The article presents the results of the analysis of the interactions between the single point incremental forming (SPIF) process parameters and the main roughness parameters of stiffened ribs fabricated in Alclad aluminium alloy panels. EN AW-7075-T6 and EN AW-2024-T3 Alclad aluminium alloy sheets were used as the research material. Panels with longitudinal ribs were produced with different values of incremental vertical step size and tool rotational speed. Alclad is formed of high-purity aluminium surface layers metallurgically bonded to aluminium alloy core material. The quality of the surface roughness and unbroken Alclad are key problems in SPIF of Alclad sheets destined for aerospace applications. The interactions between the SPIF process parameters and the main roughness parameters of the stiffened ribs were determined. The influence of forming parameters on average roughness Sa and the 10-point peak–valley surface roughness Sz was determined using artificial neural networks. The greater the value of the incremental vertical step size, the more prominent the ridges found in the inner surface of stiffened ribs, especially in the case of both Alclad aluminium alloy sheets. The predictive models of ANNs for the Sa and the Sz were characterised by performance measures with R2 values lying between 0.657 and 0.979. A different character of change in surface roughness was found for sheets covered with and not covered with a soft layer of technically pure aluminium. In the case of Alclad sheets, increasing the value of the incremental vertical step size increases the value of the surface roughness parameters Sa and Sz. In the case of the sheets not covered by Alclad, reduction of the tool rotational speed increases the Sz parameter and decreases the Sa parameter. An obvious increase in the Sz parameter was observed with an increase in the incremental vertical step size.

Stringer-stiffened panels made of aluminium alloys are often used as structural elements in the aircraft industry. The load-carrying capacity of this type of structure cannot relieve the reduction in strength in the event of local buckling. In this paper, a method of fabrication of rib-stiffened panels made of EN AW-2024-T3 Alclad and EN AW-7075-T6 Alclad has been proposed using single point incremental forming. Panels made of sheets of different thickness and with different values of forming parameters were tested under the axial compression test. A digital image correlation (DIC)-based system was used to find the distribution of strain in the panels. The results of the axial compression tests revealed that the panels had two distinct buckling modes: (i) The panels buckled halfway up the panel height towards the rib, without any appreciable loss of rib stability, and (ii) the rib first lost stability at half its height with associated breakage, and then the panel was deflected in the opposite direction to the position of the rib. Different buckling modes can be associated with the character of transverse and longitudinal springback of panels resulting from local interaction of the rotating tool on the surface of the formed ribs.

This paper presents a new product, a glass laminate aluminium-reinforced epoxy (GLARE)-based thin-walled structure with a stiffener in the form of a longitudinal rib. The stiffening rib in an outer metallic layer of a GLARE-based panel was fabricated by the incremental sheet forming technique and Alclad 2024-T3 aluminium alloy sheets were used as adherends. The strength properties of the adhesive joint between the layers of the fibre metal laminates (FMLs) were determined in a uniaxial tensile test, peel drum test, tensile/shear test and short-beam three-point-bending test. Two variants of FMLs were considered, with an adhesive film and without an adhesive film between the adherends and the epoxy/glass prepreg. The FMLs were tested at three different temperatures that corresponded to those found under real aircraft operating conditions, i.e., −60 °C, room temperature and +80 °C. It was found that the temperatures do not affect the tensile strength and shear strength of the FMLs tested. However, there was a noticeable increase in the stiffness of samples stretched at reduced temperature. An additional adhesive film layer between the adherends and the glass/epoxy prepreg significantly improves the static peeling strength of the joint both at reduced and at elevated temperatures. A clear increase in the critical force at which buckling occurs has been clearly demonstrated in the uniaxial compression test of GLARE-based rib-stiffened panels. In the case of GLARE-based rib-stiffened panels, the critical force averaged 15,370 N, while for the non-embossed variant, it was 11,430 N, which translates into a 34.5% increase in critical force.

The final accuracy of the parts formed using single point incremental forming greatly depends on the mechanical properties of the material to be formed and the residual stress formation. In this paper, an X-ray diffraction technique was used to study the distribution of the residual stress along the generating line of the conical drawpiece. A DC04 steel sheet with a thickness of 0.8 mm was used as a test material. The basic mechanical properties of the DC04 sheet metal were determined in the uniaxial tensile test according to the EN ISO 6892-1:2016. It was found that the maximum amount of the residual stress existed in a point located in the midway between a base and a truncation of the drawpiece. In the outer surface of the drawpiece, the orange peel defect associated with a rough surface appearance after SPIF a drawpiece, was observed. In contrast, the inner surface of component was characterized by linear grooves associated with the interaction of tool tip with the sheet surface. This defect is mainly influenced by vertical step size.

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The dimensional accuracy and mechanical properties of metal components formed by the Single Point Incremental Forming (SPIF) process are greatly affected by the prevailing state of residual stress. An X-ray diffraction method has been applied to achieve an understanding of the residual stress formation caused by the SPIF process of deep drawing a quality steel sheet drawpiece. The test object for an analysis of residual stress distribution was a conical truncated drawpiece with a slope angle of 71 • and base diameter of the cone of 65 mm. The forming process has been carried out on a 3-axis HAAS TM1P milling machine. Uniaxial tensile tests have been carried out in the universal tensile testing machine to characterize the material tested. It was found that the inner surface of the drawpiece revealed small linear grooves as a result of the interaction of the tool tip with the workpiece. By contrast, the outer surface was free of grooves which are a source of premature cracking. The stress profile exhibits a nonlinear distribution due to different strengthening of the material along the generating line of the truncated conical drawpiece. The SPIF parts experienced a maximum residual stress value of about 84.5 MPa.

Experimental tests to form truncated cones were carried out on a 3-axis milling machine. 0.8-mm thick low-alloy DC04 steel sheets were used as test material. The profile tool-path trajectory was generated using the EDGECAM software. The slope angle and diameter of the base of the conical shaped drawpieces were 70°-72° and 65 mm, respectively. The drawpiece heights were up to 75 mm. The full synthetic lubricant 75W85 was used to reduce the frictional resistance. The effect of selected incremental forming parameters on the formability of the DC04 sheet and the susceptibility to crack formation have been analysed and discussed. It was found that the surface roughness of the workpiece is strongly influenced by step depth. By controlling the feed rate, it is possible to prevent failure of the material.

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