Materials Science and Engineering - Experiment | EGR 250, Lab Reports of Materials science

Download Materials Science and Engineering - Experiment | EGR 250 and more Lab Reports Materials science in PDF only on Docsity! The Effects of Solidification Rates on Porosity Formation and Cast Microstructure of Aluminum Alloy A356 by Hieu Nguyen School of Engineering Grand Valley State University Laboratory Module 3 EGR250 – Materials Science & Engineering Section 01 Instructor: Dr. P. N. Anyalebechi February 1, 2005 The Effects of Solidification Rates on the Formation of Gas Porosity and Cast Microstructure of the Aluminum Alloy A356 Abstract The effects of solidification (cooling) rates on gas porosity formed in aluminum alloy A356 are discussed in this paper. The experiment was done to study how gas pores and other features of the microstructure formed in relation to the cooling rate and how the mechanical properties of the alloy were affected as a result. With four samples of the alloy cooled at different rates, a metallurgical microscope was used to observe the microsurface of the material. It was found that the formation of gas pores (and various features of the microstructure) and the strength of the alloy were dependent on the solidification rate. 1. Introduction Aluminum alloy A356 (Al-Si) has many uses in the industry due to its excellent welding characteristics and high resistance to corrosion, cracking, separating, and shrinkage [1]. The material also has excellent casting characteristics, which leads to many applications. Some of the applications that the alloy is applied to are airframe castings, machine parts, truck chassis parts, aircraft and missile components, and structural parts requiring high strength [2]. Though there are many uses for the material, metallurgists and engineers are concerned with gas porosity in the alloy, which could decrease its strength. The formation of gas porosity in A356 occurs because there are traces of hydrogen gas bubbles (pores) within the material while it is solidifying after the casting process. These pores do not form while the material is solidifying because the solubility of hydrogen is much lower in the solid state than it is in the liquid state [3]. The rate at which the heat is extracted determines the strength of the alloy. If the cooling rate is comparatively high, then the pores are small but numerous. If the cooling rate is low, then the pores are large but there are fewer of them. The effects of solidification rates on gas porosity and the microstructure of aluminum alloy A356 will be discussed in this paper. The results of the experiment will attempt to verify that the solidification rates have an effect on the formation of gas pores and the strength of cast Al-Si alloy. Also included in this paper will be a discussion on the importance of the microscopy 1 Figure 2 shows the micrographs of the samples taken at 500x magnification. The micrographs in Figure 2 show the eutectic silicon particles and the constituent phase particles. The silicon particles can be identified by its darker color and propensity to group together with other silicon particles to form bands. When solidified at a high rate, these particles contribute to the alloy’s casting characteristics. However, if cooled slowly, the eutectic particles become larger and needle-like, weakening the metal [5]. These particles are more frequent than the phase particles, which are lighter in color. Constituent phase particles, a mixture of [AlFeSi], form during solidification and influence the alloy’s plastic behavior [6]. These [AlFeSi] particles, like the eutectic particles, can weaken the mechanical properties of the aluminum- silicon alloy if the metal is solidified at a low rate. eutectic silicon particles 6.5 ºC/s 1.6 ºC/s constituent phase particles 0.7 ºC/s 0.2 ºC/s Figure 2: Micrographs of the four aluminum alloy A356, with solidification rates labeled, samples at 500x magnification showing the eutectic silicon particles and constituent phase particles 4 The features found on the microstructure of the specimens are consistent with that of cast Al-Si alloy. Previous examinations of the alloy have revealed similar findings. The examinations have also shown that shrinkage pores and a combination of gas-shrinkage pores can be found on the surface of Al-Si alloy. The formation of the former, however, occurs only at very low hydrogen gas content [7]. 4. Discussion It is more desirable to cool cast Al-Si alloys at high rates because it increases the strength of the material. Although high solidifying rates cause the amount of gas pores to increase, these pores are smaller in size than the pores created by low solidifying rates. The smaller pores take away less cross-sectional area than the larger pores, and thus greater loads can be applied to the material. The high rates also refine the [AlFeSi] particles, which, like gas porosity, affect the mechanical properties of the material. Coarse [AlFeSi] particles (and eutectic silicon particles) tend to weaken the structure of the alloy because of its brittle nature [5]. By increasing the solidification rate, the hardness value of the phase particles increase, thereby strengthening the alloy. Microscopy examination of the samples over a range of magnifications is important in describing the features of the microstructure. Features identified at a higher magnification would not have been seen at lower magnifications. For example, the micrographs in Figure 2, taken at 500x magnification, showed the constituent phase particles lying near the eutectic silicon particles. In Figure 1, the micrographs taken at 50x revealed the outline of the eutectic particles, but did not reveal the phase particles. By observing the specimen over a range of magnifications, the features on the microstructure can be identified and defined. 5. Conclusions 1. There was a relationship between the rates of solidification and the formation of gas porosity and other microstructure features on aluminum alloy A356. 5 2. High solidification rates enhance the material’s strength and other mechanical properties; low solidification rates are detrimental to the strength of the material and reduce its mechanical properties. REFERENCES 1. Aluminum Alloy Capabilities, [Online], Copyright 2001, Available: http://www.hitchcockusa.com/engineering/aluminum.html. 2. CSD: Tech Articles: A Guide to Aluminum Casting Alloys, [Online], Available: http://www.castingsource.com/tech_art_guide1.asp 3. Aluminum Alloys – Effects of Alloying Elements, [Online], 24 January 2005-last update, Available: http://www.key-to-metals.com/Article55.htm. 4. Alum-Alloy: Specs, [Online], Copyright 1999, Available: http://www.alumalloy.com/specs.html. 5. Kammer, C., 1999, Aluminum Handbook, Vol. 1, Aluminum-Verlag, Dusseldorf, Germany; 6. S. H. Choi, F. Barlat, and J. Liu: “Effect of Precipitates on Plastic Anisotropy for Polycrystalline Aluminum Alloys,” [Online], Metallurgical and Material Transactions, Vol. 32A, September 2001; Available: http://doc.tms.org/ezMerchant/prodtms.nsf/ProductLookupItemID/MMTA-0109- 2239/$FILE/MMTA-0109-2239F.pdf?OpenElement 7. J. P. Anson and J. E. Gruzleski, “Effect of Hydrogen Content on Relative Shrinking and Gas Porosity in Al-7% Si Casting,” AFS Transactions, pg. 136. 6