Simulation and automation of aluminium panel shot peen forming

We present a methodology for automated forming of metal plates into freeformshapes using shot peening. The methodology is based on a simulation softwarethat computes the peening pattern and simulates the effect of its application.The pattern generation requires preliminary experimental characterizationof the treatment. The treatment is applied by a shot peening robot. The program for the robot is generated automatically according to the peening pattern. We validate the methodology with a series of tests. Namely, we form nine aluminum plates into doubly curved shapes and we also shape model airplane wing skins. The article describes the complete workflow and the experimental results.

Determination of optimal shot peen forming patterns using the theory of non-Euclidean plates

We show how a theoretical framework developed for modelling nonuniform growth can model the shot peen forming process. Shot peen forming consists in bombarding a metal panel with multiple millimeter-sized shot, that induce local bending of the panel. When applied to different areas of the panel, peen forming generates compound curvature profiles starting from a flat state. We present a theoretical approach and its practical realization for simulating peen forming numerically. To achieve this, we represent the panel undergoing peen forming as a bilayer plate, and we apply a geometry-based theory of non-Euclidean plates to describe its reconfiguration. Our programming code based on this approach solves two types of problems: it simulates the effect of a predefined treatment (the forward problem) and it finds the optimal treatment to achieve a predefined target shape (the inverse problem). Both problems admit using multiple peening regimes simultaneously. The algorithm was tested numerically on 200 randomly generated test cases.

Improving stripping efficiency of double curvature surface on a setup with turret head

The study was performed to develop a method for selecting a rational profile of a profiled flap wheel for a turret stripping head for cleaning parts with different radius of the transverse curvature. Researchers from the Irkutsk National Research Technical University and Irkutsk Aviation Plant designed and fabricated a special PFS-4 (peen forming setup) unit to implement manufacturing technology of large-scale contour-forming components. The unit is equipped with a CNC system, two movable operating elements, a shot blaster and a turret stripping head with four flap wheels. The paper offers methods and criteria for selecting the profiled flap wheel for stripping the contour-forming surfaces of the components, depending on the curvature radius of the latter. A flap wheel with an optimal curvature radius of 40 m was chosen for analysis, which allows a sufficiently large range of profile curvature of the processed components (from 8 to 40 m) to be covered. Profiled flap wheels 100 and 200 m wide with a flap profile radius of 40 m provided uniform material removal when cleaning the surface with a curvature radius from 8 to 40 m without further overlapping with a finished strip. It was shown that wider profiled flap wheels are necessary to increase stripping efficiency. In this case, a 300 mm wide flap wheel can be used for a component surface area with a transverse curvature radius over 14 m and a 400 mm wheel for surface areas with a curvature radius of over 20 m. Thus, comparing the stripping process of a curved surface by the straight flap wheel revealed that profiled flap wheels significantly expand the workability of the PFS-4 unit turret stripping head.

Peen forming and stress peen forming of 2024-T3 aluminum sheets. Part 1: 3D scans and residual stress measurements

Aluminum skins on the lower wings of most commercial aircraft are shaped using shot peen forming. This process, which involves bombarding the skins with hard shot, uses nonuniform plastic flow to induce curvatures---in the same way that differential expansion makes metal bilayers bend when heated. Here, we investigate experimentally how constraining conditions affect the final shape of peen formed parts. We report peen forming experiments for 4.9 mm thick rectangular 2024-T3 aluminum sheets of different aspect ratios uniformly shot peened on one face with a low intensity saturation treatment. Some specimens were free to deform during peening while others were elastically prestressed in a four-point bending jig. For each aspect ratio and prestress condition, residual stresses were measured near the peened surface with the hole drilling method. Additional residual stress profiles were also obtained with the slitting method. The residual stress measurements show that the progressive deformation of unconstrained specimens had the same effect as an externally applied prestress. For the peening conditions investigated, this progressive deformation caused unconstrained strips to exhibit curvatures 33% larger than identical strips held flat during peening. Furthermore, we found that the relative importance of material anisotropy and geometric effects did determine the bending direction of unconstrained specimens.

Peen forming and stress peen forming of 2024-T3 aluminum sheets. Part 2: eigenstrain analysis

In this study we use the theory of eigenstrains to investigate how different sources of anisotropy affected the results of shot peen forming experiments reported in Part~1. The specimens consisted of 4.9 mm thick 2024-T3 aluminum sheets uniformly shot peened on one face that were either free to deform or held onto a prestressing jig during peening. Potential sources of anisotropy included the plastic anisotropy of rolled aluminum, anisotropic initial stresses that redistribute when their equilibrium is disturbed by peenning, the geometry of the specimens, and externally applied prestress. For the alloy and peening conditions considered, plastic anisotropy had no discernable influence on the resulting shape of the peen formed specimens. Initial residual stresses, on the other hand, caused slightly larger bending loads in the rolling direction of the alloy. Although the magnitude of these loads was approximately 30 times smaller than peening-induced loads, it was sufficient to overcome the geometric preference for rectangular sheets to bend along their long side and cause all unconstrained specimens to bend along the rolling direction instead. Once the sheets started to deform, larger plastic strains developed in the bending direction. We show that this effect is equivalent to that used in the variant of the process called stress peen forming where parts are elastically prestressed during peening to obtain larger plastic strains in directions in which the material is stretched.