Sheet Metal Forming in Automotive Industry - Essay Example

Table of Contents Introduction…………………………………………………….………………………….……..2 Formability of a Material.....................……………….………...…………………………….….3 Different Types of Sheet Metal Forming Processes Available........................................................4 Choices Made in Automotive Industry.................................................................................……...8 Conclusion.....................................................................................................................................10 Future Trends.................................................................................................................................10 References…..................................................................................................................................12 Introduction ‘Sheet Metal Forming is a grouping of many complementary processes that are used to form sheet metal parts’ (Gowri, (2008: 345). The process involves plastic deformation of thin metal sheet by applying force on the metal. The forces are applied by placing the sheet of metal in between dies. Plastic deformation is one of the most important techniques of manufacturing in case of metals (Boljanovic, 2004: 13). Metal is usually available in the forms ingots from the furnaces and it has to be converted into desired form or shape in a manufacturing industry. This conversion is carried out by deforming the metal permanently by the application of forces on it. The desired form or final shape of the metal defines which type of deformation process has to be applied. Physical and mechanical properties of metal such as strength, hardness, brittleness, elasticity, plasticity, malleability, toughness, grain structure, isotropic behavior etc. also play an important role in deciding which kind of manufacturing process is to be used. Metals are generally ductile materials with a large plastic range on stress strain curve. This is due to the metallic bond present in them (Askeland, 2009: 33). The stress strain curve of mild steel is shown here, (although it will be different for each metal, it will follow more or less the same pattern): The area after the yield point is the plastic range of mild steel. Clearly, it can undergo significant amount of plastic deformation before it finally fractures. Same is true for other materials. Hence, to form a material in to desired shapes, plastic deformation is a desirable process. There are many different yield criteria which tell us the stress required to cause permanent yielding in a material. Out of these Tresca criterion is considered suitable for ductile materials (Marciniak et al., 2002: 20). It suggests that yielding occurs (or plastic deformation starts) at a point when shear stress crosses a certain limit. Formability of a Metal Formability of a metal is its ability to deform in to desired shape or form without failure. Failure can be due to different physical phenomenon like shearing or necking etc. (Kalpakjian and Schmid, 2001: 424). There are different tests applicable in industries to predict formability of a sheet metal. One of the earlier developed tests is Cupping Test. In this test, a steel ball or any circular profile made of steel is pressed against the sheet with uniform increment of stress. The depth to which the sheet can be deformed is a measure of its formability. This method however has its own limitations as the results obtained are specific to the test conditions. The actual conditions in the manufacturing process may vary a great deal from the test conditions. A more recent of the techniques is plotting the Forming Limit Diagram of the sheet metal. The test is performed by taking a specimen sheet metal and marking small circles of diameters 2.5 to 5 mm (0.1 to 0.2 in.) (Kalpakjian and Schmid, 2001: 425). The specimen is then stretched over a die and is allowed to plastically deform. Different types of strain patterns are observed on the circles. The marked circles assume the shapes of ellipses with major and minor axes. The major axis is termed as major strain and minor axes is termed as minor strain. There can be a positive or negative minor strain, however major strain is always positive (Kalpakjian and Schmid, 2001: 425). A curve is then plotted between major and minor strains. This curve, called the forming-limit-diagram, is then used to predict the formability of the metal. Different Types of Sheet Metal Forming Processes Available The process can be divided into two major categories (Dhar, n.d.), Cutting (Shearing) Operations and Forming Operations. Cutting operations are used to separate a small portion of metal sheet of required dimensions from the bulk. The sheet is then formed into different shapes by performing Forming Operations. Shearing Operations have following main types (Kalpakjian and Schmid, 2001: 416), Die Cutting: Die cutting refers to a variety of processes that use sheet metal shearing techniques for punching holes, parting sheets into pieces and removing pieces from edges. Die cutting is widely used in automotive industry to obtain stamping dies for performing other operations. Fine Blanking: Fine blanking process is used to produce smooth surfaces and corners. This process has wide acceptance in the field of automotive industry in Japan and North America for cutting sheets of up to 19 mm (Fine Blanking Overview – History, n.d.). Slitting: Shearing operations carried out by a pair of circular blades similar to can opener are called slitting (Kalpakjian and