Optical microscopy of the microstructures arising from the use of differing thermo-mechanical treatments applied to the manufacture of hand forged planks as precursors to die forgings showed that a range of different grain structures could be produced. Although these differences had no significant effect upon tensile properties, as would be expected since strength is primarily controlled by precipitation within the grains, properties which are more grain-boundary sensitive such as fracture toughness and resistance to stress corrosion cracking showed a more noticeable effect. Comparison with current processing practice for the manufacture of hand forged preforms has indicated that improvements may be expected by incorporation of some of the findings in this phase of the work.
Although the proprietary nature of the results of the cold compression studies prevents disclosure of the details, exploitation of the improved understanding to have arisen has given the possibility that further property improvements may be achieved from improved optimisation of cold compression procedures.
Studies of the alloy compositional effects gave a varying degree of success. Work on the effect of a 'rich' 8090 composition has indicated that use of such material may enhance the response to heat treatment of the alloy, although care will need to be exercised in ensuring that castability and density, in particular, are not adversely affected. However, it would appear that some modifications to the basic 80990 composition specifically for forging stock may be beneficial to properties, which may not be applicable to other product forms. The effect of increased zirconium levels was somewhat disappointing compared to work carried out on the similar high zirconium-containing UL30 alloy. The reasons for this were unclear, but it is apparent that whatever the cause, a significant amount of further developmental work would be necessary to pursue this route. Finally, it has been shown that additions of vanadium/boron, indium and zinc of the levels studied in this work show no potential in improving 8090 forging properties.
It is considered that a high degree of success has been achieved in this project. As a result of the systematic approach taken in the work, it would appear that it will be possible to define a modified processing route for the manufacture of 8090 alloy die forgings after further confirmatory work has been completed.
As planned at the outset, improvements to several different aspects of the die forging procedure are indicated and are summarised below.
- Modifications to the thermo-mechanical processing in the manufacturing of hand forged preforms.
- Optimisation of cold compression parameters.
- Modifications to the basic 8090 composition to produce a variant specifically for forging applications.
The likely outcome is improved fracture toughness and stress corrosion characteristics for a given strength level than those achieved hitherto.
Although aluminium-lithium based alloys 8090, 2091, 2090 have reached commercial status, emphasis has been directed primarily at the manufacture of sheet, plate and extrusions wherein the relationship between composition, fabrication techniques and property levels is relatively well developed and understood. The similar situation for die forgings is not so well advanced and is the objective of the current project. The overall aim is to optimise the various steps in the hand and die forging processes and, together with development of an 8090-based variant specifically for die-forging, it is hoped to achieve enhancement properties in this product form. The project is comprised of the following specific tasks :
-optimisation of hand forging techniques-development of thermomechanical processing parameters to optimise grain shape, size and type (i.e. recrystallized, unrecrystallized, duplex, etc).
-mechanical working effects-maximisation of degree/uniformity of S-Al(2)CuMg precipitation by optimising post-solution heat treatment cold compression levels using special finishing dies. Correlation between experimentally measured strain distribution levels in hand and dye forgings with theoretically predicted values from FE analysis.
-compositional effects. Maximisation of degree/uniformity of S-Al(2)CuMg precipitation by modification of lithium, copper and magnesium levels. Modification of S-Al(2)CuMg precipitation and/or precipitation of further strengthening phases by minor additions of other alloying elements.
B91 3DG Solihull