异形内套一次冲压成形模具结构的改进

异形内套加工时,原使用的工艺是车削加工或冷镦加工或一般冲压加工3种。这几种工艺加工成本过高,从而影响市场竞争。对一般的冲压加工增加制件塑性变形保护机构后,解决了脱模难这一大难题。该模具结构设计方法,旨在使设计人员理清模具设计思路与步骤,提高模具设计的水平和工作效率。希望对模具设计人员有一定的帮助和指导作用。     

Oxygen influenced intergranular crack propagation: analysing microstructure and chemistry in the crack tip region

Abstract

Ni-base superalloys have for decades been studied with regard to environmentally influenced intergranular crack propagation. For high temperature fatigue frequencies <0.1 Hz, it has been shown that an oxygen-rich environment promotes time-dependent crack growth while at >0.1 Hz and/or in inert environments (e.g. vacuum) crack growth is cycle dependent. Oxygen interaction at, or ahead of, the crack tip has been pointed out as the reason for the degraded mechanical properties. While many aspects of this type of crack growth have been previously investigated there is still no consensus about the detailed mechanisms, mainly due to the lack of in-detail investigations of the crack-tip region.

Here, crack tip regions in the Ni-base superalloy Alloy 718 were studied. Specimens were subjected to 90 s hold-times at 550°C and 650°C. Crack growth was arrested before final fracture, allowing cross-sectional analyses of the crack-tip region using scanning electron microscopy (SEM). Detailed studies of the crack-tip region were performed using transmission electron microscopy (TEM) and atom probe tomography (APT). For both APT and TEM samples, site-specific focussed ion beam (FIB) sample preparation was performed in a combined FIB-SEM system. The methodology of accessing and analysing the crack tip region is shown. Initial results on oxidation, oxygen penetration and plastic deformation are shown and discussed.     

两平行近距顶管施工开挖数值模拟分析

顶管工艺由于其自身的特点,在我国有着广泛的应用领域,其对周边环境的影响一直是工程界关注的重点之一。本文以某近距平行顶管通道工程为背景,采用三维有限元建模分析了两条顶管先后顶进施工对地层变形的影响规律,所得结论可为实际施工参数及施工工艺的调整提供理论指导。     

Acoustic emission study of deformation behaviour of sensitised 304 stainless steel

Abstract

A systematic study has been undertaken to correlate the changes in acoustic emissions during tensile deformation of sensitised AISI type 304 stainless steel. Samples of a typical 304 stainless steel were sensitised at 700°C for 4, 14 and 24 h after being austenised at 1050°C for 30 min. AE signals were recorded during tensile test by using two sensors with 125 kHz resonant frequency. The results showed significant change in generation of AE during tensile deformation of sensitised AISI 304 stainless steel in compare to solution annealed material. This type of behaviour could be attributed to the microstructural changes in the sensitised specimens especially formation of continuous Cr₂₃C₆ carbides on grain boundaries which lead to increase in shearing by dislocations.     

Mechanical properties and microstructural evolution of Al 6061 alloy processed by multidirectional forging at liquid nitrogen temperature

Al–Mg–Si alloy was subjected to multidirectional forging (MDF) at liquid nitrogen temperature (LNT), to cumulative strains of 1.8, 3.6 and 5.4. The deformed microstructures were examined by optical microscopy under polarized light and transmission electron microscopy (TEM). The deformed samples showed the formation of dislocation cells structure with high dislocation density at lower strains. Composite structure consisting of lamellar morphology at deformation bands and equiaxed grain morphology was observed. Significant differences in microstructure of the deformed samples were observed with increasing strain at LNT. At cumulative strain of 5.4, the microstructure shows nearly equiaxed subgrain structure with an average size of 250 nm with high angle grain boundaries. The mechanical properties were studied through Vickers hardness testing machine and tensile tester. The hardness value of MDFed alloy at LNT has increased from 50 Hv to 115 Hv for cumulative strain of 5.4. Tensile strength has increased from 180 MPa to 388 MPa with 4.5% percentage of elongation to failure. The improvement in hardness and tensile strength of forged alloy is attributed to the formation of equiaxed sub-grain structures and the presence of high dislocation density.     

Investigations for indications of deliberate blasting on the front bulkhead of the ro-ro ferry MV ESTONIA

The roll-on-roll-off passenger ferry MV ESTONIA sank during the night of 28 September 1994 in the Baltic Sea. In October 2000, divers recovered two palm-sized test pieces from the front bulkhead of the wreck. The investigators analysed these specimens to determine whether there were any indications of deliberate blasting. Since the wreck had been submerged for almost six years, it was clear that chemical traces would not be present on the surfaces of the test pieces. Therefore, the investigators performed comparative tests on shipbuilding steel to find a microstructural criterion that exclusively characterises a blast. The shipyard Jos. L. Meyer, Germany, had built the ESTONIA and supplied shipbuilding steel plates similar to that used for building the vessel in 1979/1980. The comparative tests comprised mechanical tests, shot peening tests and blasting tests using different explosives. Testing demonstrated that blasting always formed twinned ferrite grains in the microstructure over the whole cross-section of each of the 8 mm thick comparative plates. Although one of the original test pieces of the ESTONIA showed deformation twins, this was only confined up to 0.4 mm underneath the surfaces and not spread over the whole cross-section. Comparative shot peening tests produced the very same pattern of subsurface deformation twins. Therefore, the twins detected in the microsection of the test pieces of the ESTONIA wreck traced back to the shot peening process performed by the shipyard in 1979/1980 and not to a deliberate blast.     

测斜技术在某基坑开挖过程中的应用

本文首先简述了测斜技术的重要性,并陈述了测斜技术的基本原理;接下来,本文以某基坑开挖过程中对测斜技术的具体运用为依托,详细描述了测斜的点位布置及测量结果,通过对结果的分析进一步得出规律,让施工单位掌握基坑的倾斜程度,也同时为其他同类工程提供指导意见。     

Microstructure characteristic for high temperature deformation of powder metallurgy Ti–47Al–2Cr–0.2Mo alloy

Hot compression tests of a powder metallurgy (P/M) Ti–47Al–2Cr–0.2Mo (at. pct) alloy were carried out on a Gleeble-3500 simulator at the temperatures ranging from 1000 °C to 1150 °C with low strain rates ranging from 1 × 10⁻³ s⁻¹ to 1 s⁻¹. Electron back scattered diffraction (EBSD), scanning electron microscope (SEM) and transmission electron microscope (TEM) were employed to investigate the microstructure characteristic and nucleation mechanisms of dynamic recrystallization. The stress–strain curves show the typical characteristic of working hardening and flow softening. The working hardening is attributed to the dislocation movement. The flow softening is attributed to the dynamic recrystallization (DRX). The number of β phase decreases with increasing of deformation temperature and decreasing of strain rate. The ratio of dynamic recrystallization grain increases with the increasing of temperature and decreasing of strain rate. High temperature deformation mechanism of powder metallurgy Ti–47Al–2Cr–0.2Mo alloy mainly refers to twinning, dislocations motion, bending and reorientation of lamellae.     

浅谈深基坑变形控制研究进展

深基坑工程施工的环境条件非常复杂，随着城市建设的加快，传统的深基坑设计不能满足现代的需要。因此，深基坑正由传统重视对强度的控制转为对变形的控制。本文对深基坑变形的控制研究方面进行了分析。