Structure and properties of thick plate and near surface properties after high speed machining af AA7010

Detta är en avhandling från Linköping : Linköpings universitet

Sammanfattning: 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.

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