جریان کاری صنعتی بخش به حداقل رسیده تحریف برای اجزای یکپارچه ماشین آ لات بزرگ در صنعت هوا و فضا

Part Distortion due to inherent residual stresses has resulted in recurring concession, rework and possibly scrap worth millions of Euro in the aircraft development and manufacturing life cycle. The paper presented here outlines an industrial solution based on years of fundamental research dated back to as early as mid-1990 to the development of a practical industrial solution to optimise part distortion in large monolithic components in the aerospace industry. The developed system was designed to empower manufacturing engineers at the shop floor level to help with their day to day activities from characterising residual stress profile in materials to numerical simulation to arrive at an optimised solution. The industrial technology suite includes the following technologies: (i) characterisation of inherent material residual stresses by adapting the established layer removal method for implementation on an industrial CNC machining centre; (ii) generation of residual stresses profiles using displacement measurements; and (iii) optimisation of part location in the materials through numerical modelling. The machine operator can characterise the bulk residual stresses in the materials on a standard CNC machining centre. The residual stresses profiles will subsequently be used as inputs via a user-friendly GUI, which will drive the numerical calculation to be performed remotely in supercomputers, in order to deliver an optimised solution.

Part distortion is a common problem in manufacturing life cycle and is defined as the deviation of part shape from original intent after released from the fixture. This is not caused by dimensional inaccuracy, machining tolerance or over/under-machined. Distortion is a major challenge in airframe industry [1] which costs billion of losses in profit every year. A study by Boeing, based on four aircraft programmes, estimated the rework and scrap costs related to parts distortion comes to in excess of 290 million dollars [2]. Moreover, it is estimated that distortion from heat treatment the machine tool, automotive and power transmission industries in Germany is costing an economic loss of €850 million [3]. It is known that distortion comes from several variables such as the type of material, residual stresses in bulk material [4], machining induced residual stresses, part design [5], the location of the part in the billet of which it was machined [4] etc. In aerospace industry, aero structure components are generally made up of large thin wall or web components. These components are often machined from rolled plate, forgings, extrusion or casting and up to 90 to 95% of the materials could be removed. The dominant factor of part distortion in aerospace industry is the inherent residual stresses in the part. These inherent residual stressesusually come from different manufacturing processes, i.e. quenching, stretching forging, extrusions, casting, welding, machining, forming, and etc [6]. These processes are complex combinations of heat transfer, mechanical deformation and metallurgical changes.The industrial solution detail in the paper focus on part machined from rolled plate. Different alloying elements are mixed, melted and casted into an ingot and cooled. The cast ingot is then heated, rolled, quenched orrapidly cooled to achieve desirable physical and mechanical material properties. However, quenching also induces undesirable high levels of residual stresses because of the large surface heat fluxes and high temperature gradients near the surface and between intermediate layers of the materials. These conditions usually induce thermal residual stresses at yield stress magnitude, which causes high residual stresses in the half-product and may even cause distortion or cracking [7]. The plate is then mechanically stretched in the rolling direction to 1.5 to 3% plastic deformation at room temperature to relieve these high quench-induced residual stresses [8]. The paper presents an industrial solution to minimise distortion in the following ways: (i) determination of bulk residual stresses in rolled plate; (ii) data processing to create residual stress profiles; (iii) numerical simulation to determine optimised part location for minimal distortion.

Distortion is a common manufacturing challenge in aerospace industry because of the length to thickness ratio of the parts. As it is widely known distortion can come from different reasons, from part geometry, symmetric or asymmetric design, bulk material residual stresses, or even from machining induced residual stresses. In order to minimise, or even eliminate distortion, a holistic view on residual stress distribution in the part based on the entire manufacturing history is needed. Shot peening is today’s downstream solution to introduce compressive residual stresses to correct the distortion. Unfortunately, this added manufacturing costs and lead time to the manufacturing lifecycle. As for the simulation strategy, further research needs to be done so realistic boundary conditions, progressive material removal and superficial residual stresses could be integrated because the resulted distortion is very conservative. For example, during milling, depending on the clamping, the part may distort and a subsequent milling pass will compensate.The proposed workflow will empower the manufacturing engineers to address the distortion at the shop floor prior to machining. Residual stress can be determined much earlier using the new jig and part location calculated to minimise distortion. This will lead into a more efficient resource planning, decrease of manufacturing lead times and costs and avoid rejected parts. The time for this workflow to propose an optimised offset is minimal compared to the time needed for one shot peening cycle and can be further decreased if an automated clamping system will be developed. Moreover, it can be used as a ‘Design against Distortion’ tool in the design and development of new aerospace parts as it will be possible to predict the distortion behaviour of a new concept part through simulation. Therefore, decision on plate thickness andmaterial specification to procure can be determined earlier at the design stage.