Microstructural Characterization

Aluminum 6013

Microstructural Characterization

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Processes such as heat-treating and welding often modify the microstructure of a material, which in turn is responsible for changes in the properties of the material. Taking pictures of the microstructure of samples that underwent different processes allows relating the observed changes in microstructure with the determined changes in properties. Particularly, grain size is an important indicator of strength and hardness of a material. Theoretically, higher levels of strength and hardness should be associated with smaller average grain sizes.

ImageJ allows measuring the grain size of a sample based on a picture of its microstructure. It helps calculate the average grain size based on several grains in order to reduce inaccuracy resulting from the variation in size from one grain to the other.

Method

For the microstructural analysis of Al6013 we started by etching the surface of all the samples. This process is used to accentuate the microstructural details of a metal and it ensured that our pictures would be detailed enough to appreciate the differences between the samples. Once the pictures were taken, grain sizes were calculated using ImageJ. A final comparison of microstructural details and grain sizes between the four samples (stock, cast, heat-treated and welded) allowed us relate the different microstructures to the specific processes underwent by each sample.

Results

Stock (50x)

Cast (50x)

Heat Treated (50x)

Welded (100x)

Analysis

For the grain size calculation, we started by determining the conversion between the “length” calculated by ImageJ and standard millimeters. As shown in the table below, 0.01 mm were equivalent to a length of 77 in the case of 50x magnification, and to a length of 151 in the case of 100x magnification.

To continue, we measured the width of 3 grains with ImageJ. We worked with 3 grains instead of just 1 in order to account for variability in the grain size. Then we converted the average width per grain to millimeters using the conversion factor shown in the table. The results are listed in the “Grain Size” column.

Al6013 Samples	Magnification	ImageJ length for 0.01mm	ImageJ length per 3 grains	Grain Size (mm)
Stock	50x	77	596	0.026
Cast	50x	77	819	0.036
Heat Treated	50x	77	381	0.017
Welded	100x	151	966	0.021

The results indicate that there was a significant difference in the grain size of the Al6013 stock- and cast- samples. This result was expected because, theoretically, casting increases grain size. The reasoning behind this is that, due to rapid, non-uniform solidification, nucleation occurs. This implies that new, larger, randomly oriented grains are created. The approximately 35% increase in the size of the grains when going from the stock- to the cast- sample is explained by this.

A further observation is that the grains were significantly smaller in the heat-treated sample than in the stock (and cast) one. Due to heat-treating, and particularly to age hardening, smaller grains were created on the Al6013 heat-treated sample. This is in agreement with the observation that the heat-treated samples showed the highest strength as well. The difference between grain size of heat-treated- and cast- samples was larger than between heat-treated- and stock- samples. This is explained by the fact that the cast sample had not been heat-treated at all, but the stock sample had undergone some kind of heat-treatment. Since the stock had not been age hardened, the difference between these two samples (stock and heat-treated) was still significant.

Finally, we noticed that the grain size of the welded sample was somewhere in between the sizes of the heat-treated- and stock- samples. The reason for this is that the picture was taken from a spot neat the weld but not at the weld (given that the sample had broken at the weld during the tensile test). Hence, there were traces of the effects of both welding and heat-treating on the microstructure of the welded sample.