Lab report

ABSTRACT

This experiment addresses emerging issues in precipitation hardening which is the effect of temperature on the hardness and aging time of alloys. It is a critical issue in the field of engineering since one needs to know to what extent is the hardness and strength of aluminum 2024 affected by temperature especially when it’s used in areas that experience high temperature or during welding. Five aluminum 2024 samples were subjected to different temperatures (room temperature and furnace temperature) and their hardness determined at specific aging temperatures. The data collected was analyzed by drawing graphs of hardness against aging time for each temperature and scientific deductions were made from the results.

INTRODUCTION

Aluminum (Al) is classified as a metal in the periodic table with atomic number thirteen. It has good electrical conductivity, thermal conductivity, relatively low density and relatively high strength. With modern technologies, its properties have been enhanced through formation of aluminum alloys an example being 2024 aluminum. 2024 aluminum is an alloy composed of aluminum, copper, and magnesium. The alloy is has better fracture toughness, better fatigue properties, better tolerance to damage, and higher strength to weight ratio in comparison to aluminum metal.

However, some of its properties need to be improved to make it suitable for a particular role for example its hardness. Hardness of alloys are enhanced by a process called precipitation hardening which makes use of the fine solute precipitates contained in the alloy’s supersaturated matrix. It strengthens and hardens alloys by giving rise to these dispersed precipitates present in the alloys microstructure.

The process is composed of three phases which include: heat treatment, quenching, and aging. Heat treatment involves placing the alloy in a furnace at an elevated temperature. The specimen is quenched in cooled water immediately to prevent formation of coarse grains hence increasing its hardness and tensile strength. The metal is left to age either naturally or artificially. Aging allows the precipitates to form within the supersaturated solid solution.

Aging time and hardness of 2024 aluminum are affected by ambient temperature of it immediate environment. High temperatures make atoms, which forms the microstructures of alloys, excited leading to a change in properties of the alloys. It is anticipated that the higher the temperature the less the hardness and aging time of 2024 aluminum. The extent of effect of this phenomenon influences the application of metals in engineering fields.

RESULTS

Hardness for 2024 aluminum.

Hardness results one week later.

DISCUSSION

The graph of hardness against aging time for the sample exposed to natural aging rises initially before forming a plateau with a slight negative gradient. The graph for the specimens put in the furnace at a temperature of 190 0C, rose gradually and upon reaching the peak it falls steadily with a negative gradient. During cooling, the alloy forms precipitates that are small and dispersed within its kappa matrix. These precipitate account for the initial increase in hardness.

Upon attain thermal equilibrium with the immediate environment, the hardness reaches its peak for both temperatures. However, the samples at 190 0C are mostly affected after attaining thermal equilibrium as compare to the naturally aging sample. The high temperature makes the small precipitates to reduce in size while the large precipitates increase is size. The resulting differential size causes an increase in the distance between particles leading to a decrease in both the hardness and aging of the alloy. The graph falls steadily with a negative gradient.

The samples that were maintained at high temperatures for prolonged period of time experienced further decrease in hardness and yield strength. Hardness and aging time are functions of temperature. This means that they are related using a mathematical formulae which shows inverse proportionality between independent variable (temperature) and dependent variables (aging time and hardness). An increase in temperature results to a decrease in both the adding time and the hardness while a decrease in temperature results to a decrease in both the aging time and hardness. At a temperature of 190 0C, 2024 aluminum would have low yield strength and hardness. Its aging time would be little hence rendering it useless in the manufacturing of components that experience prolonged high temperatures. Its best suited for normal temperatures hence making it a useful construction material in the aviation industry.

This practical requires personnel that is keen on minute details so as to get the desired results. Human errors contribute to collection of inaccurate data. The calibrations done on the equipment are very important. Wrong calibrations or neglecting calibration of the equipment before use results to inaccurate results. The practical should be done in pairs to reduce errors during while carrying out the experiment.

CONCLUSION

In conclusion, precipitate hardening at low temperatures is the best if the hardness and yield strength is to be maintained. The peak hardness for the artificially aging specimen was 71.6 HRB while the naturally aging was found to be 70.4 HRB. Temperature affects both the aging time and the hardness of 2024 aluminum. It was deduced that at high temperatures the yield strength and hardness of the alloy reduces hence not suitable for use in structural components that experience high temperatures continually.

REFERENCE

Alex, B. (2005). Nuclear Instruments and Methods in Physics Research Section B. New York: McGraw-Hill. Bhadeshia, H. (2006). Aluminium Microstructure and Properties. New York: Butterworth-Heinemann. William, S. (1993). Foundations of Materials science and Engineering. New York: McGraw-Hill.

A GRAPH OF HARDNESS AGAINST TIME (Natural aging)

A GRAPH OF HARDNESS AGAINST TIME (Artificial aging)

HARDNESS AGAINST TIME (natural aging)

3.0 10.0 60.0 90.0 61.7 70.4 70.0 69.8

TIME (MINUTES)

HARDNESS (HRB)

HARDNESS AGAINST TIME (artificial aging)

3.0 10.0 60.0 90.0 63.7 70.9 71.6 65.6

TIME (MINUTES)

HARDNESS (HRB)