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  A COMPARATIVE STUDY OF CONVENTIONAL FORMING PROCESS AND HYDROFORMING FOR AUTOMOTIVE INDUSTRIES 1. Introduction The overall usefulness of metals is to it can be formed in to various useful shapes. Nearly all metals go to some stage of deformation during their manufacture. Either by rolling, forging, extrusion, deep drawing etc. The deformation may be either in uniaxial, biaxial, or triaxial or combination of these in complex three dimensional [1]. These deformation causes by resulting action of tension, compression, radial, and shear stress. Hydroforming utilises these factors in combine action and deformation of metal takes place. This is done by storing the viscous fluid in closed chamber and pressure is to be applied by plunger or ram, so that pressurised fluid transfers its energy to deform the given metal. In Hydroforming deformation of material is not only the main advantage but also provides strength and better tolerance and surface finish to the metal and alloys. Metals such as aluminium, brass, low alloy steels, stainless steel which are ductile in nature are generally uses for hydroforming manufacturing process. Hydroforming allows complex shapes with multiple operation at a single feed so it is also a technique of mass  production within minimum time. 2. Literature review Hydroforming has been called with many other names depending on the time and country it was used. It is often mentioned along with tube hydroforming and sheet hydroforming, these utilize the principles of fluid dynamics to form metal parts[2]. Tube hydroforming first referenced from an early 1900‟s process for auto industry. The Cincinnati M illing machine Company (Recently- Jones Metal Products Company AS9100C certified, ISO 9001:2008 ) was incorporated in 1889, founded by Frederick V. Geiger and Fred Holz. By 1930s the company was the largest producer of machine tools in the world. In early 1950s the company produced the first metal forming machine  –    the hydroform used to „deep draw‟ case covers. Historically, the process was patented in the 1950s[3] but it was industrially spread in the 1970s for the production of large T-shaped  joints for the oil and gas industry. Vari-Form srcinated the concept and application of tube hydroforming to form body, chassis and other automotive structural parts. The company introduced Pressure- Sequence hydroforming (PSH) process in 1990 and is now industry leader which has produced 100 million parts to date. Aerospace and aircraft manufacturers search for outside processes which are assembly of highly complex products. These products are easily manufacture by sheet hydroforming. This process is  based on the 1950s patent for hydroforming by Fred Leuthesser, Jr. and John Fox of Cncinnati [4]. This was srcinally used in producing kitchen spouts because in addition to strengthening of  the metal hydroforming also produced less “grainy” parts which allows it for bett er metal finishing[5]. 3. The principle of hydroforming Hydroforming is a high-pressure deformation process that shapes metal sheets into a predefined geometry by using a fluid under high pressure. Hydroforming is similar to the conventional deep-drawing technique with a counter-mould. The specific difference from the conventional method is that a fluid is used instead of a die to form the metal sheet into its final shape. This deformation process requires fluid pressures up to 4000 bars, which means that very high tool closing forces need to be applied  –   depending on the size of the component. 4. Tubular hydroforming Tubular hydroforming begins with the placement of straight tubes in to the die but it commonly  begins with pre forming tubes by shaping the tube. Pre forming can be done before hydroforming using either rouge rotary draw bending method or up reforming die. Pre forming also can be done during hydroforming operation which is referred to as hydro bending. Celling punches within the die close the two ends while the necessary fluid pressurization at the two ends begins for hydroforming. This fluid pressurization typically removes wrinkles or imperfections in the tube. There are three surfaces that can be used as a sealing surface these are the outside diameter, inside diameter and end surface. During forming a combination of increased internal pressure and a simultaneous axial pressing on the two ends by the celling punches causes the material to flow in to the contours at the dye. Tubular hydroforming can be generally divided in to three classifications according to their operating technique [6]- low pressure hydroforming (LPH), high pressure hydroforming (HPH) and pressure sequence hydroforming ( PSH). Low pressure hydroforming is commonly defined as hydroforming at fluid pressure below 12,000 psi (pounds per square inch) or 828bar. Its production cycle time is relatively lower than high pressure hydroforming but components are carefully design to form properly using the lower fluid pressures. In high pressure hydroforming tube blanks are placed in the die and die have closed without  pressurized fluid in the blank, once closed pressurised fluid is then injected into the blank completely filling the material out to the shape of the die. In high pressure hydroforming fluid  pressure commonly exceeds 20,000 psi or 1,379bar and can reach 100,000 psi or 6,895bar. Actual pressure needed is dependent upon several factors such as material yield strength, tube wall thickness and inside radius of the sharpest cross- sectional corner. Additionally larger  pressure is needed for high pressure hydroforming which results in longer cycle times in  pressure.  