基于临界空穴扩张比理论的不锈钢管高温韧性损伤研究

针对不锈钢管在高温塑性加工过程中出现的裂纹缺陷,在Gleeble-3800热模拟试验机上对TP321奥氏体不锈钢光滑棒材进行高温拉断试验,得到了TP321在温度850~1180℃、应变速率0.01~10 s^(-1)下的真应力-真应变曲线和试样拉伸断裂瞬间的等效塑性应变。结合有限元仿真,对试验结果进行了数值模拟,得到了应力三轴度的数值,从而计算出了临界空穴扩张比参数V GC。在此基础上,验证了临界空穴扩张比参数韧性断裂准则在高温下的适用性。发现在特定温度范围内TP321断裂瞬间的V GC与应变速率相关性较小,与温度呈线性关系,并建立了奥氏体不锈钢断裂瞬间的V GC与温度的数学表达式。将得到的V GC准则应用于TP321管坯的不同辊形斜轧穿孔过程中的韧性损伤,对比分析了不同辊形斜轧穿孔仿真结果,发现V GC准则可准确预测缺陷情况。

电磁感应加热轧制制备钛/钢复合板性能研究

采用电磁感应加热对加入不同厚度纯铁中间层的钛/钢组坯进行加热,并单道次热轧复合,短时高效地制备出高质量钛/钢复合板。研究了在快速电磁感应加热至钢板居里点770℃的条件下,不同厚度纯铁中间层对钛/钢复合板界面组织和结合性能的影响。结果表明,随着中间层厚度的降低,复合板剪切强度逐渐提高,加入0.3 mm厚纯铁中间层复合板的剪切强度达到215.05 MPa。感应加热低温轧制条件下,减少了复合板界面脆性金属间化合物的生成。纯铁中间层有效地促进了结合界面两侧基体的协调变形,同时增加了界面处基体元素间的相互扩散距离,使复合板实现了良好的冶金结合,获得了综合性能较好的钛/钢复合板。

Bonding enhancement of cold rolling TA1 P-Ti/AA6061 composite plates via surface oxidation treatment

TA1 P-Ti/AA6061 composite plate was produced by oxidizing the surface of the titanium plate and adopting a cold roll bonding process.The results revealed that the oxide film(Ti6O)prepared on the surface of TA1 pure titanium was easy to crack during the cold roll bonding,thereby promoting the formation of an effective mechanical interlock at the interface,which can effectively reduce the minimum reduction rate of the composite plates produced by cold rolling of titanium and aluminium plates.Moreover,the composite plate subjected to oxidation treatment exhibited high shear strength,particularly at a 43%reduction rate,achieving a commendable value of 117 MPa.Based on oxidation treatment and different reduction rates,the annealed composite plates at temperatures of 400,450,and 500°C displayed favorable resistance to interface delamination,highlighting their remarkable strength-plasticity compatibility as evidenced by a maximum elongation of 31.845%.

双相X80管线钢塑性变形和损伤机制

针对油气管线在高压、大变形等运行工况下极易产生局部屈曲、失稳和延性断裂等与塑性损伤相关的失效行为,为了揭示管线钢塑性损伤行为的微观机制,以双相结构(贝氏体+多边形铁素体)X80管线钢为研究对象,采用有限元方法研究了双相X80管线钢微观塑性损伤演化规律。通过电子背向散射衍射技术分析了塑性损伤过程中的微观结构演化机制。结果表明,应变主要由铁素体承担,应力主要由贝氏体承担,在拉伸变形作用下,易在两相界面处发生位错堆积,导致应力集中,形成孔洞。随着应变的增加,孔洞逐渐地向铁素体内部扩展、聚集直至贯通;铁素体晶粒的主要变形机制是晶粒内部滑移系统的滑移和晶格的转动,造成了微孔洞的形成。

Evolution of bonding interface in solid-liquid cast-rolling bonding of Cu/Al clad strip

Cu/Al clad strips are prepared using solid?liquid cast-rolling bonding(SLCRB)technique with a d160mm×150mm twin-roll experimental caster.The extent of interfacial reactions,composition of the reaction products,and their micro-morphology evolution in the SLCRB process are investigated with scanning electron microscope(SEM),energy dispersive spectrometer(EDS),and X-ray diffraction(XRD).In the casting pool,initial aluminized coating is first generated on the copper strip surface,with the diffusion layer mainly consisting ofα(Al)+CuAl2and growing at high temperatures,with the maximum thickness of10μm.After sequent rolling below the kiss point,the diffusion layer is broken by severe elongation,which leads to an additional crack bond process with a fresh interface of virgin base metal.The average thickness is reduced from10to5μm.The reaction products,CuAl2,CuAl,and Cu9Al4,are dispersed along the rolling direction.Peeling and bending test results indicate that the fracture occurs in the aluminum substrate,and the morphology is a dimple pattern.No crack or separation is found at the bonding interface after90°-180°bending.The presented method provides an economical way to fabricate Cu/Al clad strip directly.

Cast-rolling force model in solid-liquid cast-rolling bonding(SLCRB) process for fabricating bimetal clad strips

Based on twin-roll casting, a cast-rolling force model was proposed to predict the rolling force in the bimetal solid-liquid cast-rolling bonding(SLCRB) process. The solid-liquid bonding zone was assumed to be below the kiss point(KP). The deformation resistance of the liquid zone was ignored. Then, the calculation model was derived. A 2D thermal-flow coupled simulation was established to provide a basis for the parameters in the model, and then the rolling forces of the Cu/Al clad strip at different rolling speeds were calculated. Meanwhile, through measurement experiments, the accuracy of the model was verified. The influence of the rolling speed, the substrate strip thickness, and the material on the rolling force was obtained. The results indicate that the rolling force decreases with the increase of the rolling speed and increases with the increase of the thickness and thermal conductivity of the substrate strip. The rolling force is closely related to the KP height. Therefore, the formulation of reasonable process parameters to control the KP height is of great significance to the stability of cast-rolling forming.

