干摩擦力学系统的建模及振动响应分析

以一具有干摩擦副的单自由度系统为研究对象,实测了其在外界谐波激励下出现的粘滑运动。并建立了基于双折线迟滞单元的干摩擦系统力学模型,模型中的参数均有明确的物理意义。针对这一力学系统,提出了相应的振动响应的求解方法,并分析了系统参数的变化对振动响应的影响。文中给出的参数识别方法和振动响应的求解方法可给工程中含有干摩擦交接副的系统设计提供借鉴。

Microstructure, mechanical properties and wear behaviour of Zn-Al-Cu-TiB_2 in situ composites

Zn-Al-Cu-TiB2（ZA27-TiB2） in situ composites were fabricated via reactions between molten aluminum and mixed halide salts（K2TiF6 and KBF4） at temperature of 875 °C. The microstructure, mechanical properties and wear behavior of the composites were investigated. Microstructure analysis shows that fine and clean TiB2 particles distribute uniformly through the matrix. The mechanical properties of the composites increase with the increase in TiB2 content. As TiB2 content increases to 5%（mass fraction）, an improvement of HB 18 in hardness and 49 MPa in ultimate tensile strength（UTS） is achieved. The overall results reveal that the composites possess low friction coefficients and the wear rate is reduced from 5.9×10-3 to 1.3×10-3 mm3/m after incorporating 5% TiB2. Friction coefficient and worn surface analysis indicate that there is a change in the wear mechanism in the initial stage of wear test after introducing in situ TiB2 particles into the matrix.

Friction stir keyholeless spot welding of AZ31 Mg alloy-mild steel

Friction stir keyholeless spot welding（FSKSW） using a retractable pin for 1.0 mm thick galvanized mild steel and 3 mm thick AZ31 B magnesium alloy in a lap configuration was investigated.The process variables were optimized in terms of the joint strength.The effects of the stacking sequence on joint formation and the joining mechanism of FSKSW AZ31B-to-mild steel joints were also analyzed.It shows that the process window and joint strength are strongly influenced by the stacking sequence of the workpieces.While the process window is narrow and unstable for FSKSW of a magnesium-to-steel stack-up,a desirable process was established for the steel-to-magnesium stacking sequence,a desirable process and higher strength joint can be got when the steel-to-magnesium stacking sequence.XRD phase and EPMA analyses of the FSKSW joint showed that the intermetallic compounds are formed at the steel-to-magnesium interface,and the element diffusion between the mild steel and AZ31 B magnesium alloy revealed that the joining methods for FSKSW joints is the main mechanical joining along with certain metallurgical bonding.

Microstructure and mechanical properties of dissimilar pure copper/1350 aluminum alloy butt joints by friction stir welding

The dissimilar friction stir welding of pure copper/1350 aluminum alloy sheet with a thickness of 3 mm was investigated. Most of the rotating pin was inserted into the aluminum alloy side through a pin-off technique, and sound welds were obtained at a rotation speed of 1000 r/min and a welding speed of 80 mm/min. Complicated microstructure was formed in the nugget, in which vortex-like pattern and lamella structure could be found. No intermetallic compounds were found in the nugget. The hardness distribution indicates that the hardness at the copper side of the nugget is higher than that at the aluminum alloy side, and the hardness at the bottom of the nugget is generally higher than that in other regions. The ultimate tensile strength and elongation of the dissimilar welds are 152 MPa and 6.3%, respectively. The fracture surface observation shows that the dissimilar joints fail with a ductile-brittle mixed fracture mode durin~ tensile test.

Structure and mechanical properties of thick Cr/Cr_2N/CrN multilayer coating deposited by multi-arc ion plating

A Cr/Cr2N/CrN multilayer coating with a thickness of 24.4 μm was deposited by multi-arc ion plating. The coating was systematically characterized by field emission scanning electron microscopy(FESEM), X-ray photoelectron spectrometry(XPS), energy dispersive spectroscopy(EDS), X-ray diffraction(XRD) and transmission electron microscopy(TEM). Hardness and adhesion were tested by nanoindentation and scratch tester, respectively. The friction properties were investigated by a reciprocating UMT-3MT ball-on-disk tribometer in air and seawater. The results showed that the multilayer coating consisted of three different layers, with Cr,Cr2N and CrN phases, respectively. Compared with CrN single layer coating, the adhesion of the multilayer coating was improved significantly, the hardness of the multilayer coating was(21±2) GPa. The corrosion resistance of the multilayer coating was also improved in artificial seawater. The friction coefficient of multilayer coating was lower than that of CrN single layer coating both in air and seawater.

