**7. References**


The fatigue strength for 1×107 cycles of 7050–T7451 aluminum alloy was increased by shot peening and laser peening. Fatigue strength of the best-laser-peened specimens is 42% higher than as-machined specimens and the fatigue strength of the best shot-peened

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Costa, L. & Vilar, R. (1996). Diffusion-limited layer growth in spherical geometry: A numerical approach. *Journal of Applied Physics*, Vol. 80, No. 8, pp.4350-4353. Damborenea, J. de. (1998). surface modification of metals by power lasers. *Surface coatings* 

Ding, K., & Ye, L. (2003). Three-dimensional Dynamic Finite Element Analysis of Multiple Laser Shock Peening Processes. Surface Engineering, Vol. 19, p.351-358. Ding, K. & Ye, L. (2006). *Laser shock peening Performance and process simulation*. Woodhead

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	- Vol. 202, pp. 1199-1203

**7** 

*Poland* 

**Microstructural Changes of Al-Cu Alloys After Prolonged Annealing at Elevated Temperature** 

The precipitation−strengthened 2xxx series Al−Cu alloys are one of the most important high-strength aluminium alloys. They have been employed extensively in the aircraft and military industries, in which materials are frequently subjected to elevated temperature. The aluminium casting alloys, based on the Al−Cu system are widely used in light−weight constructions and transport applications requiring a combination of high strength and

Al-Cu alloys are less frequently used than Al-Si-Cu grades due to technological problems in production process (e.g. high propensity to microcracking during casting). However they are the basis for development of multicomponent alloys. Typical alloys for elevated temperature application are Al-Cu-Ni-Mg alloys (containing about 4,5% Cu, 2% Mg and 2%Ni). Their good properties at elevated temperature result from formation of intermetallic phases Al6Cu3Ni and Al2CuMg, both during crystallization and precipitation hardening

Mechanism of precipitation hardening in cast and wrought binary Al-Cu alloys is well known and widely covered in literature. There are some suggestions that decomposition of supersaturated α(Al) solid solution in other precipitation hardened alloys like Al-Cu-Mg, Al-Si-Cu, Al-Mg-Si follows the same route as in the Al-Cu alloys with some specific features of the particular stages of the process (Martin, 1968;). The interest in course and kinetics of the aging process has the practical meaning as the early stages of aging leads to significant improvement of mechanical properties of the alloys. Maximum hardening effect in Al-Cu alloy is a result of in situ transformation of GP zones into transient phase θ". Increase in aging temperature leads to decrease of the hardness of solid solution α(Al) due to precipitation of equilibrium θ phase on the grain boundaries or on the θ'/matrix phase boundaries. Prolonged aging may lead to microstructure degradation related to coagulation and/or coalescence of the highly dispersed hardening phase precipitates resulting in decrease of hardening effect (Mrówka-Nowotnik et al., 2007; Wierzbińska & Sieniawski, 2010). Therefore development of the chemical composition of the alloy, especially intended for long term operation at elevated temperature, requires taking into account factors resulting in deceleration of the coagulation process and obtaining stable microstructure consisting of solid solution α grains and highly dispersed precipitates of the second phase

(El−Magd & Dünnwald, 1996; Martin, 1968; Mrówka-Nowotnik et al., 2007).

**1. Introduction** 

(Wierzbińska & Sieniawski, 2010).

ductility.

Małgorzata Wierzbińska and Jan Sieniawski

*Rzeszow University of Technology, Rzeszow,* 

