Abstract
A notch-induced high-speed splitting method was developed for high-quality cropping of metal bars using a new type of electric-pneumatic counter hammer. Theoretical equations and FE models were established to reveal the crack initiation and fracture mode. Comparative tests were conducted for notched and unnotched bars of four types of steels, i.e., AISI 1020, 1045, 52100, and 304, and the section quality and microfracture mechanism were further investigated. The results show that damage initiates at the bilateral notch tips with peak equivalent plastic strain, and propagates through the plane induced by the notch tip; the stress triaxiality varies as a quasi-sine curve, revealing that the material is subjected to pure shearing at the notch tip, and under compression at the adjacent region. High precision chamfered billets were obtained with roundness errors of 1.1–2.8%, bending deflections of 0.5–1.5mm, and angles of inclination of 0.7°–3.4°. Additionally, the notch effectively reduced the maximum impact force by 21.6–23.9%, splitting displacement by 7.6–18.6%, and impact energy by 27.8–39.1%. The crack initiation zone displayed quasi-parabolic shallow dimples due to shear stress, and the pinning effect was larger in AISI 52100 and 1045 steel; the final rupture zone was characterized by less elongated and quasi-equiaxial deeper dimples due to the combination of shear and normal stress.
Highlights
The precise cropping of blanks from long metal bars is the primary process of near-net-shape forming technologies, such as die-forging and cold-extrusion, and is widely used in the production of most mechanical parts
Along with path 1 at the neutral notch surface, equivalent plastic strain varies as a single-peak roundness error er of 1.1%, maximum bending deflection b1 of 0.5 mm, and angle of inclination of curve with a maximum value of 0.81 at the notch tip, and drops rapidly to 0 at the adjacent region
Taking the notched AISI 1045 bar as an example (Figure 10a), it is obvious that stress concentrates at the notch tip, with a maximum value of 1290 MPa; damage initiates at the lateral notch tips B and E at a splitting displacement of −0.78 mm, while the rest region is under a low level of equivalent stress and plastic strain
Summary
The precise cropping of blanks from long metal bars is the primary process of near-net-shape forming technologies, such as die-forging and cold-extrusion, and is widely used in the production of most mechanical parts. The bar material first experiences plastic deformation under bending, which deviates the maximum shearing stress plane from the vertical cropping direction. The point of precise cropping is to limit first-stage plastic deformation. The proposed precise cropping technologies can be classified into six basic types: radially constrained cropping, axially pressured cropping, torsion combined cropping, low-load cyclic splitting, high-speed cropping, and warm cropping at blue-brittleness temperature. In the first two methods, metal bars are circumferentially or axially clamped under large force to avoid bending, producing flat and vertical sections, but the roundness error is inferior due to draw-in deformation around the cropped faces [1,2], and the axially pressured cropping method is only feasible for soft metals, such as copper and aluminum [3,4]. Torsion combined cropping [5] significantly reduces bending deflection and Materials 2020, 13, 2461; doi:10.3390/ma13112461 www.mdpi.com/journal/materials
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