Sample Lab Report s1

SAMple Lab Report

(your report should be double spaced for grading purposes)

Introduction

For cast ingots, the size and shape of grains at any location is dependent on the solidification process. The progress of solidification from the mold wall to the center of an ingot may occur in three stages. Each stage is marked by a zone having a distinct grain structure. The three zones are the chill zone, the columnar zone, and the central zone. The grain morphology (size and shape) and the extent of each zone depend on the temperature of the molten metal, and that of the mold, relative to the liquidus temperature of the alloy.

(Specific Discussion of Theory with Relevant Equations)

The goal of this laboratory is to determine how the pouring temperature of an alloy can affect the grain structure of the ingot. Specifically, an Al-2.5% Cu alloy was used and the grain structures obtained by pouring the alloy at two different temperatures were examined.

Procedure

Materials and Equipment

• 75 grams AA 6063 + 1.875 grams 99.9% Cu → Al-2.5% Cu alloy

• Furnace

• Etchant - 1.0 ml HF (48%), 1.5 ml HCL (conc.), 2.5 ml HNO3 (conc.), and 95.0 ml H2O

• Keller’s reagent – 75 ml HCL (conc.), 25 ml HNO3 (conc.), 5 ml HF (48%), and 35 ml H2O

• Neophot 21 Metallograph

• Steel crucible

Two samples of Al-2.5% Cu alloy were made, put into the furnace at 850°C, and melted. They were kept there for 20 minutes in order to obtain a constant temperature throughout both samples.

After 20 minutes, sample A was taken out of the furnace and poured into a steel crucible which was at room temperature. The other sample, Sample B, was allowed to solidify without being poured. It was then put into a furnace at 690°C and remelted. Sample B was also kept in the furnace for approximately 20 minutes and then poured into a steel crucible. Each sample was allowed to cool and solidify.

Each sample was cut down at its central axis, and then ground and polished according to standard metallographic procedures. Each sample was then etched with the enchant solution, in order to obtain its microstructure.

After examination with the Neophot 21 Metallograph the samples were etched again using the Keller’s reagent, so as to obtain the macrostructure for each.

Results

(Opening Statement: General trends found in the results)

After examining Sample B with a magnification of 25X, it was determined that it has an equiaxed grain structure in its middle with a chill zone around its sides where the metal had been in contact with the crucible wall. A few dendrites were observed at the very top of the sample oriented towards the center of the sample. Photographs of the equiaxed structure in the middle of the sample and of the chill zone at the base of the sample are given in Figure 1.

Figure 1: Microstructure of Sample B - Equiaxed Structure and Chill Zone at 25X

The microstructure of Sample A was also examined with a magnification of 25X. As shown in Figure 2, it was observed that the sample poured at 850°C also had a chill zone around the edges of the sample. However, the microstructure in the middle consisted of columnar grains. The long dendrites viewed were basically all oriented towards the middle of sample.

Figure 2: Microstructure of sample A - Chill Zone at 25X

It was observed that both samples were very porous. Each sample had relatively large irregularly shaped pores throughout the observed section. An example of the pores can be seen in both Figures 1 and 2.

Discussion

The purpose of this laboratory was to study the effect of the pouring temperature of the liquid on the grain structure of the solid alloy. Sample B was poured at a temperature close to the freezing temperature of the alloy. The pouring temperature of the sample was 690°C while the freezing temperature was approximately 660°C. This is the primary reason why Sample B had an equiaxed grain structure.

The same is true of Sample A. The columnar grain structure is a direct result of its pouring temperature. The pouring temperature of sample A was 190°C above the freezing point of the alloy.

In Sample B, the equiaxed grain structure obtained was due to the pouring temperature. The liquid alloy was allowed to stabilize at a temperature before being poured into the steel crucible. The temperature of the liquid was approximately 30°C above the freezing temperature. When the liquid was poured into the crucible, which was at room temperature, the liquid alloy that touched the side walls of the crucible solidified.

