Hydroforming. Training Objective

Transcription

1 Training Objective After watching the video and reviewing this printed material, the viewer will gain knowledge and understanding of the hydroforming process and its use in manufacturing sheet metal parts. Both sheet metal hydroforming and tubular hydroforming are explained The specific requirements for each hydroforming type is featured The fundamental elements of a hydroforming system are discussed Metals used in hydroforming are explored Hydroforming uses fluid pressure applied to a tubular or sheet metal blank to form it into the desired component shape. The most commonly used materials in hydroforming include the various grades of steel. These grades of steel are used extensively because they exhibit good fatigue properties, high energy absorption, and reasonable resistance to corrosion. Other metals used for hydroforming include: Stainless Steels Aluminum Copper Copper Alloys The Hydroforming System All hydroforming systems include: Tooling and dies A hydraulic press A fluid-pressure intensification system Hydroforming dies vary dramatically with regards to sheet metal hydroforming and tubular hydroforming operations. Dies may be single or multi-cavity. Hydroforming utilizes hydraulic, rather than mechanical, presses for many reasons. Hydraulic presses are able to deliver full tonnage at any point in the stroke, not just at the bottom as is the case with mechanical presses. The hydraulic press can be adjusted to provide optimal part clearance. Also, hydraulic presses can be stopped in mid-stroke which is advantageous to some hydroforming operations. The fluid-pressure intensification pressure system consists of a low-pressure, high flow rate filling system, along with a high-pressure intensifier to raise the fluid pressure to forming levels. The forming fluid is waterbased and contains additives such as lubricants, drying agents, rust preventatives, and bactericides. In a typical hydroforming operating cycle, a blank is placed on the lower die tooling, and the press is closed. Fluid is then introduced to either one side of the sheet metal blank, or within the tubular blank. As fluid pressure is intensified the blank deforms, taking the shape of the tooling. Lubrication is especially important in hydroforming. Lubricants assist in reducing the friction and stresses as the metal flows into its final shape. Fundamental Manufacturing Processes Study Guide, DV03PUB28-1 -

2 Tubular Hydroforming Tubular hydroforming begins with the placement of either straight tubes, or more commonly preformed tubes, into the die. Sealing punches within the die close off the tube ends as fluid pressurization begins. The three surfaces of a tube that can act as sealing surfaces, include: The outside diameter of the tube The inside diameter of the tube The end surface of the tube During forming, a combination of increasing internal fluid pressure and a simultaneous axial pressing on the tube ends by the sealing punches cause the tubular material to flow into the contours of the die. Tubular hydroforming is generally divided into three operating techniques: Low-Pressure Hydroforming uses fluid pressures of 12,000 PSI/828 BAR or less. Cycle time is less than with other hydroforming methods, but components must be designed carefully to form properly using the lower fluid pressures. High-Pressure Hydroforming uses fluid pressures ranging from 20,000 to 100,000 PSI/1,379 to 6,895 BAR. The exact amount of pressure needed is dependent upon several factors such as material yield strength, tube wall thickness and the inside radius of the sharpest cross-sectional corner. When the tube is expanded by high pressure within the die cavity, material thickness may vary throughout the part. Additionally, larger presses are needed for high-pressure hydroforming and the higher operating pressures can result in longer cycle times. Pressure Sequence Hydroforming utilizes the closing action of the hydraulic press to assist in the hydroforming of the blank. The blank is first placed in the die cavity and the die is partially closed, partly deforming the tube. Low-pressurized fluid is then pumped into the blank allowing it to resist compression. The die starts to close again with the desired low-pressure maintained while part cross section reduces. Once the die is fully closed, high pressure is applied to the fluid, forcing the blank material into the corner recesses of the die cavity with no wall thinning. Maximum pressure for pressure sequence hydroforming is typically under 10,000 PSI/690 BAR. Tubular hydroforming cycles also include 'hydro-piercing' to create holes and slots in the part. The piercing tool is incorporated into the hydroforming die and activated during the forming cycle by hydraulic cylinders. The piercing action is usually inward against the pressurized fluid. Piercing can also be performed outwardly by retracting a plunger, or backup punch. The fluid pressure within the tube causes the surface material to fail, creating a hole in the unreinforced portion of the tube wall. Sheet Metal Hydroforming In sheet metal hydroforming, controlled metal flow during the operation minimizes localized stress concentrations that may cause workpiece buckling or wrinkling. Sheet metal hydroforming is slower than traditional stamping, thus its use is limited to short runs of more highly specialized parts. Fundamental Manufacturing Processes Study Guide, DV03PUB28-2 -