Microstructure and mechanical properties of dissimilar Al-Cu joints by friction stir welding

Dissimilar friction stir welding between 1060 aluminum alloy and annealed pure copper sheet with a thickness of 3 mm was investigated. Sound weld was obtained at a rotational speed of 1050 r/min and a welding speed of 30 mm/min. Intercalation structure formed at the crown and Cu/weld nugget （WN） area promotes interracial diffusion and metallurgical bonding of aluminum and copper. However, corrosion morphology reveals the weak bonding mechanism of internal interface, which causes the joint failing across the interface with a brittle-ductile mixed fracture mode. The tensile strength of the joint is 148 MPa, which is higher than that of the aluminum matrix. Crystal defects and grain refinement by severely plastic deformation during friction stir welding facilitate short circuit diffusion and thus accelerate the formation of A14Cu9 and A12Cu intermetallic compounds （IMCs）. XRD results show that A14Cu9 is mainly in Cu/WN transition zone. The high dislocation density and formation of dislocation loops are the major reasons of hardness increase in the WN.

征答

为加强编者、读者之间的交流，培养广大学生读者学习物理的兴趣，从本期开始，我刊将开展“问题征答”活动．凡在规定时间内(以邮戳为准)将正确答案寄至本刊的前50名学生作者，我刊将赠《物理实验》(中学部分)的当期杂志1册．

三锥面同步器应用原理与结构简介

本文从理论力学的角度对变速器锁环式同步器工作过程中各参数间的相互关系进行了基本分析，得出了只在变速器低档位应用三锥面同步器结构的根本原因，在此基础上对我厂三锥面同步器产品的结构作简要介绍。

Thermostability,mechanical and tribological behaviors of polyimide matrix composites interpenetrated with foamed copper

Polyimide matrix composites interpenetrated with foamed copper were prepared via pressure impregnation and vacuum immersion to focus on their thermostability, mechanical and tribological behaviors as sliding electrical contact materials. The results show that the interpenetrating phase composites（IPC） are very heat-resistant and exhibit higher hardness as well as bending strength, when compared with homologous polyimide matrix composites without foamed copper. Sliding electrical contact property of the materials is also remarkably improved, from the point of contact voltage drops. Moreover, it is believed that fatigue wear is the main mechanism involved, along with slight abrasive wear and oxidation wear. The better abrasive resistance of the IPC under different testing conditions was detected, which was mainly attributed to the successful hybrid of foamed copper and polyimide.

Influence of rotational speed on mechanical properties of friction stir lap welded 6061-T6 Al alloy

The effect of rotational speed on macro and microstructures, hardness, lap shear performance and failure mode of friction stir lap welding on AA6061-T6 Al alloy with 5 mm in thickness was studied by field-emission scanning electron microscopy （FE-SEM）. The results represent much closer hardness distribution in the upper and lower plates at the lowest rotational speed. It indicates the Fe-compounds in the fracture surface of the nugget zone by EDX.

Dynamic characteristics of gear system under different micro-topographies with the same roughness on tooth surface

The topography of gear meshing interfaces is one of the key factors affecting the dynamic characteristics of the gear transmission system.In order to obtain the contact characteristics of meshing gear pair with different surface micro-topographies,an interface feature model and a tribo-dynamics coupling model for the gear system are proposed in this paper.The effects of the gear tooth surface micro-topography on the oil film distribution,contact damping and friction are considered.The time-varying meshing stiffness and the static transmission error are included in the abovementioned models.An exemplary gear pair is analyzed using the proposed models to investigate the influence of the surface micro-topography on the dynamic characteristics of gear system under different micro-topographies and input torque conditions.Simulation results show that the effects of gear tooth micro-topography on the gear dynamic responses(including the friction and the vicious damping at the gear meshing interface and the vibration in the direction of offline of action)are highly dependent on the regularity of tooth surface.The vibration and noise can be significantly controlled by manufacturing a regular gear tooth profiles instead of random profiles.

Microstructure and mechanical properties of friction stir welded powder metallurgy AA2024 alloy

The extruded plate of powder metallurgy AA2024 aluminum alloy was successfully solid-state joined by friction stir welding(FSW) to demonstrate potential applications in the aerospace and automotive industries. For determining the optimal processing parameters of FSW, the microstructure, mechanical properties, and fracture behavior of FSW joints were evaluated. When the processing parameters were optimized with 2000 r/min rotation speed and100 mm/min traverse speed, high quality welds were achieved. The ultimate tensile strength yield strength and elongation of the joint can reach 415 MPa(85% of the base metal strength), 282 MPa, and 9.5%, respectively. The hardness of the joint gradually decreased from the alloy matrix to the heat-affected zone. The lowest strength and hardness appeared near the heat-affected zone because of the over-aging caused by heat flow from repeated stirring during FSW. The average grain size of the stir zone(2.15 μm) was smaller than that of the base metal(4.43 μm) and the heat-affected zone(5.03 μm), whose grains had <110> preferred orientation.

