer method to the more traditional method like mechanical and plasma cutting. However still laser cutting machine consume a lot of power and its efficiency is not very good so far. Power consumption and efficiency will vary greatly depending on material, thickness and cutting rate.
In the automotive industry has been in use since the 1980’s, in those times used to weld cylindrical part. On the other hand, nowadays lasers are used to complete vehicle bodies, airbags. It is also commonly used with hydroformed parts.
However in the future the technology in laser cutting should advance and become more of the norm in cutting sheet metal due to, newer laser machine being able to consume less power with greater efficiency.
Superplastic forming
Superplastic forming is when the material is heated to a certain temperature within a die. Gas pressure is then applied forcing the material to take the shape of the die. The superplastic material can be stretched at high temperatures by several times their initial length without breaking. The material most likely would be an alloy which has the superplasticity property which allows the materials to undergo extreme elongation. The process has to be closely controlled for temperature and strain rates.
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Superplastic forming has been regularly used in the aerospace industry, only recently has this process found greater use in the automotive industry. So far the automotive industry used this type of forming for low volume production. This is due to slow cycle time and cost in creating sheet metal alloy. Superplastic forming has great advantages like it being a one step process and its ability in making complex parts.
The automotive industry is expected to continue to do research on the superplastic forming and also the material behaviour. To make it more suitable high-volume production, particularly research on aluminium alloys will be significant.
Stretch forming
In this process the sheet metal is stretched and simultaneously bent in to achieve the shape. This process is not performed on presses. This method is one of the most accurate and precise method of forming sheet metal. The great accuracy allows this type of forming to make small and detailed parts. Normally the shapes made from this typed of forming are circles and parabolas. One of stretch forming great advantage is that it can reduce the amount of processes.
Mostly ductile materials are used for this process like aluminium, steel and titanium. It is also ideal for high volume production of large parts made from aluminium; this is due to its quick and efficient way. In the automotive industry the stretch forming is used to create body and door panels.
Hydroforming
Hydroforming is relatively new to the automotive industry, with greater significance given to this type of forming in recent years. At the start of the 21th century hydroforming was mainly used for prototyping and low volume production.
In the last few years hydroforming has been in high end sports cars due to its ability to create strong, lighter and more rigid structures for vehicle. Parts normally produced from hydroforming are lightweight which helps it fuel economy and performance. In automotive industry, this process delivers hollow parts such as radiator frames, engine cradles, exhaust manifolds, roof and frame rails and instrument-panel supports. Parts made through sheet hydroforming, currently a low-volume specialty process, include automotive deep-drawn fuel-tank trays and body panels as well as appliance parts such as panels and sink basins. The process also works well with smaller parts such as fittings and fuel filler necks
Hydroforming has started to replace the more traditional method like stamping and welding. As hydroforming uses pressure to forces the metal into the shape of the die, due to that it is more effective in cost and time during the production stage. Overall the industry is still continuing to change to hydroforming with major car manufactures starting to use this method than the more traditional method, Due to better strain distribution, reducing die cost and better surface finish. Nowadays parts made from hydroforming in the automotive industry are, frames, exterior body panels and also suspension. Before hydroforming is used on a material, Computer aided design (CAD) programs will be used to simulate if the material is suitable for hydroforming. Recently, technology has allowed inclusion of operations such as piercing during hydroforming.
In many cases, one-piece hydroformed parts can replace assemblies, thus increasing structural integrity while saving on material costs and reducing scrap. Hydroforming is better suited in producing parts from high-strength steel and aluminium than competing processes. High-strength steel and aluminium are the materials of choice in hydroforming parts for automotive use. But any sheet material that can be cold formed is a candidate for hydroforming.
Hydroforming is usually carried by hydraulic press than the more traditional mechanical press. The mechanical presses are not capable to carrying out the hydroforming operation, as they do not have the speed control and also the provision to stay on the bottom for performing the high pressure forming operations.
However hydroforming is still seen by many car manufacturers to be expensive. Certain material will cost more for this form of processing than the more traditional method. The Equipment required to produce hydroforming is still very expensive and maintenance of it can be complicating and not financially viable.
High Velocity Forming
High velocity forming or also known as high energy rate forming, this process uses large amount of energy which then will be applied in a very short time on the material, this results the material to undergo plastic deformation . High Velocity forming has been around since the early 20th century but it has been mainly used in the aerospace industry. There are three main types of forming which are explosive forming, electrohydraulic forming and electromagnetic forming.
High velocity forming can create very complex parts in one process which reduces the number of processes required using the more traditional method. This method of forming can create similar parts which are made from extrusion or forging. The strain distribution is much more uniform in high velocity forming to the more traditional forming methods. Materials commonly used high velocity forming are magnesium, aluminium and stainless steel.
Generally electromagnetic forming greatest benefit is formability. Most materials can be formed and also at a reduced manufacturing. Development and research are continuing in the application high velocity forming in forming large amount aluminium sheet for body panels in cars.
Explosive forming
Explosive forming is one of the newest forms sheet metal working. It is also already commonly used in the aerospace industry and expected to gain more importance in the automotive industry. Explosive forming can be used on various types of metals which include aluminium and high strength alloys.
There are two types of Explosive forming which are standoff method and contact method.
Standoff method- in this method the explosive charge is placed at a pre calculated distance form the sheet metal. When the explosion occurs the energy released is transmitted using mediums like air, oil or water.
Contact method- the explosive is placed with the sheet metal. When the explosion occurs , the pressure released makes the metal deforms.
