A Novel Biaxial Testing Apparatus for the Determination of Forming Limit under Hot Stamping Conditions

The overall goal of this experiment is to determine the forming limits of sheet metals under hot stamping conditions through the use of a novel biaxial texting system. The main advantages of this technique is that it enables forming limits of sheet metals to be determined experimentally and accomplish forming conditions. Generally conventional formability tests are not applicable to hot stamping for two reasons.

First, the cooling process occurs prior to deformation and the second, controlling the forming parameters is very difficult. We were the first in the field to have the idea of using a novel biaxial testing system to measure forming limits and the complex forming conditions. For a formability test, set different strain paths including uniaxial plain strain and equibiaxial straining states.

Machine flat and dog bone cruciform specimens from commercial material aluminum alloy 6082. Use a laser cutter and a computer controlled milling machine. From each specimen, take the average of three thickness measurements made with vernier calipers or micrometers at each specimen’s central gauge region.

The cruciform specimen should measure 0.7 millimeters plus and minus five microns and the uniaxial specimen should measure 1.5 millimeters plus and minus 10 microns. Now, spray paint the entire top surface of a cruciform specimen with one coat of flame resistant black spray paint. The paint must withstand temperatures in excess of 1, 000 degrees Celsius.

After the paint dries, use white flame resistant spray paint to add dots for recognition by the DIC system. Spray from one arm’s length away to create a stochastic pattern. Then weld a pair of thermal couples to the center of the unpainted back surface.

First, assemble all parts of the biaxial testing apparatus including a base plate, a central shaft, input and output rotatable plates, carriages, a clamp, guide rails, and rigid connecting rods. Next, attach four welding cables to each pair of stainless steel and copper grips. Thus, connect the welding cables to the electrical power supply.

Now secure the grips of the biaxial testing apparatus in the uniaxial tensile test machine, and for added security, attach the apparatus with bolts to the topside, and to the bottom side of the base plate. Now load the specimen onto the specimen holders of the test apparatus and connect the thermal couples to the feedback temperature control system of the testing machine to monitor and control the temperature. Next, set up the heating and quenching system.

Connect each terminal of the welding cables to each clamping region of the specimen. Then, tighten each clamping region to the top plate which serves as the electrode for resistance heating. For cooling, connect hoses with flared nozzles to a high flow quench system.

Set the system to produce an air pressure of 8, 000 kilograms per square meter. Now, position four nozzles to move air from the specimen’s arms to its central region. Be sure to position the nozzles so they are not blocking the camera’s view of the central zone.

Now, attach micro lens to the high speed camera of the DIC system and set it up to view the central zone. Then connect the camera to a laptop. Adjust the frame rate to the test according to the stretching strain.

Plan to collect at least 200 frames of data per test. Set the capture resolution to 1, 280 by 1, 024. Now focus the camera on the gauge section with the lens parallel to the top surface of the specimen.

For high strain tests point an additional 300 watt spotlight directly into the test chamber. To start the resistance heating uniaxial tensile test machine, click the triangular run button in the control software. Then start the camera using a trigger button so it records the deformation history.

The specimen is now heat treated and stretched at the selected constant temperature and strain rate. Perform the tests at different strain paths consisting of uniaxial, plain strain, and biaxial states by adjusting the configuration of the biaxial testing apparatus. For uniaxial tests, disconnect the two opposed connecting rods and clamp a dog bone specimen to the apparatus.

Then connect the welding cables and repeat the other preparatory steps. For plain strain state tests, fix two opposed carriages to the base plate with screw bolts. This restricts the deformation on the corresponding direction.

Then proceed to collect data. Formability tests using the novel plain biaxial tensile testing system were conducted at the designated deformation temperatures and strain rates after the heating and cooling processes. A forming limit curve identifies the boundary between uniform deformation and the onset of plastic instability or diffuse necking which leads to failure.

As the strain rate increased, the forming limit of the alloy increased. A monotonic increase in the forming limit from 370 to 510 degrees Celsius indicates that high formability of the alloy can be obtained at a higher temperature under hot stamping conditions. The three forming limit curves are quite close to each other.

Thus, the sensitivity of the temperature dependence is larger for tension tension biaxial strain paths than for tension compression strain paths. After watching this video you should have a good understanding of how to set up an experiment and conduct the formability test at various temperatures and strain rates by using the proposed system. Following this procedure, forming limits of alloy sheet metals like magnesium alloy or boron steel, can also be melded in the hot stamping conditions.