Materials Science Forum Vols. 628-629

Abstract: It is very difficult to execute the finishing process of the concavo-convex surface of female-screws. The current study offers a screw-form design electrode and a new area-finishing process using an ultrasonic generator. The ultrasonic energy generated is transmitted into the electrolyte to assist the process of electrochemical finishing. The surface of female-screws is electrochemically finished by different types of completely inserted electrodes and put through both continuous and pulsed direct current as a finishing operation. For inserted screw-form electrodes, a smaller circumference electrode on the cylinder provides more sufficient discharge space, which is advantageous for finishing. Pulsed direct current can promote the effect of electrochemical finishing, but the machining time is longer and the cost is raised. The average ultrasonic effect is much better than the pulsed current while the machining time needs not be prolonged by the off-time. The higher current density and higher current rating with ultrasonic assistance can avoid the difficulty of dreg discharge, thus reducing the finishing time. It is a great contribution that the ultrasonic-assistance electrochemical finishing after screw machining requires a shorter time than manual or machine polishing to make the surface of female screws smooth and bright.

Abstract: Mathematical models of basic parameters, force and thermal parameters of cutting were obtained by applying the thermo-elastic-plastic theory and the heat-balance theory, according to the principle of residual stress generation in cutting process. Based on the 2D finite element model of chip molding with separation surface, the direct way was applied in thermo-mechanical coupling, and residual stress of cutting GH4169 were simulated and analyzed by finite element. The results showed that the residual stresses are changed from tensile to compressive with the development of depth away from the workpiece surface. Also its values are changed according to a certain rule as varying the cutting speed. These provide a foundation for the control of cutting quality of workpiece.

Abstract: According to the results of mechanical property tests, the mechanical properties of heterogeneous composite-Patinopecten yessoensis (scallop) shells are anisotropic and related closely to the microstructure. The results of compression and three-point bending tests indicated that the compressive strength which vertical load on the shell surface was approximately 3.8 times than the parallel case. The bending strength which applied to the external of the shell was about 2.2 times than it loaded from the inner surface. Patinopecten yessoensis (scallop) shell is organic-inorganic composite and has different microstructure in different location in according to its functional requirements. Patinopecten yessoensis (scallop) shell is typical bioactive functional gradient material, and its microstructure is worthy of heterogeneous composite material parts design.

Abstract: Basing on the theoretical study on the stress intensity factor (SIF) of the crack normal to and dwelling on the interface of the cermet cladding part, the finite element analysis (FEA) of the crack SIF is made. The change laws of the SIF with the load action angle, the load Q, the clad thickness ratio h1/h and the elastic modulus ratio E1/E2 are obtained. The research results have theoretical and steering significance on the wide application of the cermet cladding part.

Abstract: Porous cermets composites were prepared from mixed powders (Ni, Al, Ti and C) by SHS (Self-propagating High-temperature Synthesis) method. When Ni-Al intermetallics bonded TiC porous cermets substitute for TiC ceramic as perform in the process of manufacturing Ni-Al/TiC composites by pressureless melt infiltration, the wetting of the ceramic material and Ni-Al intermetallics can be improved. In this study, the microstructure and mechanical properties (density and compressive resistance strength) were reported. It is learnt from the study that the relation between the open porosity and the pore size does not exist, the former depends on the green density, and the latter depends on the particle size of the nickel powder. The porous cermets with porosity ranging from 40% to 50%, pore size from 1μm to 300μm, and the compressive resistance strength about 15 MPa can be prepared by SHS. SEM observations of fracture surface suggest the fracture mode is brittleness fracture, and the microstructures of the porous cermets are very homogenous.

Abstract: Laser surface melting has been performed on high chrome steels by a 5kW CW CO2 laser using different overlapping ratios. The microstructures of laser melted steels were analysed by SEM and the hardness profiles were determined by a Vickers hardness tester. The corrosion characteristics of laser melted steels in 3.5% NaCl solution were studied by electrochemical corrosion equipment. The melted zone of a single laser track exhibits a mixed structure of dendritic, cellular and equiaxed austenite while the overlapped melted zone is only composed of austenitic dendrites. Using a large overlapping ratio (OR) gives a more uniform hardened-depth. The microhardness with an OR of 33.3% is higher compared to 16.7% and 50%. The corrosion resistance of laser melted steel is improved arising from the dissolution of carbides, the increasing of alloying elements in the solid solution and the large amount of austenite. Among all the studied specimens the one, which is laser treated with 33.3% OR presents the better corrosion resistance.

