Examination of the Effects of Solidification Rate on the Formation of Prominent Features in A356 Aluminum Alloy Specimen.

Transcription

1 Examination of the Effects of Solidification Rate on the Formation of Prominent Features in A356 Aluminum Alloy Specimen by Brad Peirson School of Engineering Grand Valley State University Laboratory Module 3 EGR 250 Materials Science and Engineering Section 1 Instructor: Dr. P.N. Anyalebechi May 31, 2005

2 Abstract Metallographic examination is a key tool in determining the microstructure of a metal sample. An experiment was run using a metallographic examination to do just that. The purpose of this laboratory was to determine the effect of solidification rate on the microstructure of A356 aluminum alloy samples. The samples as well as their cooling rates were given by the instructor. The microstructure was examined using photomicrographs taken by the metallurgical microscope. It was determined that in general as solidification rate increases the size of the prominent features in the microstructure decreases. 1 Introduction Metallographic examination can be a useful technique in determining the microstructure of a cast metal sample. The metallurgical microscope offers a magnified view of the features of a cast sample. Proper interpretation of the features can lead to a determination of the solidification rate, the composition and give an idea of the mechanical properties of the sample. The metallurgical microscope is the primary tool used in any metallographic examination. A traditional biological microscope works by reading light as it passes through a specimen. Metal samples, however, are opaque and do not allow light to pass through. A metallurgical microscope reflects light off of the specimen in order to display the features. Some metallurgical microscopes also have the ability to save a photomicrograph of the specimen for examination away from the microscope itself. A356 is a cast aluminum alloy that contains a large quantity of the alloying elements silicon (Si), magnesium (Mg) and iron (Fe). The iron and silicon form [Al Fe Si] constituent phase particles. On a photomicrograph these particles generally appear lighter in color than the aluminum dendrite cells. They form structures shaped like sand grains. The silicon also forms eutectic silicon particles. These structures are also lighter colored than the surrounding aluminum dendrite cells. The eutectic silicon particles are needle-like in shape. The aluminum dendrite cells can also be used to determine the property of the alloy specimen. Generally the smaller the cells the stronger the casting will be. A 1

3 sample that has relatively small cells will also tend to have comparatively smaller [Al Fe Si] constituent phase particles and eutectic silicon particles. In this way the relative sixe of the features of the alloy can determine its mechanical properties. 2 Experimental Procedure Two A356 samples were obtained from the instructor. Each of samples was examined on the metallurgical microscope at 50x magnification and at 500x magnification. A photomicrograph was taken of each sample at both magnifications. Another group was given two different samples. The photomicrographs were combined for a total of four at each magnification. The cooling rate of each sample was also given by the instructor. The photomicrographs at 50x are shown in Figure 1. Figure 1: 50x Photomicrographs of A356 Samples with Solidification Rates (a) 6.2 C/sec, (b) 3.8 C/sec, (c) 2.0 C/sec and (d) 0.2 C/sec 2

4 The locations of the photomicrographs were chosen to illustrate the size of the dendrite cells as well as the porosity of each sample. Figure 2 shows the four photomicrographs taken at the 500x magnification setting. Figure 2: 500x Photomicrographs of A356 Samples with Solidification Rates (a) 6.2 C/sec, (b) 3.8 C/sec, (c) 2.0 C/sec and (d) 0.2 C/sec The locations of the photomicrographs at the 500x level were chosen mainly to illustrate the dendrite cell size. 3 Experimental Results Figure 1 illustrates the effects of solidification on the formation of pores in cast aluminum alloys. As the solidification rate increased the size of the pores decreased. The photomicrographs showed that while the size of the pores decreased with an increase 3

5 in solidification rate they were far more frequent than in the slower cooled samples. Figure 1 also gave a sense of the differences in relative size of the dendrite cells as solidification rates increased. Figure 2 shows the relationship of the size of the dendrite cells to the solidification rate. As the solidification rate increased the dendrite cell size decreased. Figure 2 also shows the difference in pore frequency. There were several pores in (a) and (b) but there were none in (c) and (d) because of the decreased frequency. The [Al Fe Si] constituent phase particles and eutectic silicon particles were also present at the 500x magnification level. These features were nearly indistinguishable at the faster solidification rates. In (d), the slowest solidification rate, these features could just be seen. 4 Discussion The solidification rate effects the size of the dendrite cells because of the time the aluminum has to pull together. At the slower rates the aluminum has more time to form. It can form larger cells as well as merge cells before it becomes a solid piece. Evidence of this merging can be seen in Figure 2. The photomicrographs of the samples cooled at slower rates show cells that are not only larger but appear to have fewer arms or branches. This suggests that the larger cells were formed by smaller cells running together during solidification. The porosity of the alloy at the different cooling rates has a similar explanation. Porosity in aluminum and aluminum alloys is caused by hydrogen gas. The gas dissolves in the molten aluminum. As the metal is solidified the gas is expelled from the solid into the remaining liquid. When there is no more liquid to disperse into the hydrogen forms a pore. The slower solidification rates shown in Figure 1 have larger pores because the hydrogen has more time to move from the solid aluminum to the remaining liquid. The pores also have time to merge together while the metal is still a fair amount of liquid. The faster rates in Figure 1 have smaller pores because the opposite is true. The hydrogen does not have enough time to dissolve into the remaining liquid aluminum and is supersaturated in the solid aluminum. This is also one of the reasons that the faster rates typically have greater strength. While the pores are more frequent in the sample, 4

6 they have a much smaller combined area than the larger pores in the slow cooled samples. This means there is more metal cross-sectional area to dissipate any applied force. Solidification rate also affects the [Al Fe Si] constituent phase particles and the eutectic silicon particles. These particles form in the same way as the dendrite cells and pores. Faster rates give them less time to pull together and result in smaller particles. This is shown in Figure 2. These particles are not visible in the photomicrographs of the samples with the faster solidification rates. They only become visible, and are still relatively small, in the photomicrographs of the specimen with the slowest rate. 5 Conclusions 1. Solidification rates affect the microstructure of the microstructure of aluminum alloys. 2. As the solidification rate increases the alloy forms smaller dendrite cells. 3. As the solidification rate increases the alloy forms smaller pores. 4. Even though the pores are smaller at greater solidification rates they appear more frequently. 5. Smaller, more frequent pores have less combined volume than larger, less frequent pores. 6. As the solidification rate increases the alloy forms smaller [Al Fe Si] constituent pahse particles. 7. As the solidification rate increases the alloy forms smaller eutectic silicon particles. 8. Greater solidification rates lead to a sample that exhibits greater strength. 5