Abstract
A laser-assisted high-speed shearing (LAHSS) method has been proposed for metal bars, which prefabricates equally spaced fracture-start kerfs by Nd:Yag laser to make stress concentration, and applies a high-speed load to complete fracture separation. Comparative tests were conducted for Q235, 40Cr, and 304 steel bars, and the effects of fracture-start kerfs and axial clearance were investigated on the fracture section. Moreover, the fracture behavior was demonstrated by numerical simulation, and the micro-fracture mechanism was revealed by fractographic analysis. The numerical simulation results show that the material damage concentrates along with the kerf tips with peak equivalent plastic strain, and the corresponding stress triaxiality drops to almost zero at the kerf tip, which reveals that the material is subjected to pure shearing at kerf tip; the Max. loading force is reduced by 15.2%–29.6%, and the impact energy is decreased by 29.8%–46.9% for the three types of bar material. The experimental results showed that the fracture-start kerfs effectively inhibited the plastic deformation stage, and higher precision blanks were obtained in the LAHSS test: roundness error improved from 2.7%–10.9% to 1.1%–2.6%, Max. bending deflection decreased from 1.3–3.4 mm to 0.4–1.0 mm, and flatness error dropped from 0.9–3.3 mm to 0.3–0.7 mm. The fractographic analysis reveals that the crack initiation is related to alternative V-shape micro-notches at the laser-affected zone; the predominant fracture mechanism involves mode II microvoid coalescence at the main fracture plane; smaller and less elongated dimples were formed in 40Cr steels due to higher number density of grains and pinning effect of second-phase particles compared to Q235 and 304 steel bars.
Highlights
High-quality cropping of blanks from long metal bars is the first process of near-net-shape forming for most mechanical parts
The bar material is first compressed with a certain degree of bending deflection, and the crack initiation is caused by a progressive accumulation of plastic deformation around the upper and lower blade edges, which requires a great enough force especially for high-strength metals, and deviates the maximum shearing stress plane from the vertical cropping direction
For kerf depth h of 1 mm (Figure 6b), the material damage concentrates along with the kerf tips, and the shearing displacement S is reduced to 0.8 mm for damage initiation
Summary
High-quality cropping of blanks from long metal bars is the first process of near-net-shape forming for most mechanical parts. The industrial cropping method moves sharp blades downward to cut off blanks at a loading speed below 0.6 m/s. In this process, the bar material is first compressed with a certain degree of bending deflection, and the crack initiation is caused by a progressive accumulation of plastic deformation around the upper and lower blade edges, which requires a great enough force especially for high-strength metals, and deviates the maximum shearing stress plane from the vertical cropping direction. The pre-shearing cropping method includes two sub-stages: the bar is pre-sheared at a low speed less than 50 mm/s, and cropped at a higher speed about 600 mm/s [5].
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