The Effect of Chromium and Manganese on the Mechanical Properties and Corrosion Resistance of Al-si-fe Alloy in 0.5m HCL Solution
CHAPTER ONE
AIMS AND OBJECTIVES
The aim of the research is to investigate the mechanical properties and corrosion resistance of Al-Si-Fe alloy in 0.5M HCl solution in as-cast condition at room temperature and compare the result with those obtained when the alloys were age-hardened.
The specific aims and objectives of this research was to understand,
- The individual and simultaneous effects of Cr and Mn addition with Al- Si-Fe alloy on the mechanical properties (tensile properties, hardness and impact strength) in the as-cast
- The individual and simultaneous effects of Cr and Mn with Al-Si-Fe alloy and age-hardening treatment on the mechanical properties of Al- Si-Fe alloy.
- To determine the individual and simultaneous effects of concentration of Cr, Mn, MnCr and time of exposure on the corrosion resistance of Al-Si-Fe alloys in 0.5M HCl solution at 280C for 480 hrs in the as-cast and age-hardened
- To study the microstructural changes that occur as a result of Cr and Mn additions to Al-Si-Fe alloy in as-cast and age-hardened condition
- The correlation between the studied mechanical properties and microstructures of all the alloys
CHAPTER TWO
LITERATURE REVIEW
Al – Si – Fe ALLOY
The presence of Si, Mn, Cr and Fe in aluminium causes various precipitates to form during solidification of the alloys (Allen, 1979). The development of dendritic network, precipitation of AlFe phases, eutectic reaction involving the precipitation of Si and Fe phases and formation of complex eutectic phases of MnAl2, CrAl2, (FeCr)3 SiAl12, (FeMn)3 SiAl12 all took place during the solidification (Rajan et al, 1988). However the formation of these phases depends solely on the composition of the alloys and the cooling rates (Avner, 1974). Manganese and Chromium complexes are easily visible under the optical microscope because of their distinct grayish-white colour tinged with reddish shadings and steel-gray like colour respectively (Brumbough, 1979). While the iron phase is present in both needles (β) and Chinese script form α (Ghosh et al, 2004 and Avner, 1974). The mechanical properties of these alloys are solely dependent and guarded by the composition and the types of phases that are formed and precipitated during the solidification of the alloys immediately after casting (Metal Hand Book, 1979A). Aluminium – Silicon
– Iron alloys is also a casting alloy with major alloying element Si followed by iron. The iron content in this alloy in small amount increases the strength and hardness of the alloy and reduces hot – cracking tendencies in casting (Kenneth, 1999 ). However, it has been known since 1920’s that plates and needles of Al – Si – Fe constituent reduces the strength of aluminium – silicon-casting alloys (Smith, 1981).
TERNARY AND MULTI COMPONENT ALLOYS
Al-Si-Fe is an alloy with a ternary phase diagram. In particular, any alloy has a unique liquidus temperature for each possible composition of the liquid, and similarly, it has a unique solidus temperature for each compositions of the solid. An important difference from the two – component case is that, in systems of three or more components, the temperature does not uniquely define the solidus and liquidus compositions, in general. The simplest example is that of the ternary system; compositions in ternary systems are usually represented as points on or in an equilateral triangle, ABC (in fig. 2.1). The corners of the triangle correspond to the three pure components, and the edges represent the three binary systems. Each point within the triangle corresponds to a definite composition; the ratio of the three components for the P is equal to the ratio of the distances a,b,c, of P from the three sides of the triangle. The sum of these three distances is constant for a given equilateral triangle (Rhines, 1956). Since each possible composition of liquid corresponds to a point on or in the triangle, it follows that the liquidus is a surface, rather than a line as in the “two-component” case. In a three-component system therefore there must be a line on a liquidus surface that corresponds to any particular temperature, The ties lines aremostly used to indicate the equilibrium relationships (IAI, 2006 and Chalmers, 1964).
