Assessment of Centerline Porosity and Residual Stress in Rolled Aluminum Plates Destined for Aerospace Applications using Ultrasonic Test Techniques

The use of aluminum flat rolled products in modern aircraft structures has a history of over half a century. Rolled aluminum plates offer many advantages; for aerospace alloys, the process of hot rolling followed by heat treatment and quenching, stretching and artificial aging can produce aluminum plates of high strength (comparable to that of mild steel) and optimum ductility and fracture toughness as well as a uniform material structure and grain flow. Furthermore, in comparison with other processes for semi-finished aluminum such as casting and forging, aluminum rolled plates can be produced in large dimensions. Plates of over 20 meters length or 5 meters wide can now be produced at various thickness by many aluminum rolling mills around the globe. As such aluminum plates have been and continue to be a major structural material for modern airplanes. One of the major applications of aluminum plates has been in components that can be entirely machined out of one piece of plate, i.e. in monolithic form. This application started with smaller and simpler parts but it grew rapidly with the advancements of robotic machining. Major components such as bulk heads and floor beams were designed in monolithic form. Although in such processes, often over 98% of the plate material is machined away and sent to recycling, the costs are greatly offset by many advantages offered by monolithic structures; before the use of monolithic structures, many components consisted of sometimes hundreds of parts joined together by various joining techniques, e.g. welding and riveting, each of which adds extra weight and has a theoretical possibility of failure. A monolithic structure, on the other hand, offers a uniform and predictable material structure. The success of monolithic structures created a demand for a wider range of plate thickness. It also caused concern on the part of aluminum plate producer, as well as aircraft manufacturers, on two specific issues. These were porosity and residual stress in plates. Both of these conditions may cause damages during the process of machining and/or during the service life of a machined component. Both of these conditions may be diminished by optimizing metallurgical and operational factors. However, any improvement needed to be verified not only by standard laboratory tests but also by a nondestructive test method. The following paper describes the aluminum industry approach to these issues using NDT techniques.


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