Eric FitterlingHeat Treating of Aluminum

11/4/10Dr. Amy Robinson

Abstract

In this laboratory, the effects of heat treatments on aluminum alloys were investigated. The aluminum used was A356 which is approximately 7 wt% silicon and less than 0.3 wt% magnesium. 1 Solution treatments (S.T.) and age treatments were the two heat treatments studied. Each of these treatments was applied to a normal A356 alloy and samples with titanium and boron grain refining additions. The grain refined sample was harder after solution treatment. Upon age both samples increased in hardness and both refined and unrefined samples yielded nearly the same hardness. Microstructure analysis showed finer grains in the solution treated refined grain sample when compared to the unrefined sample. After aging the microstructures were closer in grain size, but there were more fine particles of silicon in the refined sample compared to larger particles in the unrefined sample.

Introduction

Heat treatment is defined as, “an operation or combination of operations involving the heating and cooling of a metal or an alloy in the solid state for the purpose of obtaining certain desirable conditions or properties.2” There are multiple types of heat treatments that all have different goals associated with them. Annealing is a heat treatment that’s purpose is to create a uniform microstructure and relieve stresses thus softening the material. A heat treatment leading to the hardening of an alloy is precipitation hardening which creates precipitates that disrupt dislocation movement. Overall there are many types of heat treatments many that are alloy specific. In general heat treatments are governed by three different components: soak temperature, soaking time, and cooling rate.

The soak temperature dictates the amount of energy that is available. When referring to heat treatment temperatures they are always below the liquidus line of an alloy. Heat treatments do not include re-melting and casting of alloys. The energy that results from the soak temperature can cause dissolution of soluble phases that are present in the alloy. When materials are cast different phases precipitate out during cooling and the ones that are thermodynamically unstable at the soaking temperature will dissolve into the solid solution. Thermal energy can also drive diffusion processes that form precipitates, as is the case in age hardening. Heat treatment temperatures ultimately drive the diffusion to thermodynamic equilibrium.

Equilibrium is dictated by the temperature, but all diffusion processes take time. The amount of time that the alloy is allowed to soak at affects the amount of diffusion that occurs. If an alloy is only heat treated for a short amount of time all of the atoms may not have enough time to diffuse to equilibrium. The specific properties of an alloy that arise from heat treatment can be dependent on how much diffusion occurs. This is the case with age hardening in certain alloys such as beryllium copper alloys which has a peak strength that is reached at a specific time. After that time the strength starts to go down as shown in figure 1.

Lastly the cooling rate controls whether diffusion can occur to obtain other structures or phases. The fastest method of cooling is a quench which is performed by submersing the alloy into a liquid (generally water or oil). When an alloy cools this rapidly the microstructure and phases that are present at the time of quench are locked

Figure 1. A graph of tensile strength vs time illustrating the rise and drop of strength with age treatment times.3

Figure 2. A graph temperature vs time with cooling rates in red illustrating their effect on microstructure.4

into place. Alternately, slower cooling methods include air cooling or furnace cooling. These two cooling rates can allow for other desired phases and structures. The effects of cooling rate can be seen in figure 2 which shows the structures arising in steel from cooling rate.

The different types of heat treatment cycles are classified into different categories and given letter abbreviations. Anneal (O), solution heat treat (W), as fabricated (F), strain hardened (H), and the broadest is tempers (T). Tempers can include a combination of solution treatment, artificial aging, cold working, natural aging, stabilization, shaping processes, etc. To specify what type, the letter “T” is accompanied by a numeral. Tables have been compiled that show optimal soak temperatures, times, and cooling rates corresponding to the material and type of heat treatment.

Experimental Procedures

The specific type of aluminum studied in these experiments was a 356 series (A356). This means it had an approximate composition of 7 wt% silicon and a small amount of magnesium (< 0.3 wt%). In a previous lab two different samples were cast. The first had the normal composition of A356 and the second had grain refiners added to it resulting in a composition with 0.05 wt% Ti and 0.01 wt% B. Both of these samples were used in this heat treatment laboratory.

Heat treatment conditions for our samples of A356 were determined by consulting table 1. The samples were to be solution heat treated and artificially aged which corresponds to a type 6 temper (T6). Due to the fact that these tables are set for large amounts of material, the times were shortened to accommodate the size of the samples. The temperature does not change because of size. For A356 and a T6 heat treat the temperature of 540°C was used for the solution heat treatment and 155°C for the age treatment. Soak times were shortened from 12 hours to 1.5 for the solution treatment and the minimum of 3 hours was selected for the age treatment. A foot note at the bottom of the full table 1 states that unless specifically noted solution treatments are completed with a quench in room temperature water.

Two samples of both the refined and unrefined grained aluminum alloys were solution treated in the same furnace at 540°C for 1.5 hours. When the samples were removed from the furnace they were

immediately submerged in a bucket of water until cool. Next, one of each of the different sample was aged in a furnace at 155°C for 3 hours. Aged samples were also cooled in water for convenience.

All four samples’ hardness was analyzed using a Rockwell hardness tester using the “H” scale (1/8” diameter steel ball with 60kgf force). This scale was chosen because it could record an accurate measure for both the solution and age treated samples making them easier to compare.

Once hardness measurements were taken the four samples were mounted and polished using SiC paper in the following order: 240, 320, 400, and 600 grit paper. Next 1, 0.3, and .05 micron alumina slurry on polishing wheels were used in that order. The samples were then observed under an optical microscope and images were taken for comparison at 100X, 200X, and 500X.

