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Structure and properties of thick plate and near surface properties after high speed machining af AA7010
Linköping University, Department of Mechanical Engineering, Engineering Materials. Linköping University, The Institute of Technology.
2003 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Using thick plates instead of forgings in the aircraft industry for integral construction of load carrying components is becoming more and more practice. The reasons are shorter lead-times from design of a modified or totally new component to introduction in an aircraft and smaller variations in properties for plate compared to forging. The concept of integral construction also reduces the assembly time. The complex shaped components are prepared by machining pieces of thick plate. The thicker the plate the larger components can be made in one piece. Machining components from blocks of material cut from thick plate means removal of a lot of material compared to machining of near final shape forgings. A change in machining concept to high speed machining leads to higher productivity and makes thin walled sections possible to manufacture due to decreased cutting forces.

Variation of through thickness structure and properties of 7010-T7451/2 as 100, 150 and 200 mm thick plates has been investigated. Through thickness crystallographic texture, degree of recrystallisation, distribution of inclusions, chemical composition and grain size has been mapped out. The observed structure is taken into account in order to explain variations of properties like yield strength; fracture toughness and fatigue crack growth resistance. Equipment used in the work of characterising the structure has been EBSP, SEM, X-ray diffraction and optical microscopy. Equipment used for evaluating mechanical properties is screw machines, servo hydraulic machines and hardness indentors.

The plates show a strong through thickness texture gradient that influence the yield strength. The yield strength is also dependent on chemical composition and quench rate. Recrystallisation did not show any significant influence on yield strength or fracture toughness. The grain morphology together with quench rate is of importance for the fracture toughness and the fatigue crack growth resistance.

Properties of down cut milled surfaces on thin sections using a conventional machining concept and the concept of high speed machining at various cutting speeds have been compared. The same has been done for facemilled surfaces using conventional tools and inserts at cutting speeds varying from 500 m/min up to 5000 m/min. The property of most interest is the high cycle fatigue strength. The influence of surface roughness, residual stresses and hardness on the fatigue strength has been investigated. In order to try to gain a little more information about the near surface properties x-ray diffraction studies at grazing angle incidence has been undertaken.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet , 2003. , 69 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 822
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-30057Local ID: 15517ISBN: 91-7373-652-X (print)OAI: oai:DiVA.org:liu-30057DiVA: diva2:250878
Public defence
2003-09-12, Sal C3, Linköping Universitet, Linköping, 10:15 (Swedish)
Opponent
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2013-01-07
List of papers
1. The influence of hot rolling on the variation of through thickness structure of AA7010 thick plates
Open this publication in new window or tab >>The influence of hot rolling on the variation of through thickness structure of AA7010 thick plates
1996 (English)In: Thermomechanical processing in theory, modelling and practice [TMP]2: proceedings of an International Conference organised in celebration of the 75th anniversary of the Swedish Society for the Materials Technology, 1996, 284-289 p.Conference paper, Published paper (Other academic)
Abstract [en]

The variation of through thickness structure and yield strength of AA7010-T7451 thick plate has been investigated. Grain sizes at different depths were determined qualitatively by using optical microscopy. Texture measurements were carried out using X-ray technique at the same depths as grain size studies and tensile property tests have been performed. EBSP techdque has been used to compare the degree of recovered and recrystallized grain structure at different depths. Tensile testing determined through thickness variations in yield strength. The well known W shape of longitudinal yield strength variation through the thickness was not observed. The influence of texture on the yield strength measured is discussed.

Keyword
Aluminium, texture, an isotropy, yield strength, plate, Taylor factor
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-86812 (URN)91-630-5421-3 (ISBN)
Conference
Thermo- Mechanical Processing Theory, Modelling and Practice (TMPT)2, Stockholm, Sweden 4-6 September
Available from: 2013-01-07 Created: 2013-01-07 Last updated: 2013-01-07
2. The variation of through thickness properties and structure of AA7010 thick plate
Open this publication in new window or tab >>The variation of through thickness properties and structure of AA7010 thick plate
1997 (English)In: International Conference on Thermomechanical Processing of Steel and Other Materials 1997: THERMEC '97 / [ed] T. Chandra and T. Sakai, 1997, 941-949 p.Conference paper, Published paper (Refereed)
Abstract [en]

The variation of through thickness structure and yield strength of AA7010-T7451 100 and 150 mm thick plate was investigated. Grain sizes at different depths were determined by using optical microscopy and EBSP maps. Texture measurements were carried out, using X-ray technique and EBSP, at the same depths as grain size studies and tensile property tests were performed. EBSP technique was used to compare degree of recovered and recrystallized grain structure at different depths. Tensile testing and hardness measurements reveal through thickness variations in yield strength and hardness. Variations in composition through the thickness are also possible. The influence of texture on yield strength was discussed.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-86813 (URN)0-87339-377-5 (ISBN)
Conference
THERMEC'97, International Conference on Thermomechanical Processing of Steels & Other Materials, University of Wollongong, Australia, 7-11 July, 1997
Available from: 2013-01-07 Created: 2013-01-07 Last updated: 2013-01-07
3. Through thickness fracture toughness variations of AA7021 thick plate
Open this publication in new window or tab >>Through thickness fracture toughness variations of AA7021 thick plate
(English)Manuscript (preprint) (Other academic)
Abstract [en]