The advantages of explosive forming are that it has good tolerance level and even smoothness. Explosive forming is very good method to create complicated shapes. It is also very good and utilized in creating prototypes due to how quickly the metal is created. The tooling costs are low but it does involve high labour cost. It is not suitable for high quantity production due to long cycle times and is more expensive to the conventional methods. This type of forming is generally done by specialist companies because experience and high level skills are required to do this kind of forming.
Electrohydraulic Forming
In this process,” a shock wave to deform the work into a die cavity is generated by the discharge of electrical energy between two electrodes submerged in a transmission fluid”.
The difference between eletrohydraulic forming and explosive forming is the energy source and also less energy is released. This makes electrohydraulic only suitable for small pieces.
For a larger parts, large capacitor are required to make it, this makes the process very capital intensive. However this method is more suitable high volume production due to it being better suited for automation.
Electromagnetic Forming
Electromagnetic forming is the most recognised and used type of high velocity forming.”an electrical current is rapidly imposed within an electrical conductor, it will develop a magnetic field. This change in magnetic field will induce eddy currents in any nearby conductor that generally run in a direction opposite to the primary current (like in a transformer). These eddy currents develop their own magnetic field and cause a mutual repulsion between the workpiece and actuator”
Electromagnetic forming is well suited for automation as it is very controllable. Electromagnetic forming has better tolerance level and springback in an object is significantly less than in a mechanical formed object. Current problems of this technique are that large sheet metal components can not be made due to design limitations.
Equipment and Material
Most form of metalworking uses the conventional die and press. Sheet metal working can be classified into two types processes performed on presses and not performed on presses. Stretching, spinning and High Velocity forming are process which does not use presses. Whereas, stamping, drawing and shearing use presses. Press can be classified as a machine which changes of shape of a workpiece.
The earliest presses were mechanical presses.”Mechanical presses use a mechanical flywheel to store the energy, transfer it to the punch and to the work piece. Mechanical presses are mainly used for blanking, drawing and for making precision parts”.
The more common presses nowadays are Hydraulic presses particularly in a production line”. A machine in which a large force is exerted on the larger of two pistons in a pair of hydraulically coupled cylinders by means of a relatively small force applied to the smaller piston”. Hydraulic presses are flexible and easy for both punching and hydroforming operations.
The greater advances of technology make hydraulic presses more viable and better. The benefits of using hydraulics press than the mechanical press is that there is more control in speed and force. Furthermore the advances in fluid management, gives the user greater controls. Additionally the end product is better and greater accuracy is achieved.
Probably the most important advances in the automotive manufacturing have been the growth in use of Computers. Nowadays most car plants depend on Computers in the process of metal forming. They have two major roles in the forming process.
Guide the process- A computer run machine will make it a lot easier to create complex shapes. Furthermore it can control the flow of the work piece during the production state.
Simulate the process- Computers are used to simulate the process of the metal forming before it is done actually due to highly sophisticated software nowadays available. This helps reduce waste, time and more importantly money.
Materials have also evolved over the in the industry. The automotive industry has moved from being steel depending to a more aluminium favoring industry. Aluminium better strength to weight ratio and better forming methods has made it more suitable for the industry. Materials which show superplastic properties have also found greater use in the industry as greater researched is conducted on them.
Conclusion
In the current automotive industry, the more traditional metal forming methods are still used at a greater level than the modern method. The traditional methods have been updated to suit their more current needs. The great advances in computer technology and machinery have helped the traditional method to be more up to date to current circumstances. These advances have allowed those methods to become suitable for high volume production in the automotive industry. For example stamping has evolved from blacksmith job, to a more computer controlled automatic process in a car plant to match the industry need.
The newer method of sheet metal forming are gaining more importance in the industry but are still not as much in use compared to the traditional method. The high cost and being unsuitable for high volume production due to slow life cycle in most of the forming method outweigh the benefits they have in the current circumstances.
Aluminium sheet metal is definitely the most important type of metal in the automotive industry due to great strength to weight ratio. The modern sheet metal forming technologies are favouring the production of aluminium objects than the more traditional materials like steel.
At the moment the automotive industry uses all kinds of sheet metal forming processes. Each kind of forming process brings in own advantages and disadvantage depending on the object requirement and viability the processes are chosen in the industry.
Future
The future of sheet metal in the automotive industry will largely depend on the future needs of the industry. With current tendency on lean manufacturing and cost cutting, the sheet metal forming process which suit that the most will be the most valuable to the industry.
The traditional method like forging, extrusion and bending will still be active but most likely at lower rate. However the traditional method will need to be updated to match the future problems. In the coming years there will greater use of more modern sheet metal forming processes. Hydroforming and High Velocity forming are expected to gain greater recognition and importance in the automotive industry with more research being carried out to make these processes more feasible for high volume production. Superplastic forming might become the biggest type of forming in the future years, with technology already available and greater research being carried around metal behaviors, with one major breakthrough only required. As a result of this more car parts will be made from super plastic forming.
Greater advances in technology will enable more sophisticated simulation programs. Furthermore better and more accurate machinery will be available to the manufacturers. For example laser cutter will become more accurate and more efficient. Even more new method are being tested to see if they can become suitable in the future like vibrostamping or a press free metal stamping. Only time will tell if these newer method will succeed.
Carbon fibre has been great use in the last few years and is expected to gain more importance in the industry as it is becoming more commercially viable. The industry will continue researching carbon fibre, This will have great significance on usage of sheet metal in the automotive industry. If Carbon fibre becomes more commercially viable, there will be less usage of sheet metal. However, research is being conducted on metal alloys which will significantly impact the processes of sheet metal being used in the automotive industry.
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