3 The two most common methods of sheet metal hydroforming are: Rubber Diaphragm Forming Active Hydro-Mechanical Drawing In Rubber Diaphragm Forming, a fluid forming chamber is attached to the slides of a hydraulic press. Fluid is retained within the chamber by a flexible rubber diaphragm, which serves as a universal die capable of accommodating any shape. Attached to the rubber diaphragm is a wear-pad which acts as a blankholder and protects the diaphragm. A single tool, typically a punch, but in some cases a cavity die, is mounted to a hydraulic cylinder on the bed of the press. A blank is placed on the lower blankholder of the rubber diaphragm forming press. The forming chamber is then lowered and initial pressure is applied. The punch moves upward and into the flexible die member, shaping the sheet metal material. Typical forming pressures range from 5,000 to 15,000 PSI/345 to 1,034 BAR. After forming is completed, pressure is released, the forming chamber is raised, and the punch is stripped from the finished part. Active Hydro-Mechanical Drawing is used in fabricating large sheet metal panels and can improve the buckling strength of the finished part. The lower blankholder is a reservoir containing an oil and water emulsion as a fluid medium, while the top die holds the punch. As the press closes over the blank it forms a water tight seal between the die halves. At this stage, a part-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 of the blank over its entire surface, resulting in work hardening of 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 punch is lowered into the blank. The sheet metal flows rather than rubs against the punch and female die by fluid pressure. Active Hydro-Mechanical Drawing is cost effective since only one die half needs to be machined to achieve the required part shape. Post Hydroforming Operations Although many secondary operations are performed during hydroforming, many post-hydroforming operations are needed to finish tubular and sheet metal parts. Common post-hydroforming processing includes: Additional holemaking using conventional drilling, laser drilling, punching, or cutting. Trimming to remove surplus metal from the hydroformed part using cutting lasers, trimming dies, machining, sawing, or shearing operations. Joining operations, such as the various welding processes and mechanical fasteners, are used to assemble hydroform components to existing structural assemblies. Fundamental Manufacturing Processes Study Guide, DV03PUB28-3 -

4 Review Questions 1. A specific advantage of using a hydraulic press in hydroforming is: a. high speed operation b. quick change dies c. operator safety d. full tonnage throughout the stroke 2. To raise fluid pressure to hydroforming levels: a. large electric pumps are used b. fluid conduits are restricted c. higher viscosity fluids are used d. a high pressure intensifier is used 3. In tubular hydroforming, most often: a. straight tubes are used b. preformed tubes are used c. softer aluminum is used d. pre-pierced tubing is used 4. In High Pressure Hydroforming, fluid pressure ranges from: a. 8,000 to 12,000 PSI/550 to 822 BAR b. 20,000 to 50,000 PSI/1,379 to 3,425 BAR c. 20,000 to 100,000 PSI/1,379 to 6,850 BAR d. 100,000 to 150,000 PSI/6,850 to 10,275 BAR 5. An advantage of Pressure Sequence Hydroforming is: a. higher production rates b. stronger alloys can be formed c. there is no wall thinning of the tube d. less expensive dies can be used 6. In Rubber Diaphragm Forming, the diaphragm: a. acts as a universal die b. is attached to the press bed c. is mounted to a hydraulic cylinder d. must be replaced after 6 to 12 cycles 7. Active Hydro-Mechanical Drawing: a. allows very large panels to be formed b. hardens the workpiece c. needs only one die half to be machined d. all of the above Fundamental Manufacturing Processes Study Guide, DV03PUB28-4 -

5 Answer Key 1. d 2. d 3. b 4. c 5. c 6. a 7. d Fundamental Manufacturing Processes Study Guide, DV03PUB28-5 -