The Effects of Pressure on the Mechanical Properties and Microstructure of Die Cast Aluminum Alloys

The Effects of Pressure on the Mechanical Properties and Microstructure of Die Cast Aluminum Alloys

Chapter One

AIM AND OBJECTIVES

The aim of this research is to study the effects of pressure on the microstructure and mechanical properties of aluminum die castings which will be of better qualities and free from defects. The specific objectives are to:

Evaluate the influence of different applied pressures on the mechanical properties and microstructures of die cast aluminum A380 and
Compare the mechanical properties of both alloys
Study the grain size and numbers of both alloys
Establish the level of porosity in both alloys

CHAPTER TWO

LITERATURE REVIEW

INTRODUCTION

Die casting is a manufacturing process that can produce geometrically complex metal parts through the use of reusable molds, called dies. The die casting process involves the use of a furnace, metal, die casting machine, and die. The metal, typically a non-ferrous alloy such as aluminum or zinc, is melted in the furnace and then injected into the dies in the die casting machine. There are two main types of die casting machines mainly hot chamber machines (used for alloys with low melting temperatures, such as zinc) and cold chamber machines (used for alloys with high melting temperatures, such as aluminum). However, in both machines, after the molten metal is injected into the dies, it rapidly cools and solidifies into the final part, called the casting. The castings that are created in this process can vary greatly in size and weight. Metal housings for a variety of appliances and equipment are often die cast. Several automobile components are also manufactured using die casting, including pistons, cylinder heads, and engine blocks. Other common die cast parts include propellers, gears, bushings and valves. (www.webcitaion.org)

In the aluminum die casting process, solid ingots of aluminum are melted in furnaces at approximately 650 – 720 ⁰C. Once liquefied, the aluminum metal is picked up using a ladle and poured by hand or robotically into a steel shot sleeve. The molten aluminum is then injected with hydraulic pressure into the two halves of the die. The molten aluminum metal is then held under high pressure until the metal solidifies, usually within a matter of 2-15 seconds depending on the size of the parts. The die halves are then opened and the part ejected and removed by hand or robotically (www.webcitaion.org).

Hot Chamber Die Casting Process:

Hot chamber machines are used primarily for zinc, lead and other low melting point alloys that do not readily attack and erode metal pots, cylinders and plungers. The injection mechanism of a hot chamber machine is immersed in the molten metal bath of a metal holding furnace. The furnace is attached to the machine by a metal feed system called a gooseneck. As the injection cylinder plunger rises, a port in the injection cylinder opens,

allowing molten metal to fill the cylinder. As the plunger moves downward it seals the port and forces molten metal through the gooseneck and nozzle into the die cavity. After the metal has solidified in the die cavity, the plunger is withdrawn, the die opens and the casting is ejected.

A complete die casting cycle can vary from one second for small component to three minutes for the casting of lager components. This makes the die casting process the faster technique for producing precise non ferrous metal parts. (www.webcitaion.org).

The schematic arrangement of the process is shown in Figure 2.1.

CHAPTER THREE

MATERIALS AND METHODS

MATERIALS

The materials shown in figures 3.1 and 3.2 that were used in this work are aluminum alloys A380 (used mostly in aeronautics) and A1350 (used mostly for electric distribution lines) these were procured, cut and melted in an electric furnace of capacity of 500kg available at the Scientific and Equipment Development Institute (SEDI), Enugu.

X- Ray Florescent test

X-ray fluorescence (XRF) is the emission of characteristic “secondary” (or fluorescent) X- rays from a material that has been excited by bombarding with high-energy X-rays or gamma rays. The phenomenon is widely used for elemental analysis and chemical analysis, particularly in the investigation of metals, glass, ceramics and building materials, and for research in geochemistry, forensic science and archaeology. The use of a primary X-ray beam to excite fluorescent radiation from a metal sample was first proposed by Glocker and Schreiber in 1928. Today, the method is used as a non-destructive analytical technique, and as a process control tool in many extractive and processing industries.

When materials are exposed to short-wavelength X-rays or to gamma rays, ionization of their component atoms may take place. Ionization consists of the ejection of one or more electrons from the atom, and may occur if the atom is exposed to radiation with energy greater than its ionization potential. X-rays and gamma rays can be energetic enough to expel tightly held electrons from the inner orbitals of the atom. The removal of an electron in this way renders the electronic structure of the atom unstable, and electrons in higher orbitals “fall” into the lower orbital to fill the hole left behind. In falling, energy is released in the form of a photon, the energy of which is equal to the energy difference of the two orbitals involved. Thus, the material emits radiation, which has energy characteristic of the atoms present. The term fluorescence is applied to phenomena in which the absorption of radiation of a specific energy results in the re-emission of radiation of a different energy (generally lower) (Beckhoff et al, 2006).

