BS in Materials Engineering
Materials Engineering Department
7050 alloy (6.2Zn-2.3Mg-2.3Cu-0.12Zr) is a structural aluminum alloy commonly used in the aerospace industry. Its rich chemistry makes it susceptible to incipient melting. Incipient melting is a phenomenon where areas with high local alloy content melt before the rest of the material. This can cause a degradation of mechanical and corrosion properties. A six by four factor solution heat treatment trial was conducted. Six solution heat treatment temperatures, 890- 940°F in 10°F increments, and four times, 1, 2, 4, and 6 hours, were used. Incipient melting was observed in the 910°F, 6 hour solution heat treatment and all 920-940°F solution heat treatments. Solution heat treatment temperature was shown to be the more important factor. Little microstructural variance was seen among the different solution heat treatment times at a single temperature. Literature suggested that incipient melting would pit preferentially and increase susceptibility to intergranular attack. All 4 hour solution heat treatments were tested for pitting corrosion susceptibility by immersion of metallographic samples in a 0.1M NaCl solution. Samples with incipient melting present did not show preferential pitting. The same samples were tested for intergranular attack susceptibility according to AMS 2772 and ASTM G110. Samples with incipient melting showed no increase in sensitivity to intergranular attack. Neither of the corrosion properties tested degraded in the expected manner.