An Overview of High-Performance Aircraft Structural Al Alloy-AA7085

doi:10.1007/s40195-015-0275-z

Abstract

Abstract:

AA7085 Al alloy is a new-generation, high-strength, quench-insensitive alloy that features in airframe applications such as massive bulkhead and wing spars structures in Airbus A380 and Joint Strike Fighter aircrafts. This alloy has a great potential in replacing currently used high-strength AA7xxx alloys in many international and national military aircraft programs [Hawk, Su-30, MiG series, light combat aircraft (Tejas), Hindustan turbo/jet trainer, light combat helicopter, advanced light helicopter, and fifth-generation fighter aircrafts]. An effort has been made to highlight the processing, heat treatment, properties and applications of the AA7085 Al alloy from the existing literature on the AA7085 alloy. The potential applications of this alloy are presented and compared with the currently used AA7075 Al alloy. A typical process cycle involved in the manufacturing of wrought AA7085 alloy products is described in detail. Particularly, the precipitation mechanisms involved in the different temper conditions are critically reviewed with the supporting microstructures. The mechanical properties with a particular emphasis on tensile, fatigue, and fracture toughness properties are discussed in detail based on the analysis of experimental data. The effects of temper conditions on the stress corrosion cracking and exfoliation corrosion have also been reviewed critically.

Key words: Al, alloys, Heat, treatment, Material, processing, Mechanical, properties, Corrosion

T. Ram Prabhu. An Overview of High-Performance Aircraft Structural Al Alloy-AA7085[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(7): 909-921.

Figures/Tables 14

Fig. 1 Flow stress curves of the homogenized AA7085 Al alloy at 350 °C a, 400 °C b, 450 °C c and the solution-treated AA7085 Al alloy at 350 °C d, 400 °C e, 450 °C f under strain rates of 0.0001-1 s-1 [24]

Fig. 2 Typical microstructure of AA7085 Al alloy: a homogenized at 450 °C for 24 h and 470 °C for 30 h, b solution-treated at 470 °C 1 h [24]

Table 1 Typical heat treatment cycles, tensile, toughness, and corrosion properties of the AA7085 alloy subjected to different temper and ageing conditions [7, 26, 32, 39]

Temper conditions	Ageing cycle	I _SSRT	Yield strength (MPa)	Tensile strength (MPa)	Elongation (%)	Max. exfoliation corrosion depth (μm)	Unit initiation energy (N/mm)
T6	120 °C, 24 h	0.6	530	582	13	78	148
T74	110 °C/6 h + 160 °C/10 h	0.8	457	525	10	52	182
RRA	120 °C/24 h + 180 °C/0.5 h + 120 °C/24 h	0.95	528	578	11	60	169
DRRA	120 °C/24 h + 180 °C/0.5 h + 120 °C/12 h + 180 °C/0.5 h + 120 °C/24 h	0.95	528	578	12	53	184
HLA	200 °C/5 min + 120 C/24 h	-	430	510	13	130	-

Fig. 3 Typical transmission electron micrographs of AA7085 Al alloy subjected to T6 a, T74 b, RRA c, DRRA d, HLA e temper conditions [26]

Table 2 Physical properties of the AA7085-T7452 die forging [7]

Property	Typical value
Density (g/cm³)	2.85
Electrical conductivity (%IACS)	24-41.5
Melting point (°C)	552-635
Specific heat at 100 °C (J/(g °C))	0.884
Thermal conductivity at 100 °C (W/(mK))	159.5
Coefficient of thermal expansion between 20 and 100 °C (10^-6/°C)	24.7

Fig. 4 Effect of air and corrosive medium (3% NaCl + 0.5% H2O2) on tensile properties of the AA7085 Al alloy under different temper conditions (T6, T74, and DRRA) [39]

Table 3 Mechanical properties of the AA7085-T7452 die forging for the section thicknesses of 25.1-300 mm, and the properties are measured in the samples of longitudinal orientation [17]

Property	Typical value
Yield strength (MPa)	448-462
Ultimate tensile strength (MPa)	496-503
Elastic modulus (GPa)	69.6
Elongation (%)	7-10
Shear strength (MPa)	282
Shear modulus (GPa)	26.9
Possion ratio	0.33
Fracture toughness (MPa m^1/2)	137-214
Fatigue strength of a smooth specimen for R = +0.1 condition for the life of 10⁷ cyc (MPa)	289-344
Fatigue strength of an open hole (K _t = 2.3) specimen for R = +0.1 condition for the life of 10⁷ cyc (MPa)	165-193

Table 4 Effects of section thickness and sample orientation on tensile properties of the AA7085-T7452 die forging [17]

Section thickness (mm)	Direction	Yield strength (MPa)	Tensile strength (MPa)	Elongation (%)
25.4-50.8	L	448	496	10
	LT	427	482	8
	ST	407	482	5
102-152	L	448	496	9
	LT	427	482	7
	ST	407	482	4
254-305	L	448	496	7
	LT	427	482	4
	ST	407	482	3

Fig. 5 Typical stress-strain curves of AA7085 Al alloy [7]

Fig. 6 Fracture surface morphologies of AA7085 Al alloy under conditions of T6 a, T74 b, RRA c, HLA d temper conditions [32]; high-magnification images of fracture surface of AA7085 Al alloy show the cracking along the grain boundary triple points, dimples, and voids e, cracking around coarse intermetallic particles f [42]

Fig. 7 Typical S-N curves of as machined and laser peening-treated AA7085 Al alloy (R = +0.1, K t = 1, RH >90%) [43]

Fig. 8 Fracture surface of an as machined AA7085 Al alloy, and surface crack initiation site is clearly observed [43]

Fig. 9 Effect of stress on the stress corrosion cracking behaviour of the AA7085 Al alloy [46]

Fig. 10 Surface morphology of an AA7085 Al alloy affected by exfoliation corrosion under different temper conditions: a T6, b T74, c RRA, d DRRA, e HLA [26]

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