Schmid, 2001: 417). Slitting finds its applications in automotive industry shearing processes where excessive burr formation creates problems as new techniques in this process is relatively burr free. Nibbling: This process uses a small machine called nibbler which moves a small straight punch up and down rapidly into a die. This process is occasionally used in automotive industry for cutting sheets etc. Lancing: This is a process in which a flap is formed by creating partial hole in a sheet and then one side is turned over. Forming operations can have following main types (Kalpakjian and Schmid, 2001: 431-451). Press Brake Forming: Bending process may be carried out through press brake forming in which a press machine is used to exert pressure on the sheet metal that will then cause it to undergo plastic bending. Press brake forming is so extensively used in manufacturing automobile parts that it has become a norm in automobile industry (Brake Press Forming, 2010). Bending process can also be carried out in a variety of other ways e.g. Roll Bending: In roll bending, plates are bent by passing them through a series of rollers. Roll bending is used to convert sheet metals into hollow tubes etc. The process finds its applications in manufacturing of low cost mufflers etc. Bending in a 4-Slide Machine: In this type sheet of metal is placed on one part of die and force is applied from top as well as from both sides i.e. from four different sides. This process is also used in automotive industry. Beading: In this process, the desired shape is obtained by inserting the sheet metal with pressure in a hollow space shaped according to the desired output. The process is used in manufacturing of parts of automobile doors. Flanging: In flanging, sides of sheet are bent at right angles. It further has many types like shrink flanging, stretch flanging etc. Stretch flanging is widely used in automotive industry. Hemming: This process is adopted when strength of sheet is to be increased by increasing thickness. The sheet is bent over itself to increase thickness by applying pressure in a press. The hemming process is used in the manufacturing of different parts such as “hoods, decks, lids, fenders, and tailgates” (Saboori et al, 2009: 1). Tube Forming: Tubes and hollow cross-sections are formed through somewhat modified processes from those mentioned above. These processes are commonly designated as tube forming. Tube forming is used to manufacture all sorts of tubes used in an automobile. Deep Drawing: It refers to a whole class of processes that are used to create cylinders, cans, pots etc. of certain depths from a sheet metal. Deep drawing is also a process extensively used in automotive industry. Roll Forming: When long lengths of sheets are to be bent into a particular shape or form then roll forming is used. In this process, long lengths of sheet are passed through rollers to impart a certain shape to it. Roll bending is used in manufacturing of certain types of materials such as Advanced High Strength Steel (AHSS) (Steel Roll Forming Process Simulation, 2010). Stretch Forming: Sheet metal is formed in to different shapes by stretching it over tool of desired shape. This process is used to produce aluminum parts in automobile industry or titanium parts in aerospace industry (Stretch Forming Equipment, n.d.). Rubber Forming: If one of the parts of die which is used in sheet metal forming is made up of flexible material then the process is called rubber forming. Rubber forming is not very widely applied process as far as automotive industry is concerned Choices Made in Automotive Industry An automobile industry uses nearly all the above mentioned sheet metal forming processes for manufacturing the body parts both exterior and interior. But some are more commonly used than others. In the following lines the most common choices of manufacturing process, material and designs are discussed. Manufacturing Process: Sheet metal forming is mainly used in the manufacturing of steel panels in car body. Although other parts are also manufactured using steel metal forming techniques but their contribution to overall sheet metal forming activities in an automotive industry is relatively low (Sheet Metal Stamping in Automotive Industry, n.d.: 1). The sheet metal manufacturing process in an automotive industry starts with the Fine Blanking process. Sheets are obtained for the manufacturing of body parts such as door, bonnet, roof etc. as shown in the figure. The blanking process is carried out on blanking presses which create the desired output in the form of a metallic sheet of desired shape and size. The rate at which blanking is carried out can be as high as one thousand strokes per minute (Custompar.net, n.d.). Clearances of the order of one percent are possible with sheet thickness 0.5 mm to 13 mm (0.02 in. to 0.5 in). Suitable sheet hardness can be between 50 and 90 HRB (Kalpakjian and Schmid, 2001: 417). The operation is normally performed on hydraulically controlled presses which have three degrees of freedom i.e. die, pressure pad and the punch are free to move. The reason of preferring fine blanking on any other process of shearing and cutting is that there is no possibility of fracture zone in case of fine blanking. This is because whole part is pressed at a time with equal stress on whole of the sheet (Degarmo, 2007: 425). The next major step in the manufacturing is drawing. The sheet metal obtained in the previous step is drawn to the required depth in this process. The general method employed in this step is