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Ball-end milling cutters are one of the most widely used cutters in the automotive, aerospace, die and machine parts industries. In addition, milling chatter will reduce the surface quality and production efficiency, resulting in noise. It is particularly important to model the cutting force and analyze the flutter stability of ball-end milling cutters. In this paper, a simplified milling force model of ball-end milling cutter with three degrees of freedom was established based on Merchant bevel cutting theory. The model simplified the milling force coefficient. The expressions of instantaneous milling area considering the vibration displacements in X, Y and Z directions were derived. The nonlinear dynamic cutting force model of ball-end milling cutter with three degrees of freedom was established. The nonlinear chatter vibration mechanical model of ball-end milling cutter with three degrees of freedom was established by introducing the time delay term, the stability analysis is carried out by time domain simulation. The proposed models were experimentally verified.

Selective laser melting (SLM), as a revolutionary technology for metal manufacturing, attracts tremendous attention because it can produce complex components to benefit the customised production. H...

Hot workability and deformation mechanism of β-solidifying γ-TiAl alloy with a nominal composition of Ti-44Al-5Nb-1Mo-2V-0.2B (in at%) were systematically studied under uniaxial hot compression. The strain rate sensitivity exponent (m) and processing maps at strains of 0.2 and 0.6 were developed. Based on an analysis of the processing maps and the corresponding microstructure, 1200 °C/0.1 s−1 was defined as the optimum deformation condition. At low temperatures and high strain rates, pileup of dislocations at lamellar colonies was found to be responsible for deformation, which provided the driving force for sub-grain boundary formation. As deformation progressed, twinning-induced dynamic recrystallization (DRX) and near complete breakdown of lamellae through the rearrangement of sub-boundaries were the main deformation mechanism. Moreover, nano-lamellar structure and stacking faults (SF) were generated. The hot isostatically pressed (as-HIPed) alloy had relatively good workability, and crack-free sheets were obtained from the as-HIPed alloy with different reductions. A multiple phase, homogeneous as-rolled microstructure without coarse residual lamellar colonies (RL) or shear bands was obtained when the total rolling reduction was 78.8%. The ultimate tensile stress (UTS) and ductility at 800 °C ranged from 523 to 553 MPa and from 21% to 32%, respectively, for different deformation reductions.

Fatigue performance and reliability application have always been the key issues restricting the development of disruptive additive manufacturing technology due to intrinsic defects caused by the manufacturing process. In the present work, tensile tests and fatigue tests of 304 L stainless steel (SS) fabricated by selective laser melting (SLM) with various laser power levels were carried out with in-situ monitoring of acoustic emission (AE) and thermal imager (TI). Microstructural characterizations were performed to measure the surface roughness, porosity, and residual stress of as-built SLMed 304 L SS. A fatigue life prediction model considering surface roughness, porosity, and residual stress was established based on experimental data. In addition, direct evidence is offered to show the strong correspondence between the temperature variable of material surface induced by energy dissipation caused by entropy generation during cyclic deformation and the degradation of material properties caused by fatigue micro-damage. This work indeed provides a novel fatigue life prediction model for SLMed 304 L SS, and advances our current understanding of underlying fatigue micro-damage mechanism of additively manufactured materials.

This study reports the comprehensive effects of building direction and scanning speed on the fatigue performance of 304L austenitic stainless steel (SS) fabricated by selective laser melting (SLM) through a series of detailed microstructural characterizations, including optical microscopy (OM), scanning electron microscopy (SEM), Transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), acoustic emission (AE), and thermal imager (TI). The fatigue tests were performed at a single stress level (90% of yield strength). The correlation analyses between fatigue life and the factors of surface roughness, porosity, and residual stress were performed to elucidate the comprehensive influence of these factors on fatigue lifetime. The combination of fractography analysis, AE signals, and TI signals were used to reveal the underlying microstructural mechanisms during cyclic deformation. Direct evidence is offered to show that the process of fatigue crack initiation promoted by cyclic deformation occupied most of the fatigue history. The synergistic effects of part densification, residual stress, and microstructure variable dominate the fatigue performance of SLM 304L SS. Clear evidence is shown that the higher density of high angle grain boundaries (HAGBs) on the top surface can hinder dislocation flow rate, promoting the dislocation piling-up at HAGBs, thus weakening the fatigue performance. In addition, the stronger texture on the top surface can improve the twinning behavior, strengthening the fatigue crack initiation resistance.

