Effects of Solidification Rate on Porosity Formation and Cast Microstructure in Aluminum Alloy A356. Dan VanderBoon

Transcription

1 Effects of Solidification Rate on Porosity Formation and Cast Microstructure in Aluminum Alloy A356 by Dan VanderBoon School of Engineering Padnos College of Engineering & Computing Grand Valley State University Laboratory Module 3 EGR 250 Materials Science & Engineering Section 2 Instructor: Dr. P. N. Anyalebechi February 1, 2005

2 1 I. Abstract The purpose of this report was to analyze the effects of solidification rate on the microstructure of the cast aluminum alloy A356. Four specimens of A356 that solidified at different rates were examined with an optical microscope. Photomicrographs of each specimen were taken at high and low magnifications. The photomicrographs revealed a trend in porosity formation. Decreasing solidification rate caused fewer pores, bigger pores, and more area of the photomicrograph was taken up by the pores. It was also observed that an increase in solidification rate resulted in finer microstructure. In other words, all the microstructure features (porosity, dendrite cells, eutectic silicon particles, and [AlFeSi] constituent phase particles) became smaller. II. Introduction Metallography is the science of evaluating metal structures [1]. This is done on the macro and micro levels. The optical microscope is useful on the micro level to provide extensive information about a materials microstructure. The class of the material involved, method of manufacturing, whether the material has been heat-treated or not, and other inferences can be gathered from an optical microscope [1]. In this laboratory, an optical microscope was used to examine the effects of solidification rate on the microstructure of aluminum alloy A356. A356 is used for aircraft pump parts, automotive transmission cases, and water-cooled cylinder blocks [2]. Aluminum alloys are favorites of the transportation industry because they can decrease fuel consumption as a result of a high specific strength. High corrosion resistance, ductility, and electrical and thermal conductivity are other favorable properties of aluminum [2]. A356 is a cast, heat-treatable alloy [2]. It is subjected to a solution heat treatment followed by artificial aging [2]. This process is referred to as precipitation hardening [2]. In solution heat treatment all the solute atoms are dissolved to form a single-phase solid solution [2]. This is accomplished by heating the alloy to a temperature within a phase field and waiting for all other phases present to dissolve into that particular phase [2]. After this the solution is cooled. The cooling step in the alloy treatment process affects the microstructure of A356. However, it is not the cooling rate but the solidification rate that is taken into account in this laboratory. This

3 2 distinction must be realized. Cooling rate is the slope of the cooling curve before solidification begins [3]. Solidification is the phase transformation of the non-crystalline liquid to the crystalline solid [3]. At the freezing temperature, where solidification begins, there is a thermal arrest that levels out the cooling curve, which is caused by the latent heat of fusion [3]. In other words, the latent heat of fusion keeps the remaining liquid at freezing temperature until all of the liquid has solidified [3]. Because of this, the cooling rate is considerably different than the solidification rate. The four specimens of A356 examined in the laboratory were solidified at four different rates resulting in considerably different microstructures. This report lays out how the microstructures of the specimens differed and the cause of this variation. This is done in the following sections. Section III describes the experimental procedure. Section IV reveals the photomicrographs and results. Section V gives a discussion on these results. Section VI is the conclusion of the report. III. Experimental Procedure The average composition of A356 based on weight is 7.0% silicon, 0.3% magnesium, and the remainder aluminum. Specimens of A356 solidified at 6.2 Kelvin/s, 3.8 K/s, 2.0 K/s, and 0.2 K/s were examined at a range of magnifications with the Olympus IX50 optical microscope. Photomicrographs were taken of each sample at 50X and 500X magnification. Care was taken to include porosity and dendrite cell features in the 50X photomicrographs. Eutectic silicon particles and [AlFeSi] constituent phase particles were revealed in the 500X photomicrographs. IV. Results Porosity, dendrite cells, eutectic silicon particles, and [AlFeSi] constituent phase particles are the four microstructural features that were expected from the cast aluminum-silicon alloy. All four features were seen through the optical microscope. Porosity (pores of hydrogen gas) appeared as dark spots in the photomicrographs because they reflected light away from the eyepiece. The bottoms of the holes could be focused into, making the rest of the viewable area blurry. [AlFeSi] constituent phase particles are grayer and have irregular boundaries. The cell boundaries of the dendrite cells are defined by the small dark dots called eutectic silicon particles.

4 3 It can be observed from Figure 1 that as the solidification rate increases, so does the number of pores. However the pore size and the volume fraction of porosity decrease with an increase in solidification rate. Porosity 6.2 K/s 3.8 K/s Dendrite Cell 2.0 K/s 0.2 K/s Figure 1: Effect of Solidification on Aluminum Alloy A356 at different cooling rates (50X) In Figure 2 the eutectic silicon particles and [AlFeSi] constituent phase particles are more visible. These two features as well as the dendrite cells are larger at slower solidification rates. From this, it is evident that faster solidification rates result in finer microstructure.

5 4 6.2 K/s 2.0 K/s [AlFeSi] Constituent Phase Particle 3.8 K/s 0.2 K/s Eutectic Silicon Particles Figure 2: Effect of Solidification on Aluminum Alloy A356 at different cooling rates (500X) V. Discussion Porosity was not revealed in the 500X magnification photomicrographs. This shows the importance of viewing specimens over a large range of magnifications. A certain trend was discovered relating to porosity. Increasing the solidification rate increased the number of pores, and decreased pore size and the volume fraction of porosity. The reason for this is that when the alloy is cast, the outside of the casting is first to solidify. The result is that the hydrogen gas trapped in the molten metal is retained inside the casting. The longer it takes for the inside of the casting to solidify, the more time there is for the many small hydrogen

6 5 bubbles to merge and diffuse creating pockets of hydrogen gas. So when the casting solidifies faster, there are more pockets, and the pockets are smaller. When the casting cools slowly, the smaller pockets merge with each other to form larger pockets and as a result there are less pockets. Also, a casting with a slow solidification rate has more porosity volume because the hydrogen has more time to diffuse. VI. Conclusions Increased solidification rates result in: 1. Finer microstructure 2. More pores 3. Smaller pore size 4. Reduced volume fraction porosity VII. References 1. Dr. P. N. Anyalebechi. Essentials of Materials Science & Engineering Laboratory Manual. January William D. Callister, Jr. Fundamentals of Materials Science and Engineering: An integrated approach. 2 nd edition. John Wiley & Sons, Inc. Hoboken, New Jersey, 2005, pp Dr. P. N. Anyalebechi. Essentials of Materials Science & Engineering. January pp