Review on underwater friction stir welding: A variant of friction stir welding with great potential of improving joint properties

Friction stir welding（FSW） is a solid-state welding process which is capable of joining materials which are relatively difficult to be welded by fusion welding process. Further, this process is highly energy-efficient and environmental-friendly as compared to the fusion welding. Despite several advantages of FSW over fusion welding, the thermal cycles involved in FSW cause softening in joints generally in heat-treatable aluminum alloys（AAs） due to the dissolution or coarsening of the strengthening precipitates leading to decrease in mechanical properties. Underwater friction stir welding（UFSW） can be a process of choice to overcome these limitations. This process is suitable for alloys that are sensitive to heating during the welding and is widely used for heat-treatable AAs. The purpose of this article is to provide comprehensive literature review on current status and development of UFSW and its importance in comparison to FSW with an aim to discuss and summarize different aspects of UFSW. Specific attention is given to basic principle including material flow, temperature generation, process parameters, microstructure and mechanical properties. From the review, it is concluded that UFSW is an improved method compared with FSW for improving joint strength. Academicians, researchers and practitioners would be benefitted from this article as it compiles significantly important knowledge pertaining to UFSW.

Mechanical properties of underwater friction stir welded 2219 aluminum alloy

Underwater friction stir welding of 2219 aluminum alloy was carried out in order to further improve the joint performances by varying welding temperature history.The results indicated that the tensile strength of the joint can be improved from 324 MPa by external water cooling action in normal to 341 MPa.However,the plasticity of the joint is deteriorated.The underwater joint tends to fracture at the interface between the weld nugget zone and the thermal mechanically affected zone on the advancing side during tensile test,which is significantly different from the normal joint.

Effect of welding speed on microstructure and mechanical properties of Al−Mg−Mn−Zr−Ti alloy sheet during friction stir welding

Effects of welding speed on the microstructure evolution in the stir zone(SZ)and mechanical properties of the friction stir welding(FSW)joints were studied by OM,XRD,SEM,TEM,EBSD and tensile testing.Compared with the base metal(BM),an obviously fine dynamic recrystallization(DRX)microstructure occurs in the SZ and the DRX grain size decreases from 5.6 to 4.4μm with the increasing of welding speed.Fine DRX microstructure is mainly achieved by continuous dynamic recrystallization(CDRX)mechanism,strain induced boundary migration(SIBM)mechanism and particle stimulated nucleation(PSN)mechanism.Meanwhile,the geometric coalescence and the Burke−Turnbull mechanism are the main DRX grain growth mechanisms.Among all the welding speeds,the joint welded at rotation speed of 1500 r/min and welding speed of 75 mm/min has the greatest tensile properties,i.e.ultimate tensile strength(UTS)of(509±2)MPa,yield strength(YS)of(282±4)MPa,elongation(El)of(23±1)%,and the joint efficiency of 73%.

Thermal elastohydrodynamic lubrication of modified gear system considering vibration

The thermal elastohydrodynamic lubrication characteristics of a modified gear system under a dynamic load were investigated,including the influence of the modification coefficient and vibrations.Based on the dynamic theory of gear systems,a six-degree-of-freedom tribo-dynamics model was established.Thermal elastohydrodynamic lubrication characteristics of a modified gear system under vibrations and a static load were analyzed.The results showed that the positive transmission gear system exhibited the better lubrication effect compared with other transmission types.A thick lubricating oil film could be formed,and the friction coefficient between the teeth and the oil film flash temperature were the smallest.As the modification coefficient increased,the lubrication condition was continuously improved,and the scuffing load capacity was enhanced.The increment of the modification coefficient increased the meshing stiffness of the gear system but reduced the stiffness of the oil film.

Friction and wear behaviors of TiCN coating based on electrical discharge coating

Titanium carbonitride (TiCN) coating was prepared on 45# carbon steel by electrical discharge coating (EDC), and the compositions, morphology and microstructure of the coating were studied. In addition, its friction and wear behaviors relative to the physical vapor deposition (PVD) TiN coating were investigated. The results show that the TiCN coating features a thickness of 15μm with a primary phase of TiC 0.3 N 0.7 . The wear rates of the two coatings have no clear distinction at low applied loads. However, severe abrasive wear appears in the PVD TiN coating when the applied load exceeds 30 N, while the TiCN coating features better wear resistance. The abrasive wear with coating peelings is found to be the predominant wear mechanism at high applied loads.

Microstructure and mechanical properties of friction stir welded AA7075-T651 aluminum alloy thick plates

Friction stir butt welding of AA7075-T651 plates with thicknesses of 10 and 16 mm was investigated. Defect-free, full- penetration welds were obtained after careful process parameter selection. While the nuggets in both welds exhibited very fine reerystallized grains, and finer grains were observed in welds made on 10 mm thick plates. Microhardness surveys revealed that significant loss in hardness occurs in the heat-affected zone. The reduction in hardness due to the welding process is higher in the case of welds made on 16 mm thick plates. Welds made on 10 mm thick plates exhibited superior tensile properties compared with those made on 16 mm thick plates. Fracture during tensile test occurred in the heat-affected zone in both cases. TEM images of specimens revealed that the heat-affected zone consisted of widened precipitate-free zones along grain boundaries and partial dissolution of precipitates in the grain interiors. It is concluded that defect-free single pass welds can be made on AA7075-T651 thick plates using friction stir welding and the welds made on 10 mm thick plates exhibit high joint efficiency.