that a die- cavity of the required shape is created and the sheet metal is pressed in to the die cavity by application of force on it in a press. There are standard procedures established for proper execution of the drawing process. The process is carried out with pressure of die and blank holder with in specified limits in order to avoid failure. The depth to which the sheet can be effectively drawn depends on plastic anisotropy of the material. A measure of ability of a material to be deep drawn is called Limiting Drawing Ratio (Kalpakjian and Schmid, 2001: 437-441). The process of drawing can be replaced by cold working or casting in the manufacturing but both of these processes have their own draw backs and hence cannot be used in the automotive industry. Casting is expensive process for complex shapes and forms which are required in automotive body parts. It also compromises the strength of parts obtained by strain hardening. Cold working has a disadvantage of residual stresses. Also the time taken for both of these alternatives considered here is greater than drawing process. Hence, drawing process is the most suitable choice. The process of conventional drawing is now gradually being replaced by hydro mechanical drawing due to its greater efficiency, high quality and less possibility of failure (Onder, 2005). After Blanking and Drawing, design specific processes are carried out on the automotive body part which gives fine details to it required for its incorporation in the automotive body. The processes may include stretch forming, piercing and hemming etc. Hence the most commonly used processes in automotive industry are fine blanking and drawing with relatively less use of stretch forming, piercing and hemming etc. Materials: The materials selected for the automotive body panels and other parts manufactured by sheet metal forming should have acceptable physical properties as depicted by their strain hardening exponent (material with better strain hardening properties will have more strength), plastic anisotropy (better values of plastic anisotropy means better drawability) and Forming Limit Diagrams (which explains the formability of a metal). The most common materials used are High Strength Low Allow Steels (HSLA). These alloy steels have good strength hence the thickness of sheets for a particular application can be reduced to decrease the weight of automobile. This in turn increases fuel efficiency which a desirable characteristic. These steels also have good formability and strain hardening properties which make them ideal choice for the automobile body. Other types of steels used are Low Strength Ultra Soft Steels used for parts with complex geometries (Sheet Metal Stamping in Automotive Industry, n.d.: 4). Design: While designing an automotive part there are a lot of considerations taken in to account. The weight of the part must be low but it must not compromise with the strength of the part. The part must also be easy to manufacture and cost must be low. The design must easily lend itself to the sheet metal forming process to be used in its manufacturing. Hence the body panels of automotives are designed with a rib structure resembling that of a honey comb. This gives extra strength to the body part but also helps in saving material which will cause a decrease in weight hence fulfilling the design requirements for an automobile. Conclusion Sheet metal forming is one of the most extensively used processes in automobile manufacturing industry. The process is cost efficient, adaptable and reliable especially for the production of automobile body parts (panels) as most of these parts are made up of thin metal sheets. The process can take many different forms to obtain parts of varying geometries and sizes. Out of all these types and forms, most commonly employed in automotive industry are fine blanking and drawing accompanied by trimming, hemming, roll forming etc. The material selection for the process is carried out by keeping in mind the formability of the materials and their strength. Design of the parts is also adapted to suit the manufacturing process being employed. Future Trends: The future trends in sheet metal forming process in automobile industry are shifting towards automation as in any other manufacturing process. The process parameters are controlled by computers (CAD/CAM) and roll of worker is being reduced (Lin and Kuo, 2008). Moreover, talking specifically about sheet metal forming, the techniques such as incremental sheet metal forming are gaining recognition in the industry as cheaper techniques that require simple tooling (Ready and Cao, n.d.: 1). In the incremental sheet metal forming the researchers and designers are introducing better techniques such as Electro-Magnetic Assisted Stamping (EMAS) for better productivity (Okoye, Jiang and Hu, 2006: 1-4). Manufacturers are interested in increasing the productivity and decreasing the losses in the process. This leads to investment in automation and other better controlling techniques. At the same time, the operation has to be kept simple and cost efficient which poses a great challenge for the researchers in this field. Reference Gowri, S., Hariharan, P. and Babu A.S. (2008) Manufacturing Technology-I, India: Pearson Education Boljanovic, V. (2004) Sheet Metal Forming Process and Die Design, New York: Industrial Press Inc. Askeland, D.R., Fulay, P.P. (2009) Essentials of Materials Science and Engineering, Canada: Cengage Learning Marciniak, Z., Duncan, J.L., Hu, S.J. 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