Materials Science Forum Vols. 704-705

Abstract: Aiming at traditional peak aging of 7A04 Aluminum Alloy with high strength but low corrosion resistance, innovative gradually cooling three-step aging heat treatment process were designed, conventional mechanical properties and corrosion resistance of 7A04 aluminum after three kinds of aging treatments was studied through microstructure. The results show that compared with the traditional peak aging process, two gradually cooling three-step aging process is to improve materials tensile strength, plastic and impact toughness in different degrees. Meanwhile corrosion resistance of materials has improved significantly. And extension of low temperature aging time is conducive to the further improvement of strength.

Abstract: Probability of traffic accidents grows when the driver is under situation of tendency to accidents. This paper introduces a new vehicle model under internet based shared control technology, modeling towards the internet based shared control of steering system. Influence of the network transmission delay on the steering performance is simulated, and a closed loop control circuit model is created. A systematic controller algorithm and input gain range from the viewpoint of the sufficient condition for delay dependent robust stability and steady state error are put forward. Finally, a dynamic self-switching delay-dependent robust controller is designed and showed at the end of the paper. Simulation and experimental results show the validity of the proposed method.

Abstract: To investigate the drilling process of titanium alloys, the three dimensional finite element model of two flutes twist drill was established. The drilling process of Ti6Al4V was simulated based on the presented model. In addition, the drilling experiments were carried out under the different spindle speed and feeding speed. The drilling forces of the experiment and simulation were achieved. The result of simulation agrees with the result of experiment well. It proves that the finite element model is correct. Simultaneously, the results show that more high drilling speed can improve the quality of hole in drilling process of Ti6Al4V obviously.

Abstract: In this paper, the reliability of welded pressure pipe with circumferential surface crack was calculated by using three dimensional stochastic finite element method for A131 pipe steel. This method has overcome the shortcomings of conservative results in safety assessment with deterministic fracture mechanics method. The calculation of reliability was based on three dimensional elastic-plastic stochastic finite element programs which were developed by ourselves. The calculation results indicate that the external load has great effect on the reliability of the welded pipe. When the mean value of bending moment changes from 10000N.m to 20000N.m, the failure probability of the welded pipe will change from 3.3750×10^-9 to 1.6794×10^-1. When the mean value of moment is changed from10000N.m to 20000N.m, the failure probability of the welded pipe increases dramatically for the same circumferential crack opening angle, inner pressure and the crack depth. The method has put forward a new way for safety assessment of welded pipe with circumferential surface crack.

Abstract: In the study three dimensions finite element mathematical model of MIG welding with longitudinal magnetic field was established. By ANSYS FEA software the temperature and other physical characteristics of the arc were obtained including the distributions of current density and arc pressure on the anode surface. The simulated results show that when the additional longitudinal magnetic field was introduced into welding process, the temperature of arc decreased remarkably and peak value of temperature changed from 16 950K to 13 700K at a welding current of 120A. Under the action of longitudinal magnetic field, on the one hand, heat flux density and current density at the anode surface decrease in the arc core and rise at the edge of arc, on the other hand, arc pressure decrease and arc potential increase. Keywords: Numerical simulation; MIG welding arc; magnetic field

Abstract: With considering the Marangoni convection in the molten pool on plasma direct metal forming process, a finite element model posed to describe and reflect the flow in the molten pool. Results of temperature distribution modeling prepared by plasma direct metal forming process of metal powders in an Ar environment were numerically obtained and compared with experimental data. Powders of Fe314 and base plates of R235 steel were taken as sample materials. In the experiment a multi-stream nozzle capable of delivering metal powder coaxially with the plasma arc was used. The model revealed that the velosity of the front part of the pool is a little slower than aft part. Marangoni convection reinforced the convection and enhanced the heat transfer. Profile of the model is the same as the experimental data. This allows us to conclude that the model can be applied for preselecting the process parameters. Keywords: plasma, rapid forming, temperature field, Marangoni convection.

Abstract: A two-dimensional mathematical model has been developed to simulate the impinging and solidification process of a single droplet onto substrate in uniform droplet spray rapid prototyping. Droplet free surface is tracked by volume-of-fluid (VOF) algorithm. The effect of surface tension on the droplet is taken into consideration by means of considering surface tension to be a component of the body force. The governing equations are solved using a finite volume formulation. The calculation results predicted the final shape of a molten droplet impacting onto a solid substrate, and revealed that the solidification process began at the leading edge with the spread process of droplet. The simulation results provide insight and information not easy available from experimental. Keywords: numerical simulation, droplet, rapid prototyping

Abstract: The morphology and growing behavior of Cu₆Sn₅ intermetallic compound (IMC) of low Ag content Sn-2.5Ag-0.7Cu-0.1RE/Cu solder joint interface are investigated by adopting the X-ray diffraction, JSM-5610LV scanning electronic microscope and energy spectrum analysis. The results show that the cross-section morphology Cu₆Sn₅ of the solder joint interface is scallop-like and its section morphology is circle-like grain. With the aging time increasing, the cross-section Cu₆Sn₅ morphology of the solder joint interface can be changed from the scallop-like to the shape-layer, and the growing kinetics is coincidence with the law of parabola and its growing behavior is controlled by diffusion. With adding a small amount of rare earth elements in the Sn-2.5Ag-0.7Cu solder alloy, the growing rate of the Cu₆Sn₅ can be reduced.

Abstract: The coarsening behavior of Ti (C_xN_1-x) particles in CGHAZ for Ti-microalloyed steel has been studied after different welding thermal cycles corresponding to heat inputs 30, 60, 100kJ/cm using Gleeble-3800 thermo/mechanical simulator and transmission electron microscopy. Cuboid TiN and a lot of ellipsoidal or elongated particles VC, M₃C and M₂₃C₆ are observed in the base metal. After welding thermal cycle with peak temperature of 1350°C, irregular particles existed in base metal have also been dissolved even at the lowest heat input of 30kJ/cm. TiN Particles coarsening are observed in CGHAZ with the increase of heat inputs that is just related to dissolution and reprecipitation of particles and irrelevant to isothermal growth of particles. Keywords: microalloyed steel; coarse grain heat-affected zone; TiN particles

Abstract: The Fe-Cr-Mn-N hardfacing alloy was deposited on a low carbon steel substrate by the weld hardfacing technique. Ti and Nb as the most effective nitrogen-fixing elements were added in the hardfacing alloy. Carbonitride precipitates were systematically studied by canning electron microscopy and electron probe microanalyser. The thermodynamics and the effect on the matrix of the formation of carbonitride were also discussed. It was found that carbonitride precipitates were complex carbonitride of Ti and Nb distributing on grain boundary and matrix of the hardfacing alloy. In as-welded condition, primary carbonitride particles were readily precipitated from the hardfacing alloy with large size and morphology as they were formed already during solidification. In heat-treated condition, a large number of secondary carbonitrides can precipitate-out with very fine size and make a great secondary hardening effect on the matrix. As a result, precipitation of carbonitride in the hardfacing alloy can prevent the formation of chromium-rich phase on grain boundaries and increase the wear resistance of the hardfacing alloy. Keywords: carbonitride precipitate, hardfacing alloy, deposit, thermodynamics