8

Bend Formability Study

Submitted by: Samir KhodaIntern07/20/2015

Quality AssuranceResearch and Development/Process EngineeringPMX Industries, Inc.5300 Willow Creek Dr. SWCedar Rapids, IA 52404

Table of ContentsIntroduction1Materials and Methods2Results and Discussion3Conclusions4References5Appendix6

IntroductionThe bend formability of a metal is its ability to be shaped or manipulated from its original position with minimal or close to no loss in its performance. When a metal is bent, the thickness goes through a differential loss while the length has a differential gain. When the bend radius is increased, the thickness of the metal has a larger decrease; and therefore, the length has a larger gain. If the decrease is large enough compared to the original thickness of the metal, there is a large probability for cracks or breakage of the piece. Cracks are shown to be an early indicator of a future breakage, and therefore, assumed to be a failure for the alloy. Due to the increase in demand for smaller connectors, terminals, etc., bend formability studies are becoming more important to display the potential usability and limits of various copper alloys.These tests are done in accordance with ASTM B820 (Appendix). The different alloys have bend tests done in both the longitudinal (good way) and the transverse (bad way) direction at the specified bend radius. Figure 1 shows an example of what this means.

Figure 1 Longitudinal (good way) direction vs. transverse (bad way) direction

The longitudinal bend is done in the direction of the roll, and usually has a better bend formability rating when the test is conducted in this direction rather than the transverse direction. There are exceptions to this rule though, which is why both tests are done so that the optimal (the smallest) thickness for the specific alloy is deemed durable enough for usage in a companys products.The interpretation of the bend tests are done in a subjective manner with the following rating system in Table 1.Table 1 Rating System for Bend TestRatingDescription

1Very low amount or no orange peeling

2Medium amount of orange peeling

3High amount of orange peeling

4Small amount of cracks with not much depth

5Medium level of cracks

6Large and deep cracks

Ratings of 1-3 determine a passing grade for the strip of copper, while 4-6 is failure for the strip. The bend tests are observed under a microscope with 30X magnification. The ratings are standardized with the bending radius (R), the width (W), and the thickness in a graph of the W/t ratio vs. the R/t ratio. The only points recorded in the graph are the ones with the ratings of 2 and 3 to show the customer the recommended and mandatory limits of the alloy.Materials and MethodsA sample strip of each alloy is collected from production and cut up into 47 mm by 47 mm squares. After the squares are collected, forty of them are marked either good way or bad way (so there are 20 of each square) and then taken to the punching die. The punching die make ten samples of varying widths (2 0.33 mm samples, 2 0.635 mm samples, 2 1.22 mm samples, 2 2.413 mm samples, and 2 5 mm). Other samples that are made for more data points are 8 mm and 12 mm in width. Since the R/t value is plateaued after a W/t value of 30, the 12 mm sample is sometimes not included in the data for the specific alloy if the thickness is too low. Once the samples are cut and prepared, they are cleaned with an alkaline detergent to remove any grease which could have accumulated from the production line or sample preparation. The squares are cut in half as well so that each sample section can be run in different trials. An example of a sample is shown in Figure 2 with each sample being roughly 30 mm in length.

Figure 2 Sample used for bend testThe bending die is set to the appropriate setting based on the punch radius and material thickness, and then each sample is bent to the desired angle. Once the metal is bent appropriately, it is placed under the microscope for observation and then a rating is determined based on Table 1. Even though a 90 bend test is usually included in the report, for some companies, a 180 test is acceptable on its own because it shows the durability of the alloy in the most extreme case of bending for that specific bend radius. Results and DiscussionOne trend that was easily seen right away was that the rating for the samples would get worse as the bend radius got smaller. This makes sense because the smaller the bend radius, the more stress and strain the sample will have because they are both proportional to the reciprocal of the radius as shown in Equation 1 and 2 with consideration to the y, the distance from the neutral axis of the material. = bending strain=bending radius=bending stressE=Youngs Modulus