Abstract: The green presetting of powders was performed on the substrate by introducing a novel technique namely squash presetting method, and cladding coatings were prepared by crosscurrent CO2 laser in this work. Based on the concept of laser energy efficiency which being accepted generally by insider, the energy efficiency of squash presetting laser cladding was determined. Meanwhile, effects of processing parameters including specific energy, laser power, scanning speed and spot diameter on energy efficiency were investigated through an orthogonal test. The results show that the energy efficiency increases at first and then decreases with the increase of specific energy, and the energy efficiency is relatively higher when laser specific energy ranged from 90 J/mm2 to100 J/mm2. Among three single-factors, the effects of spot diameter on energy efficiency are most significant, laser power takes second place, and scanning speed comes next. It is considered through analysis that the relationship between energy efficiency and processing parameters is closely related to powders melting and heat-conduction course of squash presetting laser cladding.

Abstract: Plasma arc, a kind of high energy density beam, is proposed as one kind of surface treatments in this paper to improve the adhesive properties of the chromium coatings to the steel substrate. Scratch tests are used to obtain the critical load (Lc) of the coatings. The wear behaviors are evaluated by a reciprocating ball-on-flat wear machine and the wear tracks of the coating were characterized by scanning electronic microscope (SEM). Results show that plasma arc treatment could promote the adhesive and the wear performances of the chromium coatings. Optimal value of Lc and the wear resistance of the treated coatings could be obtained when the average output energy density of the plasma arc (E) grow to 1.05×105J/m2. The comparative study indicates that the promotion of the adhesion could be attributed to the formation of the inter-diffused alloyed layer and the improved hardness distribution. This promotion then contributes to the improvement of the wear performance, which makes up, even exceeds the loss of it caused by the drop of the coatings hardness.

Abstract: Typical specimens of AZ31B Magnesium alloy were processed by single point and continuous laser shock peening (LSP). The selected laser energy was 25 J, spot diameter was 8 mm, peening spacing was 8 mm and peening times were 2. The obtained value of residual compressive stresses were -144.3 MPa and -230 MPa for single and continuous LSP respectively, and the magnitude of residual stress was in direct proportion to the depth of deformation in definite micro-deformation range. The average surface micro-hardness in the laser spot zone was 92.42 HV, which increased by 26% as compared to 73.2 HV of substrate, the depth of hardened layer was about 0.3 mm, and the maximum micro-hardness was about 109.86 HV beneath surface of 0.05~0.075 mm. Large amount of crystal chunks appeared at the crystal grain boundaries and inside the grains, and the average grain size decreased from the untreated 7 μm to the peened 4 μm. The results show that the nucleation of fatigue crack can be retarded and the mechanical properties of AZ31B magnesium alloy sheet can be improved greatly with LSP process.

Abstract: Microstructure, microhardness and tribological properties of laser hardened GCr15 steel were investigated in this paper. The wear resistance under lubricated sliding conditions was compared between specimens treated with laser and those of conventionally hardened. The tribological properties of laser surface-quenched GCr15 steel specimens were slightly better due to the effects of the microstructure hardening, high hardness and toughness, with the wear rate (in the order of 10-6mg/Nm) lower than that of the conventionally treated specimens. At the steady state, the frictional coefficient of laser-treated samples had no obvious difference from that of the conventionally treated samples. The wear mechanism for both cases was similar, generally involved surface fatigue wear and slight abrasion wear. LeiQ.K. Liu S. Lei Introduction H. Li In recent years, among the various surface modification methods, laser-induced surface modification has gained much attention for achieving the desired properties for applications[1]. This method is mainly used for ferrous alloys which undergo martensitic transformation and thus form a very hard surface layer with negligible surface roughness and distortion[2]. Some ideas demonstrated that the wear rate at a particular contact pressure can be strongly influenced by the microstructure of the steel, but there is also contrary idea that under the conditions of mild wear, the microstructural constituents of steels have no significant influence on the wear rate, although they affect the rate of severe wear. Previous studies of the authors demonstrated that under the dry sliding wear conditions, laser surface-hardened specimens of ferrous alloys exhibited enhanced wear resistance than conventionally hardened specimens. The aim of the study is to investigate the lubricated sliding wear behaviors of laser surface hardened GCr15 steel specimens and to compare the effect of the different microstructure compositions for laser transformation hardening with those of conventionally hardened and quenched. Moreover, the wear properties of the GCr15 steel and its corresponding wear mechanism under the lubricated wear conditions will also be studied.