The Hybrid and 30% SiC composites showed a different behavior. Strengths were maintained or improved up until six hours of solution treatment at which point the strength decreased. The failure strain remained under 1% so it is diffi- cult to make conclusions on this data. Bonding of the particles and fibers ap- pears good in the SEM micrographs as the solution time increases, but further micro-analysis is required to determine the reasons for the decrease in strength after the six hour mark.
CONCLUSIONS
Heat treatment plays a major role in reinforced aluminum alloys. The most significant results were ob- tained with an alumina-silica (Saffil) fiber reinforced 359 alloy containing magnesium. The indication of the me- chanical and SEM results is that the magnesium in the alloy is forming a compound (most likely a spinel) at the fiber-aluminum interface. Initial- ly, the interaction of the alloy with the fiber increases the bond strength. This is evident in the SEM micrographs of the samples with three hours of solu- tion treatment. There are many areas of aluminum well bonded to the fiber, and reactions evident on a very jag- ged fiber surface. This is not seen in either of the samples solutionized for one hour or six hours. The one hour fibers are very clean with minimal interaction between fiber and alumi- num matrix. At six hours, a shale type structure dominated at the fiber sur-
Figure 4. UTS and failure strain data verse solution time of 359 composites.
face. The strength of the final composite was not signifi- cantly less than the three hour samples; however, the strain to failure was decreased by approximately 45%.
Silicon carbide based reinforcements do not respond like the fiber reinforcements in the loadings used in this study with A359 (30 and 40%). The effect of longer solution treatment doesn’t have a positive effect on the strength of the final composites initially in either one of the alloys tested. As the solution time was increased, a rebound of the UTS was realized. The solution times needed to reach adequate strength was roughly the same required for mono- lithic/unreinforced cast alloys.
In the 319 alloys, Saffil composite responded favorably to increase solution times up to ten hours, while 30% SiC and
62
the Hybrid MMCs increase or maintained strengths until six hours of solution time. After this point a decrease in strength was seen.
Overall, the solution treatment parameters play a large role in final strength and failure strain a composite can obtain. This current work demonstrates that the composites can be over or under solutionized if standard monolithic alloy pa- rameters are followed.
JUSTIFICATION AND ECONOMIC SIGNIFICANCE TO INDUSTRY
This work benefits casting technologies in which MMCs are cast, such as squeeze casting, stir casting, low pressure permanent mold casting, and high pressure die casting. Dif-
International Journal of Metalcasting/Winter 11
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87