Ball passage vibration arising during recirculation of rolling elements is a limitation of high-precision performance associated with ball screw. A relatively deep understanding of this phenomenon when rolling balls leave load area and enter circulation channel has been achieved by now. However, few mechanical models are established for calculation of rolling ball contact deformation and force inside nut while moving by taking into account transient phase at import and export of circulation channel. In this work, we present a general time-dependent dynamic model for description of ball motion and axial periodic micro fluctuation within ball screw subjected to external load. The numerical simulation is employed to deal with equation of motion including segmented restoring force function. Effects of applied load, preload and screw rotational speed on dynamic response are discussed theoretically. Reduced vibration amplitude may be realized by small ball diameter and phase difference between ball groupings. Proposed model is experimentally illustrated.

The fatigue performance has always been the key problem hindering the development of disruptive additive manufacturing (AM) technology. We recently found the stored energy induced by SLM process and cyclic strain during fatigue testing can trigger grain boundary migration and local recrystallization behavior. In addition, the junctions between the recrystallized grain tribes and the original coarse grains can lead to the plastic strain localization, promoting the fatigue crack initiation during cyclic deformation, seriously affecting the fatigue lifetime of SLMed 304L SS. Therefore, this study carried out a series of microstructural characterizations to investigate the formation mechanisms of local recrystallization and the role of local recrystallization on the fatigue performance of SLMed 304L SS, which can advance the current understanding of fatigue failure mechanisms of additively manufactured materials. The experimental results show that the detrimental effect of local recrystallization during cyclic deformation on fatigue properties of SLMed 304L SS exceeds that of small pores.

This study carried out the fatigue tests of selective laser melting (SLM) 304L austenitic stainless steel (SS) made by different scanning speeds. The purpose is to deeply understand the microstructure, fatigue failure process and fatigue failure mechanism of SLMed 304L SS. For this purpose, the microstructure, surface roughness and porosity were tested by field emission scanning electron microscope (FESEM) and electron backscatter diffraction (EBSD), while the residual stress was obtained by μ-X360 residual stress analyzer. In addition, the fatigue failure process of SLMed 304L SS was in-situ monitored through acoustic emission (AE) technique and thermal imager. The results show that SLMed 304L SS has excellent fatigue performance. The ‘critical’ value is defined to distinguish the critical influence of surface roughness and porosity. In addition, the temperature rise caused by the stored energy deriving from the active dislocation motions is affected by the loading direction and scanning direction by regulating the dislocation motions during fatigue testing.

The purpose of this paper is to investigate the failure mechanisms of carbon fiber reinforced composites (CFRP) single-lap bolted, bonded and hybrid bolted/bonded joints during compression. The failure process and the failure mechanisms of the compression tests were analyzed using field emission scanning electron microscope (FESEM) and acoustic emission (AE) technique. A novel method is proposed to obtain the relationship between the FESEM analysis and AE signals. The crack data is obtained by identifying, segmenting and quantizing FESEM images based on image processing technology. The final results give effective AE parameters for identifying crack damage in composite joints, and show the superiority of the proposed method. The research in this paper puts forward important suggestions for the application of AE technique in composites and fault diagnosis.

This study investigated the effect of countersunk hole depths (0.65, 0.90, and 1.20 mm) on the fatigue performance of the riveted AA2024-T3 alloy. The fatigue fracture of the samples was observed by scanning electron microscopy. The strain at the countersink hole was analyzed by finite element (FE) method. The stress intensity and stress concentration factor of the countersunk holes were calculated. The rationality of the FE analysis and theoretical calculation was verified by measuring the strain near the countersunk holes. The high cycle fatigue life of the samples with a countersunk depth of 1.20 mm was far less than the other samples. In contrast, the low cycle fatigue life changed insignificantly.

Thin Ti–44Al–5Nb–1Mo–2V-0.2B alloy sheets were processed under different pack-rolling process directly from HIPed ingot in (α+β) phase region. The corresponding microstructure evolution, deformation mechanisms, and high-temperature tensile properties were systematically studied. As the pack-rolling progressed and thickness of plates is reduced, the microstructure was characterized by elongated lamellar colonies with B2+equiaxed γ grains at colony boundaries, that transformed into nano-lamella colonies along with γ platelets within B2 grains. The nano-lamella colonies were divided by a number of α mechanical twins and deformation bands. During the initial stage of rolling, mechanical twins, continuous, and discontinuous dynamic recrystallization of γ were the primary deformation mechanisms in the multi-phase microstructure. When the total rolling reduction was 74%, α2 phase dominated the deformation instead of γ. Two kinds of straight HAGBs with ~60.6° - 65.5° and ~71.0° - 72.7° formed in pairs in α2 colonies indicating the presence of twin boundaries and shear bands, respectively. While both the as-rolled alloys were produced via 4 and 6 rolling passes, exhibited high performance at 800 °C with ultimate tensile stress and ductility of ~600 MPa and 60%–70%, respectively. Nano-lamellae, nanoscale DRX grains, and ω precipitation in B2 grains at colony boundaries contributed positively to the high-temperature tensile properties.