X-ray is a type of electromagnetic wave such as visible light ray, but the key difference is its extremely short wavelength, measuring from 100A to 0.1A. And compared to normal electromagnetic waves, X-ray easily passes through a material and it becomes stronger as the material’s atomic number decreases. X-ray fluorescence analysis is a method that uses the characteristic X-ray (fluorescent X-ray) that is generated when X-ray is irradiated on a material. The fluorescent X-ray is the excess energy irradiated as electromagnetic field, which is generated when the irradiated X-ray forces the constituent atom’s inner-shell electrons to the outer shell and the vacant space (acceptor) falls on the outer-shell electrons. The generation of fluorescent X-ray is shown in Figure 3.3

CHAPTER FOUR

RESULTS AND DISCUSSIONS

RESULTS

The results of the various tests carried out are shown below in figures 4.1 to 4.14 and tables A1 to A16 in the appendix.

CHAPTER FIVE

CONCLUSIONS AND RECOMMENDATIONS

CONCLUSIONS

From the results of this research, the following conclusions can be drawn:

The hardness of both alloys increased and percentage elongation decreased with applied pressure. Also the model that was fitted to the experimental data showed linearrelationship with the actual data in view of the small error generated by
Tensile and yield strengths of both alloys also increased with applied pressure. Also the model that was fitted to the experimental data showed linear relationship with the actual data in view of the small error generated by

The impact strengths of both alloys were observed to vary in similar manner across the different applied pressures in the casting process as the impact strengths of both alloys decreased as hardness increased with applied pressure. Also the model that was fitted to the experimental data showed linear relationship with the actual data in view of the small error generated bythem

The number of grains increased with applied pressure for both alloys. Also the grains became finer with applied pressure for both alloys. Also the model that was fitted to the experimental data showed linear relationship with the actual data in view of the small error generated by
The microstructures obtained of the samples of both alloys at different pressures also showed that at applied pressure of 1400kg/cm², the eutectic reactions wererestrained, and the final solidified structure was (η+ε) phases instead of eutectic phase (β+η+ε) signifying the effect of applied pressure, also the primary aluminum reaction (α) was promoted in the samples that solidified at 1400kg/cm² and fine microstructures were obtained .

The micrographs showed the different morphologies that were distributed across the samples of both alloys under the different applied pressures. The fine grains which were homogenously distributed on micrographs of both alloys at 1400 kg/cm²can effectively block the movement of dislocations, thus increase the strength and plasticity of both

Microstructures of the samples of both alloys showed structural changes (granular, lamellar, coarse e.t.c ) due to pressure
Porosity susceptibility in the samples of both alloys decreased with applied pressures due to fine grains and more numbers of grains as no pore was seen on the micrographs and microstructures of samples that solidified at pressures of 1400kg/cm²in both

For all the models developed, a close relationship with the experimental results were underlying in view of the small errors generated by them and can be used to predict the experimental values of this

RECOMMENDATIONS FOR FURTHER WORK

In the research work, A380 alloy and A1350 alloy have been studied. Other alloys should also be investigated for different reasons:

Effect of pouring temperature on grain refinement and mechanical properties of die cast A360 and aluminum magnesium
Effects of various cooling mediashould also be carried out on mechanical properties of die cast aluminum
Pressure effects should be carried out on porosity and grain refinement of die cast alloys.
Effects of other input parameters on the microstructure of die
The influence of pressure on the cost of energy consumed in theprocess

Contributions of the research work to knowledge

The results of this research can contribute to improving the quality of aluminum die castings by:

Using the model equations to predict variable responses to experimental values of the mechanical properties and other properties and can be used to predict future variable responses in future
Decrease inporosity
Increase in strength evidenced by mechanicalproperties
Higher quality due to reduced grain size and more number ofgrains
Expected assurance that die castings are less prone to rejection and are of higher standard of soundness through microstructure

References

Adler, L., Nagy, P. B., Rypien, D. V., and Rose, J. H., (1989): Ultrasonic Evaluation of Porosity in Aluminium Cast Materials, Ohio State University, Columbus, OHIO, USA.
Ambardar, R., Jayakumar, T., Pathak, S. D., and Prabhakar, O., (1996) : Effect of Surface Roughness on Ultrasonic Echo Amplitude in Aluminium-Copper alloy Castings in the 14th World Conference on Non Destructive Testing, New Delhi, india, vol. 2, pp. 981-984.
An-ming, L., Hai-rui, W., (2008): Effect of Silicon and Manganese on Mechanical Properties and Microstructure of As-Cast ZA-27 alloy [J]. Foundry, 57(6): 608−610. (in Chinese)
Anon, (1978): Aluminium Technology Book 6: Casting Aluminium, Canberra: Aluminium Development Council of Australia Ltd.
Aleksandr, A.S., ĭMikhaĭlov, A.P., (2002): Principles of Mathematical Modeling: Ideas, Methods, Examples Volume 3 of Numerical Insights, Taylor and Francis publishers, Moscow.
Allen, D. M., (1971): Mean Square Error of Prediction as a Criterion for Selecting Variables, Technometrics, 13, 469–475.
Aris, Rutherford, ( 1994 ): Mathematical Modeling Techniques, New York. ASM handbook volume 15, casting (2008 Edition).

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