CHAPTER THREE
MATERIALS AND METHODS
Materials
The materials used for this research for the production of Al-Si-Fe (control), Al-Si-Fe-Cr, Al-Si-Fe-Mn and Al-Si-Fe-MnCr alloy systems includes high purity aluminum obtained from Northern Cable Company NOCACO Kaduna and ferroalloys (FeSi, FeCr, FeMn), etchants, baker, hydrochloric acid, thread (for suspending the coupons in media), measuring cylinder, silica sand and bentonite, moulding boxes from National Metallurgical Development Center (NMDC), Jos.
Equipments
The following equipment were used, Muffle electrical resistance furnace, Denison impact tester, Rockwell Hardness B Scale machine, Tinius Olsen tensile testing machine, Polishing machine, Metallurgical microscope, Digital weighing balance.
Methods
Sixteen different compositions of alloys were produced in which the amounts of Mn and Cr in the alloys were varied from 0.1 to 0.5% in step of 0.1% while the percentages of Si and Fe were kept constant (Table 3.1). Each composition was melted separately in alumina crucible. The melting was done using a muffle resistance furnace that was allowed to heat to 7500C before the crucible was removed and other alloying elements (ferroalloys) added. Then the crucible was returned in to the furnace for further 30 minutes during which the furnace temperature was raised to 8000C for superheat to occur; then 0.01%NaCl was added and stirred thoroughly before pouring into the mould. The cast samples were machined to standard tensile, hardness, impact, and corrosion test specimens.
CHAPTER FOUR
RESULTS AND DISCUSSION
Results
The results obtained are presented in tabular (table 4.1-4.21) and graphical form (Appendix B). Tables 4.1-4.21 are given the determined mechanical properties and Corrosion rates of the alloys. Figures 1 –15 show the variation of mechanical properties with percentage Cr and Mn addition while in Figures 16 – 27 are given the variation of corrosion rates with exposure time and percentage Mn and Cr addition for the categories of alloys (as-cast and age-hardened). The microstructures of the as-cast and age-hardened alloys are presented in Appendix A (Plates 1 – 32).
CHAPTER FIVE
SUMMARY
From the experimental results, the mechanical properties; tensile properties, hardness of the Al-Si-Fe alloys produced showed an improvement for all levels of the % Cr, Mn, and MnCr additions in the as- cast condition. But a decrease in impact energy of the alloys was observed. However, further improvement in the tensile strength, hardness and impact energy were noticed after age-hardening the alloys. For the corrosion resistance of these alloys in 0.5M HCl solution at 280C, the corrosion rate decreased as the percentage of additions were increased respectively with exposure time in the as-cast condition. Age-hardening of the alloys shows that there was significant resistance of the alloys to corrosion for all levels of additions and exposure time considered. However, the alloys with improved mechanical properties and corrosion resistance can be used in automobile industry as oil pan, flywheel and rear-axle housings, cranks cases, transmission cases, clutch components and in aerospace and aviation industry as tankage for storage of liquid fuels and oxidizers, airframes, engines, propellers, accessories, etc.
Conclusion
From the results obtained in this research it can be concluded that
- Addition of Cr, Mn separately and simultaneously to Al-Si-Fe alloy improvedthe tensile properties and the hardness and corrosion resistance in 0.5M HCl solution in both as-cast and the age-hardened conditions subject to a maximum of 0.5% (Mn and Cr) addition.
- Heat-treatment (Age-hardening) improves the mechanical properties and corrosion resistance of the alloys in 0.5M HCl solution at room temperature.
- While the addition of Mn and Cr lowers the impact resistance of the as- cast alloy but it improves after age-hardening and simultaneous addition of these elements is better than individual additions in terms of mechanical properties investigated and corrosion resistance of the alloys.
Recommendations
Based on the research output obtained, and analysis, the followings are recommended:
- Typical automobile and aerospace prototype parts should be produced out of alloy compositions with optimum mechanical properties and corrosion
- The fatigue properties of Al-Si-Fe alloys with Cr, Mn, and MnCr should be investigated for purpose of application in cyclic loading
- The effect of varying ageing temperatures and time on the mechanical properties of the alloys should be looked
- The effects of heat treatment and different quenching media on the mechanical properties and corrosion characteristics of the produced alloys should be
- Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and other equipments should be used to examine the microstructures for better understanding of the concept of dislocation and grain boundary
REFERENCES
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