Verification of Heat Treatments Effect on the Microstructure of Aluminum Alloy (Al-Zn)

VERIFICATION OF HEAT TREATMENTS EFFECT ON THE MICROSTRUCTURE OF ALUMINUM ALLOY (Al-Zn) *1 I. A. LATEEF, and 2 M. O. DUROWOJU 1Department of Mechanical Engineering, Osun State College of Technology, P. M. B. 1011, Esa – Oke, Nigeria. [email  protected] com 2Department of Mechanical Engineering, Ladoke Akintola University of Technology, Ogbomoso, Oyo,State, Nigeria *(Address of correspondence) ABSTRACT: This study focused on the effect of heat treatment on the microstructure of Aluminum – Zinc Alloy. The composition of the Aluminum alloy used is eighty percent (80%) of Aluminum and twenty percent (20%) of Zinc. The method of casting employed is Sand casting, while annealing and quenching were heat treatment processes it’s subjected to. The microstructure is taken. It is concluded that heat treatment have effect on the microstructure of Aluminum – Zinc alloy and responded well to Oil quench. [Keywords: Aluminum-Zinc Alloy, Heat treatment, Microstructure, Tensile test, Hardness test, Annealing and Water quenching. INTRODUCTION The extensive application of metals in the field of Engineering has necessitated the need for metal heat treatments in order to meet the taste of firm, industries and individual as a result of their wide engineering application. One of the major engineering fields where the application of various metals was used is Automobile engineering. In order to reduce motor vehicle weight, many automotive components have been redesigned to take advantage o f material such as lightweight and polymers. To this end aluminum casting alloys are being widely used for many automotive components (CAD, 2006), the alloys have good casting characteristics, reasonable mechanical properties, and are heat treatable (Melo, Rizzo and Santos, 2005). Material science and engineering today has developed to a stage where correlations between microstructure, properties and application can be established for many commonly used alloys. The structure of a material is related to its composition, properties, processing history and performance (Argo and Gruziesk,(1988) and Rooy,(1993)). And therefore, studying the microstructure of aluminum alloys provides information linking its composition and processing to its properties and performance interpretation of microstructure requires the understanding of the process by which various structures are formed. There are many processes by which aluminum alloys can be formed. The major process that is common and the focus of this study is the Sand casting process (Albert, (1957)). One of the common defects in aluminum castings is porosity (Monroe, (2005) and Tyler, 1981)). It is a clear fact that the quantity and the appearance of the porosity are very crucial to the mechanical properties of the aluminum alloy casting, most especially the fatigue properties because the pore in micro scale are primary source of initial cracks for the final failure of the aluminum parts. Due to this the aluminum alloys structure need to be improved by metal treatment to have the required properties. Olanrewaju (2000), stated that metal treatments are classified into two groups namely-: Heat treatment and surface treatment. Surface treatment as corrosion resistance operations includes phosphating chroming nickeling, anodization and so on, While heat treatment as structural adjuster includes hardening, tempering toughening and so on (Ojediran and Alamu, (2004) and Vincet,(1968)). In this study, the effects of such heat treatment are determined to know the extent of their impact on the aluminum – Zinc alloys (Al- Zn). MATERIAL AND EXPERIMENTAL TECHNIQUES Materials used The targeted materials composition, uses and forms are as follows: TABLE 1 THE COMPOSITION, USES AND FORMS OF ALUMINUM ALLOYS |Composition | Uses | Forms | |Al – Zn 20% |Armored vehicle, Military | Rod (10mm x 150 mm) | | bridge, Motorcycle and | | | |Bicycle frames, | | | |Air frames etc | | Equipments used The following equipments were used for the study: Pit furnace, Sand mould, Electrical furnace, and Crucible Pots at FIIRO, Nigeria. And Mosanto Tensometer, Wild metallurgical Microscope, Polishing Machine,and Manual Grinding Machine at Obafemi Awolowo University, Ile-Ife, Nigeria. Others are-Digital Cameral, Patterns, Grit papers, Emery Cloth/Paper, Silicon carbide solution of different grade and NaOH Solution Methods Casting Process A Crucible pots is used, 1 kg of commercial purity Aluminum (99. 7 % pure by weight) and 250g of Zinc is used. The alloying element Zn is put first in the Pot because of its high melting point (1108oc) which is higher than that of the base metal aluminium (660oc). So, the Zinc (Zn) in the pot is lowered first into the furnace while the aluminium is added after Zn has commenced melting. The mass of each of the alloying elements used for each sample can be obtained as follows: Total mass of mixture for the samples= frames Mass of Al + Mass of alloying element = 1kg + 0. 250kg = 1. 250kg Percentage of Aluminum (Al) in the mixture = mass of Al/Total mass x 100 =1/1. 250 x 100 = 80% Percentage of alloying element (Zn) = Mass of alloying element/Total mass x 100 =0. 25 / 1. 50 x 100 = 20% The dimension of one sample is as following: Height of the sample =150mm (15cm) Diameter of the sample = 12 mm (1. 2 cm) Radius of the sample =60mm (0. 6cm) Shape of the sample= Cylindrical Volume of the sample (V) =? r2h = 3. 142 x (0. 6) 2 x 13 = 16. 