Session: 03-05-03: Design, Material Processing, and Applications of Metal and Ceramic Composites

Paper Number: 95883

95883 - The Effect of Multi-Stage Age Treatment on Mechanical Properties of 7075 Al Alloy 

7xxx aluminum alloys (AA) are attractive in aerospace, and ground transportations industries because of their high specific strengths and better fatigue properties. Zinc is the primary alloying element in 7xxx series alloys along with copper and magnesium. These 7000 series aluminum alloys are heat treatable and the strength is increased by precipitation method. One of the widely used aerospace alloys is 7075 aluminum alloy because of its high tensile and compressive strength and good response to exfoliation corrosion. A solution heat treatment followed by artificial ageing is known as T6 temper designation was initially used to obtain peak strength for 7075 AA. The artificial aging, done at 115°C to 130°C (T6 temper), increases strength of 7075 AA to a peak level then decreases, however, decreases resistance to stress-corrosion cracking. During quenching supersaturated solid solution forms, and during aging sequentially coherent Guinier–Preston (GP) zone and stable precipitates (η) form. The mechanical strength greatly depends on the homogeneous distribution and refinement of precipitates. T73 temper improves stress-corrosion cracking resistance, but it decreases the peak strength by 10% - 15%. T76 temper has been developed to improve exfoliation corrosion resistance. The overaging temperature range is 160°C to 175°C. Retrogression and reaging (RRA), a multistage heat treatment process, has been applied as a trade-off between T6 and T73 tempers. RRA consists of T6 temper plus aging at higher temperatures for shorter time and another last stage t6 temper. Another factor to be noted is the room temperature delay between solution treatment and artificial aging. Highest strength can be achieved with minimum delay. The effect of room temperature delay between quenching and aging can be eliminated by double-stage aging. Recent trend shows that the strength can be increased more by applying multi-stage aging process. This research focuses on the effect of multiple aging temperature, time, and pre-age strain hardening on the mechanical properties of 7075 aluminum alloys. ASTM standard coupons were machined from an as received aluminum plate and applied different combination of age treatment and strain hardening. The initial results on strength, ductility, relationship with phase transformation and precipitation behavior are reported. The tensile strength and ductility were measured using uniaxially tension test. The hardness was measured using a micro hardness tester. The microstructures were investigated using optical microscopy (OM) and scanning electron microscopy (SEM). Acoustic emission signals were obtained during uniaxial tension test to correlate acoustic signal with crack initiation and propagation. These signals were again correlated with the different RRAs.

Presenting Author: AHM RAHMAN Pennsylvania State University - Harrisburg

Presenting Author Biography: Dr. Rahman is an assistant professor of mechanical engineering in the Pennsylvania State University, Harrisburg campus. His research interests are light alloy technology, metal matrix composites, joining of specialty alloys and hybrid bioceramics.

Authors:

AHM RAHMAN Pennsylvania State University - Harrisburg
Issam Abu-Mahfouz Pennsylvania State University - Harrisburg
Amit Banerjee Pennsylvania State University - Harrisburg
Johnmark Wisniewski Pennsylvania State University - Harrisburg