Bonding strength of Invar/Cu clad strips fabricated by twin-roll casting process

Based on traditional twin-roll casting process,Invar/Cu clad strips were successfully fabricated by using solid Invar alloy strip and molten Cu under conditions of high temperature,high pressure and plastic deformation.A series of tests including tensile test,bending test,T-type peeling test and scanning electron microscope(SEM)and energy dispersive spectrometer(EDS)measurements were carried out to analyze the mechanical properties of Invar/Cu clad strips and the micro-morphology of tensile fracture surfaces and bonding interfaces.The results indicate that no delamination phenomenon occurs during the compatible deformation of Invar/Cu in bending test and only one stress platform exists in the tensile stress-strain curve when the bonding strength is large.On the contrary,different mechanical properties of Invar and Cu lead to delamination phenomenon during the uniaxial tensile test,which determines that two stress platforms occur on the stress-strain curve of Invar/Cu clad strips when two elements experience necking.The average peeling strength can be increased from13.85to42.31N/mm after heat treatment at800℃for1h,and the observation of the Cu side at peeling interface shows that more Fe is adhered on the Cu side after the heat treatment.All above illustrate that heat treatment can improve the strength of the bonding interface of Invar/Cu clad strips.

Microstructural characteristics of as-forged and β-air-cooled Zr-2.5Nb alloy

Multiple characterization and analysis techniques including electron backscatter diffraction（EBSD）, electron channeling contrast（ECC） imaging, transmission electron microscopy（TEM） and microhardness test were jointly employed to investigate microstructural characteristics such as local composition, morphology, grain boundary characteristics and interphase orientation relationship of a forged Zr-2.5Nb alloy before and after β-air-cooling. Results show that the as-forged specimen is composed of equiaxed and lamellar α grains and continuous net-like β-Zr films. After the β-air-cooling, the microstructure of the specimen is featured by basket-weave Widmanst？tten structure, in which the inter-α-plate second phases are nanoscale β-Zr. Analyses for crystallographic orientations reveal that the Burgers relationship has been strictly followed during the β→α cooling. Compared to the as-forged specimen, the hardness of the β-air-cooled specimen is higher, which could be attributed to the decreased structural sizes of both α and β phases, and the increased fraction of high angle boundaries as well.

Interfacial pressure distribution and bonding characteristics in twin-roll casting of Cu/Al clad strip

The mechanical properties and product thickness specifications of bimetallic clad strip prepared by twin-roll casting are tightly related to the mechanical behavior of bonding interface interaction.The thermal−flow coupled simulation and the interface pressure calculation models are established with the cast-rolling velocity as the variable.The results show that the interface temperature decreases,the interface pressure and the proportion of the thickness of the Al side increase with the decrease in cast-rolling velocity.The thinning of Cu strip mainly occurs in the backward slip zone.The higher pressure and longer solid/semi-solid contact time make the interface bonded fully,which provides favorable conditions for atomic diffusion.The inter-diffusion zone with a width of 4.9μm is attained at a cast-rolling velocity of 2.4 m/min,and the Cu side surface is nearly completely covered by aluminum.Therefore,the ductile fracture occurs on the Al side,which prevents the propagation of interface delamination cracks effectively.Meanwhile,shear effect becomes more significant at high interfacial pressure and large plastic strain,and the microstructure on Al side is composed of slender columnar crystals.Thus,the metallurgical bonding and refinement of grains on the Al side can result in higher bonding strength and tensile properties of the clad strip.

Effect of Surface Treatment on Bonding Strength of Aluminum-Magnesium Hot-Rolled Composite Plate

The effect of different surface treatments on the bonding strength of composite plates was investigated under the conditions of 400℃ and reduction ratio of 45%.Results show that the wire brush grinding treatment can only eliminate the oxide film on the plate surface,but it can hardly produce a hard layer on the plate surface.The bonding effect depends on the element diffusion promoted by the close contact between the metals on both sides of the interface.After anodic oxidation,there is a hard layer on the metal surface,and the hard layer broken during the rolling process forms a mechanical occlusion at the bonding interface.However,the hard layer cannot form an effective combination with the metal at the interface,and the bonding can only occur in the fresh metal bonding area at the crack of the hard layer.The acid-alkali washing treatment can completely remove the hard layer on the surface of both alloys without increasing the surface roughness of the plate,and the metal on both sides of the interface is more closely bonded during the rolling process.The optimal bonding strength can be obtained by surface treatment of acid-alkali washing for the aluminum-magnesium hot-rolled bonding.

模拟轧制过程的镁合金带材动态接触传热机理

改进接触传热测量装置,利用有限元模拟镁合金轧制过程辊缝界面的瞬态换热特性,以准确分析温度、压力和粗糙度对接触传热系数的耦合影响。结果表明,接触传热存在2个明显的临界阈值。当温度在150℃以下且界面压力低于22.1 MPa时,存在良好的线性规律。当超过第一阈值后,接触传热明显增强,呈现显著的非线性特征。另外,当界面压力超过50 MPa(第二阈值)且温度超过300℃时,接触传热很快趋于稳定。显然,此时的第二阈值与镁合金带材表面摩擦峰的弹塑性变形直接相关,通过增加微接触面积和摩擦峰的交互扩散,从而形成高压接触传热。基于这一现象的规律特征,有助于精确控制辊缝的接触界面温度,便于设计合适的轧制参数或优化轧制工艺。