96 cm3 Density of Aluminum = 2. 69g/cm2 (Olagoke, 1999). Mass of sample used = ? x V = 2. 69 x 16. 96 = 45. 64 g Therefore, 80 % of Aluminum = 36. 51 g for each sample Also, 20% of Alloying element (Zn)= 9. 13 g For each sample (rod form) 36. 51 g Al + 9. 13 g of alloying element (Zn) were used. The samples for heat treatment are: Sample 1 =as received from Casting process. Sample 2 = for quenching in Water. Sample 3 = for Oil quench Sample 4 = for annealing (Furnace cool). The method adopted in casting the samples is sand molding (Sand casting). The casting process is performed at FIIRO Nigeria Limited, Lagos. The pattern is made from wood with the following dimensions Height – 15 cm Diameter – 1. 2 cm The sand used is the traditional-green sand’ which is a mixture of sand grains and clay particles, clean sand with oil and binders). The mould used is a conventional vertical sand mould. The drag is placed upside down on a firm flat surface and the pattern is placed face down and pattern is placed face down and then, dusted with a parting powder. Handfuls of sifted sand are then thrown at the pattern, covering every detail (Sand slinging). The sand is rammed. The drag is turned upside down with a swift movement. The cope is fitted onto the drag. The cope is rammed with sand and strictly making the top smooth and firm. And the cope is removed. The crucible is held about half way down with tongs and withdrawal from he furnace. Dross is raked away from pouring lip with heated skimmer and the metal poured in one continuous stream until it appears at the head of the riser. When the casting has solidified and cooled, the sand is knocked out and the casting fettled. Heat treatments process Heat treatment is also carried out at FIIRO Nigeria limited, Lagos. The types of heat treatment carried out are: Annealing and Quenching. Three each of Al-Zn casting samples were put in the electrical furnace (up to 3300oc). The soaking time for the sample in the furnace is one hour, after which two of the Al-Zn casting samples is removed and quench in both water and oil while the last one of Al-Zn is allowed to cool in the furnace atmosphere (annealing). Microstructure Analysis Process. The microstructure was done at Obafemi Awolowo University, Ile-Ife, Osun State. The samples were machined and grounded to gauge 240, 320, 400, 600 each using Grinding Machine and Grit paper. Each sample is initially polished, using Polishing machine, emery cloth and Silicon Carbide. The final polishing is done with the aid of Polishing machine, Emery cloth and Silicon carbide of different grades while etching took effect using 5% NaOH Solution. Each sample is examined using the Optical Microscope to check that etch reveal clearly the Microstructure of the sample. The photograph of the resulting microstructure of the aluminum-Zinc (Al- Zn) alloy samples were taken using Optical microscope with x100 magnification and Digital Cameral. RESULTS AND DISCUSSION Discussion on the Microstructure Results of Al- Zn Alloys. The figure 2 below shows the microstructure of Al-Zn alloy, As received sample i. without heat treatment. It can be observed that the grains were not homogeneous and pores are numerous towards the peripheral zone Figure 2 Microstructure of As Received; Al – Zn alloy Sample 1. The figure 3 below shows the microstructure of Al –Zn alloy, Oil quenched sample. It can be observed that the grains are more homogeneous and well distributed towards the core. The pores are not numerous compare to Al- Cu alloy, As received sample. Figure 3 Microstructure of Oil Quench; Al – Zn Alloy Sample 3.. Figure 4 below shows the microstructure of Al- Cu alloy, Furnace cooled sample. It can be observed that the grains were deformed at peripheral more than at the core of the structure and there are more pores than that of As received sample and Oil quenched sample. this indicates that there is no significant improvement in the microstructure arrangement of grains when Al-Cu alloy is oil quenched Figure 4 Microstructure of Furnace Cooled; Al – Zn Alloy Sample 4. Figure 5 below shows the microstructure of Al- Zn alloy, Water quenched sample. It can be observed that the grains were deformed throughout the structure and there are more pores than that of As received sample and Oil quenched sample. This indicates that there is no significant improvement in the microstructure arrangement of grains when Al-Zn alloy is Water quenched. Figure 5 Microstructure of Water Quench; Al – Zn Alloy Sample 2. . CONCLUSION From the result of the experiments, it can be deduced that: (i) The arrangement of the microstructure grains and pores are more even with the Oil quench method than any other heat treatment methods in Al – Zn alloy. (ii) There is relationship between the heat treatments method (Oil quench) and microstructure of Al- Zn alloy. iii) Oil quench Aluminium Zinc alloy (Al – Zn ) is more appropriate for the production of Armored vehicle, Military bridge, Motorcycle frames and Bicycle frames and Air frames ACKNOWLEDGEMENT The workers of FIIRO Nigeria limited,Lagos and Mr Alo of Metallurgy and Material Department, Obafemi Awolowo University Ile- Ife, Osun State, Nigeria are acknowledge for providing the facilities for this research work. REFERENCES Ojediran, J. O . and Alamu, O. J. (2005) Fundamental concepts in Materials Science and Engineering; Kunle (Nig) Printers; Ijebu-Ode, Nigeria. Vol. 1, 1st ed, pp. 46. Vincet, A. (1968) Workshop Theory for G. C. E. 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