Solid nuclei formed heterogeneously on the walls of the crucible. As the nuclei began to grow in preferred directions the protrusions intercepted each other. As each protrusion grew, they began to run into each other and stop growing. The conglomeration of nuclei whose growth was stunted was called the chill zone. This chill zone can be seen in Figure 1. However, some protrusions that were oriented towards the center of the crucible were not intercepted by other protrusions and, therefore, continued to grow. The equiaxed structure which formed in the middle of the sample was due to protrusions of this type. These protrusions, which were developing into primary dendrite arms, were, somehow during the pouring, broken off and swept into the middle of the liquid alloy.

There are two possibilities as to how the protrusions were broken off. The first possibility is that the turbulence in the liquid, as it was being poured, broke the protrusions off. The second possibility is that as the mould walls increased in temperature the base of the protrusion was remelted without melting the rest of the protrusion, in that manner, the solid nuclei was allowed to mix with the rest of the liquid.

Since the pouring temperature of the liquid was so close to the freezing temperature, the solid nuclei that were swept into the middle of the liquid did not melt. The temperature of the liquid was not high enough to remelt the solid nuclei. Therefore, the solid nuclei provided heterogenous nucleation sites for the rest of the liquid to nucleate on. These particles began to grow in every direction through the development of protrusions. These protrusions could not develop into dendrites because the material was solidifying from both the inside and the outside. The protrusion could not grow for very long in a preferred direction before they ran into other protrusions which stopped their growth.

Due to the larger volume of space in between particles, however, the equiaxed grains were allowed to grow larger than the chilled zone grains. This difference in size can be seen in Figure 1.

The few columnar dendrite grains at the top of sample B were due to the fact that there were not any particles near the top to stop the growth of the protrusions. Therefore, they developed into dendrites. The protrusions which were swept into the center of the liquid may have sunk more to the bottom of the crucible. In that case they would not have been near the top to prevent the columnar growth.

Sample A developed in the same manner as Sample B as far as the chill zone was concerned. However, when protrusions were broken off, whether from turbulence or melting, they were remelted when they were swept into the liquid. For sample A, which had a temperature 190°C above the freezing temperature, as shown by the phase diagram, none of the protrusions could remain solid. Due to the high pouring temperature of the liquid the protrusions were remelted.

Because of the high pouring temperature, Sample A developed columnar grains. The columnar grains were due to dendrite formation in preferred directions. The dendrites formed because of the stead removal of heat through the mould walls. Those protrusions which were oriented towards the center of the sample grew in the direction opposite to the flow of heat. Since heat was removed in every direction away from the mould walls, all of the dendrites formed in a direction pointing towards the center of the sample.

Solid nuclei developed heterogeneously along the walls of the crucible forming a plane of solid. As the temperature of the liquid began to cool and approach the freezing temperature given of the phase diagram the plane of solid began to grow. As temperature gradients moved through the liquid some parts of the solid plane were surrounded by a cooler liquid than others. This variation in temperature allowed some of the solid in the solid plane to grow and form faster, thus producing protrusions. These protrusions were allowed to grow as the liquid became cooler and cooler.

The development of dendrites occurred when the protrusions became very long and began to develop secondary arms. The formation of the dendrites was opposite to the direction of heat flow. The heat flow outward in all directions around the crucible made the dendrites grow towards the center of the crucible.

Columnar grains are made up of several dendrites. All of the dendrites must have the same preferred orientation. This leads to the columnar grains being oriented similarly. Therefore, the columnar grains should also be pointed in the direction opposite to heat flow, or rather, towards the center of the crucible.

One final point about the structure of both ingots was the pores that developed when the liquid solidified. The pores were mainly due to shrinkage. When the liquid solidified, it shrank. The dendrites formed by protruding out into the liquid where they shrank leaving more space for the liquid to occupy; however, there was less liquid because the dendrites were forming. The liquid that remained was all that was left to fill in the spacing between the dendrite arms. This liquid when it solidified also shrank leaving large holes in between the dendrites. The result was a large number of pores throughout the structure of the ingot. An example of the pores can be seen in Figures 1 & 2.

Conclusion

(Key Observations/Conclusions Relevant to the Goal)

In conclusion, it was determined that the pouring temperature for casting and for making ingots controls the grain structure of the final solid.

References

Phase Transformations in Metals and Alloys, Porter and Easterling, Van Nostrand Reinhold, Berkshire, England, 1981.

Effect of Pouring Temperature on Grain Structure, Materials Department, Mat. E. 3084, Lab Report, 11/29/89