Bernard Bes

Damage Tolerance Capability of an Al-Cu-Mg-Ag Alloy (2139)

The effects of dispersoid forming elements on the mechanical properties of Al-Cu- Mg-Ag alloys are examined. It is found that a small amount of Zr addition is detrimental to the damage tolerance of Al-Cu-Mg-Ag alloys in artificially aged temper, while Mn addition is beneficial. The superior damage tolerance capability of Alloy 2139 is demonstrated by comparing it to other high performance alloys used for DT critical applications

Microstructural interactions during stress ageing a 7475 aerospace alloy

In industrial process, like creep-ageforming, materials are aged under load. To investigate the influence of an applied stress on the ageing behaviour of Cu bearing Al-7xxx series alloys, a 7475 alloy was aged under a constant tensile stress and analysed by means of Small Angle X-ray Scattering (SAXS) and TEM. Mechanical testing was also employed, to determine if there was any effect on the materials strength. The results show that during the early stages of ageing significant interactions takes place, which preferentially aligns one type of GP zone, as well as affecting their size and volume fraction. During the second stage ageing treatment, the applied stress was observed to cause more rapid over-ageing, by promoting the formation of the η phase.

Through Thickness Microstructural Gradients in 7475 and 2022 Creep - Ageformed Bend Coupons

In creep-ageforming a material experiences continuously variable bending stresses through its thickness, from tensile to compressive, which are maximum at the surfaces. This can potentially result in through thickness microstructural gradients, due to interactions between the bending stresses, creep, and precipitation occurring during ageing, that can alter a component’s performance. The aim of the work reported here was to develop an understanding of these effects in an industrial creep ageforming process. For this purpose two aerospace alloys, 7475 and 2022, were ageformed by Airbus UK using industrial scale ageforming tools. Their microstructures were analysed in detail by TEM, and SAXS, which revealed significant through thickness microstructural changes after forming.

Effects of Combined Zr and Mn Additions on Dispersoid Formation and Recrystallisation Behaviour of AA2198 Sheet

The present paper focuses on the influence of combined additions of Zr and Mn on the recrystallisation resistance of aluminium alloy 2198 sheet. Dual additions of these dispersoid forming elements have previously been reported to be beneficial for reducing recrystallisation during solution treatment, as they exhibit opposing microsegregation partitioning on solidification. Contrary to expectation, it was found that the addition of Mn, to a standard Zr-containing 2198 sheet material, reduced recrystallisation resistance. The reasons for this behaviour are explored by analysis of the morphology, size, chemistry, and distribution of the dispersoid families formed, as a function of the Mn and Zr level, traced back to the homogenisation stage.

PREDICTION OF SPECTRUM FATIGUE CRACK GROWTH IN LATEST GENERATION HIGH DAMAGE TOLERANCE ALLOYS

An attempt was made to predict the variable amplitude fatigue crack growth performance of aluminium alloys using existing simulation software. The experimental basis for validation consisted of variable amplitude tests for a large transport wing spectrum for 2024-T351 and the latest generation of high damage tolerance alloys 2024A-T351, 2027-T351 and IS237T851. For 2024-T351 and 2024A-T351, there was also data from a business jet spectrum test. A first simulation approach was to fit the retardation models to the behaviour of 2024, then verify the predictive capabilities for the other alloys using the same spectrum. Results were quite satisfactory with two exceptions. The models that performed the best in this exercise were the Boeing/Northrop Constant Closure model (NASGRO) and the closure model from AFGROW. In general, it can be concluded that a first assessment of the performance of new high damage tolerance alloys can be made on the basis of parameter fits done for 2024 – for a given spectrum. Subsequently, the spectrum and test specimens were changed, and the parameters of the retardation models adapted according to the authors’ current best judgment. Again it proved impossible to predict crack lives for 2024A. For 2024, the model that came closest was the Generalised Willenborg model as implemented in NASGRO. However, this result must be seen against the background of unexpected no-retardation predictions from NASGRO on the new spectrum & specimens. In general, it must be concluded that until understanding of both the underlying phenomena and the ways of modelling and simulating these is improved, a change of spectrum necessitates re-fitting of many model parameters.