Theory:

To measure the hardness of the samples, we use the Rockwell hardness test. The Rockwell test determines the hardness by measuring the depth of penetration of an indenter under a large load compared to the penetration made by a preload. There are different scales used, each of which uses a different load or indenter. The hardness measurements are dimensionless numbers and should only be compared to hardness measurements taken using the same scale.

To measure the microhardness of the samples, we use the Knoop hardness test. Microhardness testing measures the hardness of a material on a microscopic scale. A precision diamond indenter is impressed into the material at loads between a few grams and 1 kilogram. The impression length is measured microscopically, and the test load is used to calculate a hardness value.

The hardness values obtained are useful indicators of the strength and expected behavior of aluminum 6013. Hardness can also be used to monitor the precipitation process, since the strength is related to the number, type, and spacing of precipitates.



Method:

We measured the hardness and microhardness of each of our 4 samples (stock, cast, heat treated, and welded). This was done so that we could compare how the processes performed on the samples affect their hardness. For the Rockwell hardness test, we made sure that the sample lay flat on the platform of the machine and took a measurement. After each measurement, we moved the sample and took another measurement at a different location on the sample. This was done 3 times for each sample. For the microhardness test, each sample had to be mounted and polished before taking measurements. The samples must be mounted so that the readings taken are accurate. They must also be polished so that the measuring microscope can clearly resolve the microscopic hardness impression. Again, we took 3 microhardness measurements on each sample.


Results:

Rockwell Hardness Test
Stock = 71.7 B
Cast = 10 B
Heat Treated = 40.7 B
Welded = 39.3 B

Knoop Microhardness Test
Stock = 198 KHN
Cast = 150 KHN
Heat Treated = 177 KHN
Welded = 74 KHN


Analysis:

KHN (Knoop Hardness Number) = PC/L^2 , where P=applied load (kg), C = constant relating to the area of recovery of the indent, L = length of indentation (mm).
L = # divisions * conversion factor. C = 14.229


From the results we see that the stock sample has the highest hardness, followed by the heat treated, welded, and then the cast sample. We can see here that the hardness of a material depends upon its processing history. The stock sample has the highest hardness because it is not processed at all. Casting decreases the hardness of aluminum 6013. The aluminum becomes very soft during casting because it solidifies so fast and forms large grains. The grains are also non-uniform because the cooling is non-uniform (i.e. surface cools much faster). The heat treated sample has a higher hardness than the cast sample, which makes sense since heat treatment is done to increase strength and hardness. The hardening may be a result of precipitation hardening or a smaller grain size. The welded sample has a lower hardness than the heat treated sample because the thickness could have decreased at the weld and the microstructure may have changed. The typical hardness of aluminum 6013 is 80 on the Rockwell B scale, and 163 on the Knoop microhardness scale. Our results are consistent with these numbers.