In pressure sequence hydroforming the closing action of the press assists in the hydroforming of the blank. The blank is placed in the die cavity and die is partially closed and partially deform the tube. Low pressurised fluid is then pumped in to the blank allowing it to resist compression, and at this point blank is like a formable solid. The die starts to close again with the desired low-  pressure maintained while cross section reduces. Low pressure during die closure discourages  pinching between the die this liquid pressure also resist unwanted inward deformation. Once the die is fully closed high pressure is applied to the fluid forcing material into the corner recesses of die cavity. When properly designed manage complex shapes can be generated from tubes with less pressure than needed for other hydroforming techniques. Maximum pressure is typically under 10,000 psi or 690bar also with pressure sequence hydroforming the tube is forced into the desired location and shape with no wall thinning and leave no room to expand. High pressure hydroforming and pressure sequence hydroforming both effect forming mechanism of material differently so this reason selection of material is fundamental to successful production. Hydroforming also allows for many secondary operations to be performed within the process. The most dimensionally stable and robust and economical way to generate holes or slots in tubular hydro formed parts is to punch them in the hydroforming die during the forming cycle this is referred to as hydro piercing. Hydro piercing tooling is built into the hydroforming die and uses a hydraulic cylinder to activate the punch. 5. Sheet hydroforming In sheet metal hydro forming controlled metal flow during the operation minimises the localized stress concentrations that may cause workpiece buckling or wrinkling. Sheet metal hydro forming cycle times relatively slow as compared to mass production stamping. This may limit its use to low-volume production. The common method of sheet metal hydroforming include Rubber Diaphragm Forming and Active Hydro Mechanical Drawing. In rubber diaphragm forming a fluid forming chamber is attached to the slides of the hydraulic press. Fluid is retained within the chamber by a flexible rubber diaphragm which serves as a universal die which is capable of accommodating any shape. A blank is placed on the lower blank holder above the rubber diaphragm forming press and the forming chamber is then lowered and initial pressure is apply. The punch moved upward into the flexible die member which shaping the sheet metal material. Typical pressures for rubber diaphragm forming range from 5,000 psi (345 bar) to 15,000 psi (10,034 bar). After forming is completed pressure is released and the forming chamber is raised and the punch is stripped from the finished part. Large panel sheet metal components, such as automobile hoods, doors and quarter panels have minimum buckling strength at their centers this is due to the low level of defamation that occurs during their manufacturing which results in minimum strain hardening. This deficiency can be  overcome by using higher strength materials or reinforcing element, both of which can add cost and wait to a vehicle. Active hydro mechanical drawing is a process that is typically used in forming large sheet metal panels while improving their buckling strength. The lower blank older is a reservoir that holds the oil/water emulsion fluid medium and the top female die holds the punch. A large size blank is placed over the reservoir and the press is closed and clamped all around with a water tight seal between the female die and the blank holder. At this stage a partially specific gap exists between the clamped blank and the punch. As soon as the blank holding force has built the fluid is brought to a defined pressure this pressure causes a controlled bulging at the blank over its entire surface resulting in work hardening at the workpiece and a substantial improvement in  buckling strength of the part. Bulging continues until the blank comes to rest against the center of the punch surface. After this pre stretching process the punches lowered into the blank the sheet metal flows rather than rubs against the punch and female die by fluid pressure. The blanks is formed against fluid pressure to achieve the required product shape. Many secondary operations are performed during the hydroforming process i.e. post hydroforming operations are needed to finish tubular and sheet metal parts. Common post hydroforming processing includes additional whole making trimming and joining operations. this hole making process include conventional drilling, laser drilling, punching, cutting and machining processes. Trimming  process removes the surplus forming material from hydroformed part to produce the desired part. It uses cutting laser, trimming dies, machining sawing and shearing operations. Joining operations are performed to assemble hydroformed components to obtaine required shape. It includes various welding techniques and mechanical fasteners. 6. Advantage of Hydroforming- Hydroforming is similar to the conventional deep-drawing technique, but has significant advantages for the formed part and keeps the tooling costs and hence the component production costs low. 1.) Better degree of deformation for the formed part- By applying a uniform force to the metal sheet, the fluid shapes it into the form of the tool. In this process, a uniform distribution of sheet thicknesses is achieved, which allows for maximum degrees of deformation. Abrupt changes in stress are avoided  –   a factor that ensures high dimensional accuracy and reduces the tendency of the material to return to its srcinal size and shape when the applied load is removed. Conventional deep-drawing a) strong local thinning of the material  b) inhomogeneous distribution of material thicknesses
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