In this work, a variation of fracture toughness as a function of position and orientation in commercial thick plates of a high strength aluminium alloy has been investigated. Structural and mechanical property variations have been investigated in order to understand the result of the fracture toughness measurements. 100, 150 and 200 mm thick plates of the aluminium alloy AA7010 were studied. The material was averaged and stress relieved by stretching (100 and 150 mm) and cold compression (200 mm), T7451/52. The material studied is frequently used to produce load-carrying airframe components. The knowledge of fracture properties throughout the thick plates is of importance since components are machined from them. The understanding of the varying fracture properties is valuable for development of material and or processes for producing thick plate of high strength aluminium alloys. It is of course also important to know the limitations of the thick plate material and the best way to produce a certain component from a piece of the plate material. Investigated parameters influencing fracture properties are inclusions, recrystallisation, yield strength, chemical composition and quench rate.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-86816 (URN)
Available from: 2013-01-07 Created: 2013-01-07 Last updated: 2013-01-07
4. Fatigue crack growth rate in commercial thick plates of AA7010
Open this publication in new window or tab >>Fatigue crack growth rate in commercial thick plates of AA7010
(English)Manuscript (preprint) (Other academic)
Abstract [en]

A mapping of fatigue crack growth rates in commercial thick plates of a high strength aluminium alloy, AA7010-T7451/52, has been done. The investigated plate thicknesses were 100, 150 and 200 mm. Material from near edge and mid-width at near surface and mid-thickness has been investigated. Measurements of crack length have been perfonned using DC potential drop. Cyclic condensation is used in order to be able to investigate local crack growth after fracture. Influence of crack closure, crack branching and slow growing side cracks on fatigue crack growth rate of S-L and L-T oriented CT specimens are discussed. S-L specimens show the highest fatigue crack growth rates. Beach marks on the fracture surface due to the cyclic condensation reveals locally straight crack fronts. A difference in growth rate between near surface and mid-thickness positioned L-T specimens are found. At a nominal ΔK of - 7 MPa√m the mid-thickness samples showed a sudden crack growth rate acceleration. The result of the investigation is explained in tenns of variations in structure, which is a function of position in the plates and plate thickness.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-86819 (URN)
Available from: 2013-01-07 Created: 2013-01-07 Last updated: 2013-01-07
5. Property comparison of thin walled sections machined using high speed machining and conventional machining
Open this publication in new window or tab >>Property comparison of thin walled sections machined using high speed machining and conventional machining
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The influence of two different machining concepts has been compared. Pockets were machined in a thick plate of AA7010-T7451 by using a high speed machining concept at three different cutting speeds and a conventional machining concept. The pockets were machined using down cut milling and of the remaining thin walls of material fatigue samples were taken out. Some high-speed machined surfaces were anodized using chromic acid in order to see if differences in fatigue properties remained after this surface process. Fatigue properties at constant amplitude and random spectra loading were tested. Properties like residual stress, surface roughness; peak broadening at grazing incidence and micro hardness of machined surfaces were investigated and correlated to the fatigue results. A low cutting speed using the high speed machining or the conventional machining concept yielded the best fatigue properties when fatigue initiation is of concern. The higher fatigue strength is not explained by the residual stresses measured at the machined surfaces using chromium and copper radiation or by the somewhat larger surface roughness. The penetration depth of the residual stresses was found to be higher at high cutting speeds. Fatigue initiation took most often place at near surface iron-rich inclusions. The points of fatigue initiation were not found to differ for different machining concepts and cutting speeds.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-86820 (URN)
Available from: 2013-01-07 Created: 2013-01-07 Last updated: 2013-01-07
6. Face milling of AA7010 at high cutting speeds
Open this publication in new window or tab >>Face milling of AA7010 at high cutting speeds
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Face milling using different commercial tools and inserts at varying cutting speeds on a high strength aluminium alloy has been performed. The surface integrity of the machined samples has been investigated in terms of surface roughness, residual stresses, hardness and peak broadening from x-ray diffraction at grazing angle incidence. Some fatigue testing of the machined surfaces has been done. The cutting chips from the different machining parameters are investigated and compared to one another. The results show a strong influence of tool insert on surface roughness, residual stress, peak broadening, and hardness profile and fatigue properties. The influence of cutting speed on the surface integrity is much smaller. The cutting speed does however influence the size and shape of the cutting chips. There is also a general decrease in peak broadening from x-ray diffraction very near the machined surface after high cutting speeds. This could be explained by a higher local heating of the work piece at very high cutting speeds.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-86822 (URN)
Available from: 2013-01-07 Created: 2013-01-07 Last updated: 2013-01-07

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