Results & Discussion

The hardness results from the four samples are displayed in table 2 and show that after solution

Table 2

treatment the grain refined sample was harder. Grain refined A356 is harder than unrefined A356 because it has smaller grains. This follows the Hall-Petch relation:5

σ y=σ i+k y√D

This equation states that yield strength (σy) is inversely proportional to grain size (D). Figure 3 and figure 4 picture the unrefined and refined samples respectively, and it is discernable that the grain refined sample has smaller grains. These smaller grains account for the higher hardness.

Figure 3. A 100X micrograph of unrefined A356 in the S.T. condition showing large grains.

Figure 4. A 100X micrograph of refined A356 in the S.T. condition showing small grains.

Solution Treated Hardness

(HRH)

S.T. & Aged Hardness

(HRH)

Percent Change

Theoretical T6 Hardness(*HRH)

Refined 98 115 16.95%

Unrefined 71 113 60.14% 90-105

In both the unrefined and refined samples the hardness value rises after the aging process. This increase is due equilibrium phases that arise from the solution treatment and the age treatment. The solution treatment at 550°C is at a temperature just below the α-aluminum and liquid two-phase region as seen as a red line in figure 5. This means that material is still solid during solution treatment and still in the Al + Si region, but when we look at the same temperature on the Al-Mg2Si pseudo-binary phase diagram (figure 6) it becomes apparent that dissolution occurs. The Mg2Si particles present dissolve into the α-aluminum matrix.

It was found that the maximum dissolution of Mg2Si particles was 1.4 wt%.1 With less than 0.3 wt % magnesium in A356, all of the Mg2Si will dissolve. When the alloy is water quenched, after solution treatment, the magnesium stays dissolved inside the matrix because it does not have time to diffuse, thus supersaturating the aluminum. The age cycle is at a much lower temperature, pictured in blue in figure 6, well below the dissolution temperature for Mg2Si. This means that the magnesium will diffuse back out and form Mg2Si particles again. This is what is known as precipitation strengthening. The particles impede dislocation movement and harden/strengthen the material which explains why both samples were harder after age treatment.

When observing the differences in microstructure from solution treated sample to aged sample there is clearly more gray flakes in

the aged samples. This can be seen when comparing unrefined and solution treated to the unrefined and aged (figure 7). The refined solution treated sample and its aged counterpart (figure 8) show the

same phenomena. An increase in gray flakes is not due to the presence of Mg2Si in the aged samples.

The amount of magnesium is so low that these particles are not discernable at these magnifications. In addition to Mg2Si dissolving during solution treatment it has been shown that up to 1.5 wt% silicon will also dissolve into the matrix.1 The increase in gray flakes, which are silicon clumps, is due to the increase in silicon that comes out of supersaturation during aging.

Figure 5. An equilibrium phase diagram of aluminum and silicon with the solution treat temperature in red .1

Figure 6. A pseudo-binary phase diagram of aluminum vs Mg2Si with the S.T. temperature in red and the age temperature in blue.1

Figure 7. 500X micrographs of unrefined A356 and S.T. on the left and S.T. and aged on the right.

Figure 8. 500X micrographs of refined A356 and S.T. on the left and S.T. and aged on the right.

The reason that the unrefined and refined A356 converge after age treatment is because there is a switch in the mechanism dictating hardness/strength. Grain size dictated the hardness in the solution treated samples, but after precipitation hardening occurs during the age treatment that overrides the effects of grain size.

Theoretical T6 hardness for an A356 alloy was projected to be in the range of 90-105 HRH after converting from Brinell scale. The values that obtained in this laboratory were higher than the maximum. This could be explained by the fact that we were solution treating and ageing small samples relative to the samples that the approximations were probably set from. Our small samples may have reacted different to the heat treatment or high readings due to the close proximity of the edge of the sample could have resulted.

Conclusion

The investigation into the hardness and microstructures of A356 under different heat treatments answered many questions. Refined grain samples are harder than unrefined grain samples because of the Hall-Petch relation relating strength to grain size. Age treatments raised the hardness for both samples due to Mg2Si particles causing precipitation hardening. A more dense silicon flaked microstructure arose in aged samples due to supersaturated silicon diffusing out. The converging hardness values for the unrefined and refined samples after aging arose from the change in mechanism that affects hardness. It switched from grain size dependant to precipitate dependent. Lastly, the error from theoretical hardness values was due to our sample sizes.

References

1 Hernandez Paz, Juan F. “Heat Treatment and Precipitation in A356 Aluminum Alloy.” Montreal: McGill University, Sept. 2003. Web. <http://digitool.library.mcgill.ca/R/BIGMRDGQ8MAVJ1P9S5VV8IR A1RV1KHVTD8VJITQNSUAPUYL26T-01968?func=search>.

2 "Heat Treating Terms and Definitions - Engineers Edge." Engineers Edge - Design, Engineering & Manufacturing Solutions. Web. 03 Nov. 2010. <http://www.engineersedge.com/heat_treat.htm>.

3 "Age Hardening." Berylliumcopper-china|Strip、Rod、Plate / Bar、Wire. Web. 03 Nov. 2010. <http://www.berylliumcopper-china.com/technical/indexx.htm>.

4 "CCT Curve Steel" Wikipedia, the Free Encyclopedia. Updated: 07 Jan. 2010. Web. 04 Nov. 2010. <http://en.wikipedia.org/wiki/File:CCT_curve_steel.svg#filehistory>.

5 Murty, Dr. K. L. “The Hall-Petch Relation.” Raleigh: North Carolina State University, Feb. 2004. Web. < http://www4.ncsu.edu/~murty/NE509/NOTES/Ch5d-Strengthening.pdf >.