Kristián Máthis

Incremental sheet forming (ISF) has been adopted as a low-cost technique that can be utilised to create three-dimensional sheet forms with extremely short lead times. Unlike traditional forming, ISF is a die-less process which uses CNC and CAD files. It enhances the plastic deformation of hard-to-deform alloys such as titanium and magnesium by localised deformation. Magnesium (AZ31B) was tested using Erichsen cupping and the ISF to determine the maximum formability in 1.5 mm sheets, at ambient temperature. The depth obtained using the Erichsen cupping test was 2.88 mm. ISF with a spiral toolpath was performed until fracture with a range of feed rates (F) (50, 1000 and 2500 mm/min), spindle speeds (S) (50, 500 and 3000 rpm), and pitch sizes (P) (0.1, 0.5 and 2.5 mm). The optimum condition was found to be F50S3000P0.5, and the forming depth was 8.31 mm, which is approximately three times more than the conventional test. The experiments showed that an increase in the pitch size above 0.5 mm and feed rate above 50 mm/min tends to decrease the formability, whereas an increase in spindle speed to 3000 rpm increased the formability. The as-received sheet was found to be a strong basal texture material using electron backscatter diffraction (EBSD) and thus could not be strongly activated to undergo (10–12) tension twins. Nevertheless, in the ISF process, plane strain, biaxial stretching and frictional heating at high spindle speeds of 3000 rpm increased non-basal slip systems and extensive recrystallisation. These processes were highly effective in enhancing the formability of the alloy at ambient temperatures. Overall, the results can be used to demonstrate that the ISF process can form a 3-fold increment on magnesium and other alloys that are hard to form without losing their significant strength and properties, and it can be viewed as a potential alternative to traditional methods of sheet forming.

The extruded Mg–3 wt.% Y alloy exhibits serrated flow during compression tests along the extrusion direction at intermediate temperatures (150-250 ºC) and a wide range of strain rate (10-2 to 3x10-5 s-1) due to dynamic strain aging (DSA). The values of the m + β constant and the activation energy were 2 and 100 kJmol-1, respectively. To elucidate the underlying deformation mechanisms, in-situ synchrotron radiation diffraction and acoustic emission (AE) experiments were conducted at 200 °C. The deformation process involves the activation of -dislocation slip on both basal and non-basal planes, as well as extension twinning, all occurring concurrently with DSA. The characteristic serrations arise from the dynamic interaction between diffusing yttrium atoms and mobile dislocations and twin boundaries. Yttrium atoms are shown to effectively pin -dislocations and inhibit twin boundary propagation, thereby influencing the overall mechanical response during deformation. The unpinning of twin boundaries from yttrium atoms induces a new additional cluster in the acoustic emission signal.