(1)(2)This makes it more likely for more severe orange peeling or cracks to happen at smaller radii. Another trend is the correlation between a bigger width and a lower rating. This is due to the fact that at a smaller width, the plane strain is more dominant than the plane stress and allows for relief because of allowing the dislocation movement which enables plastic deformation. A dislocation is essentially a stacking error for atoms where some of the atoms deviate from the crystal lattice structure which increases the volume of the material but maintains the same amount of matter. At a larger width, plane stress is more dominant because the transverse axis for plane strain decreases to 0 (finally reaches that value when width/thickness > 8) and therefore prevents the dislocations from moving. If the dislocations cannot move, cracks form in the material because of the high internal stress.Dislocation is also behind the reasoning for plastic deformation when an alloy is heated up. The heat causes more vibrations in the atoms which cause more space in between them to allow the dislocations to move. At colder temperatures, when the atoms are closer together, the dislocations have a lower possibility to move which would promote cracks and breakage. This is why forging and shaping of alloys occurs at very high temperatures so that plastic deformation can easily occur.An example of the results from the bend tests are shown in Figure 3. The curve for rating 3 provides information on combinations of radii and widths which would be deemed passable by PMX. This means that there are no cracks formed in the alloy when subjected to a bend of a specific radius. Any combinations below the curve have a very high chance of forming cracks or breaking which is useful information for customers who use the alloy in their products. Especially because companies are making connectors and terminals, for example, at smaller sizes, it is necessary to show the customer the limits of the alloy so that breakage does not occur in the product. The curve for rating 2 provides information on the recommended limits for combinations of widths and radii. This gives the customer a set of conditions below the curve which would not form cracks or break which provides a safety net for PMX in guaranteeing quality of the alloy. Most customers request the R/t value at a W/t of 10 for both curves because that is when the quality of the alloy is maintained at any width greater than 10t because the curve plateaus at that specific R/t. Providing the entire curve also enables the customer to see other options not usually known if only the data for W/t > 10 is available. Knowing this data helps PMX with selling the alloy by providing options for product formation if the customer requires the alloy to be at a smaller radius than the one provided at W/t=10.

Figure 3 Data from Bend Test for Coil #E5M5BA-AConclusions

The bend formability tests are necessary to not only determine the minimum radii for usage of an alloy, but it also allows for the determination of the maximum width of the material. Because copper is a ubiquitous material used in industry and the possibilities for alloys are endless, it is necessary to insure that the product will not break if bent in a certain way or exposed to various stresses and strains. Especially since a lot of alloys are strengthened via strain hardening and cold rolling, it is necessary to understand the limits of these procedures because while preventing the dislocations from moving as much would allow for a more uniform stress distribution, it will also increase the chance of breakage and cracks at certain widths and radii.Because of this, there are other ways to bypass ASTM temper codes to improve feasibility, efficiency, and profitability from material formation for the customer while also maintaining the safety features and integrity of the alloy as well. Examples of this include thermal stress relief annealing, thermal stress relief annealing + bend/tension leveling, tension annealing, precipitation hardening v. cold rolled mechanical property development, particle size/distribution control, modification of particles to effect precipitate coherency and therefore energy for dislocation shear. Even though this allows for more flexibility in product formation, other factors still need to be held in consideration for product performance, such as tightening gauge tolerance, controlling frictional forces, controlling oxide film, and adding coatings to support lubricant distribution or reduce sliding forces and adhesive buildup.

ReferencesAl-tahat, Mohammad.Experiment 6 Spring Back in Bending of Sheet Metals and Plates. Tech. N.p.: U of Jordan, n.d. Print.Committee B05, ed.Standard Test Method for Bend Test for Determining the Formability of Copper and Copper Alloy Strip. Tech. no. ASTM B820. West Conshohocken: ASTM International, 2004. Print.Dieter, George E.Mechanical Metallurgy. New York: McGraw-Hill, 1976. Print.Gedeon, Mike.Formability and Bend Testing. Tech. Vol. 2. Cleveland: Brush Wellman, 2000. Print. Ser. 3.Wojnicz, Lawrence.Bending Properties. Rep. Vol. 5. N.p.: Molex Engineering Specifications, 2009. Print. Ser. 5.

Appendix Page 6 of 9