Investigation On Tube High Pressure Shearing

Control over grain size has long been recognized as a way to obtain materials with outstanding properties.One possible avenue for grain refinement is the use of severe plastic deformation(SPD)which is an important technique for bulk ultrafine-grained or nano-grained materials preparation.Several different SPD procedures are now available but to date most attention has been given to the procedures of equal-channel angular pressing(ECAP),high-pressure torsion(HPT)and accumulative roll-bonding(ARB).ECAP can produce bulk samples with considerable size,and the equipment can be scaled up easily.But,the effect on grain refining is not as good as HPT,multi-passes operation reduces the efficiency.HPT,based on rotational movement under high pressure,can produce ultra high strain in single operation and ultra fine structures.But,samples processed by HPT have a small size and the scaling up of HPT equipments are difficult.ARB is close to the industrial process.No special mold is needed.But,the quality of ARB products lie on the quality of the interfacial bonding.All these interfaces that need to be boned show high probabilities of contact with the environment.The onerous and repetitive cleaning,cutting and stacking of sheets are required in ARB to prevent the contamination in bonded interfaces.In addition of the problem of contamination,the bonding of the last interface in the middle of the whole sample is weaker since it suffered only one pass of rolling.Consequently,the development of new SPD methods with high capacity of grain refinement,high efficiency and high possibility of scaling up has become the common goal of many researchers in this area.The analysis of severe plastic deformation is a typical plastic problem solving process which is usually necessary to solve the stress equilibrium differential equation,strain geometry equation,material constitutive equation and yield condition equation under appropriate initial conditions and boundary conditions.The analytical solution should also include the characteristic parameters of the material.However,so far,the strain calculation of severe plastic deformation,such as ECAP,HPT,ARB,depends only on the geometric parameters of the deformation,and does not include any material characteristic parameters.This objectively hinders the theoretical analysis and essential understanding of the severe plastic deformation process.In order to solve the above problems.,based on the existing methods of SPD,a new method of SPD,"tube High Pressure Shearing(t-HPS)",which can enlarge the size of sample,is proposed.During t-HPS,the high hydrostatic pressure is applied to improve the grain refinement effect and the machinability of the material,and the single pass large deformation or high efficiency of continuous processing is realized by the use of rotary motion.Starting from the initial concept of t-HPS,the new method was analyzed by the plastic theory,and the experimental devices were established,and several kinds of materials were processed.The following innovative results are achieved:(1)Based on the basic equations of plastic theory and the constitutive relation of materials,the analytical solution of t-HPS strain field is given.The solution includes not only the geometric parameters of deformation zone that the previous solutions the strain of SPD depend on,but also the characteristic parameter of material:strain hardening exponent,which reflects the intrinsic effect of the material parameters on strain,the core physical quantity,of SPD.(2)Based on the above strain field analytical solution of t-HPS,the intrinsic characteristics of traceline elongation in deformation zone were revealed.The traceline equation was established,and the(virtual)interface multiplication theory based on traceline elongation was proposed.A one-step process was developed for synthesizing multilayered laminates by t-HPS for the first time.(3)After 1 turn(average equivalent strain?45)of t-HPS at room temperature,except for grain refinement,5N A1(99.999%wt.)evolved into an unreported one-component strong texture {110}<110>with the<110>glide directions near the shear direction has been obtained,which is totally different from the shear textures usually found for f.c.c.metals.Based on the strain energy release maximization(SERM)model,{110}<110>texture can be obtained by the recrystallization texture evolution from the "exceptional"{110}<110>f.c.c.shear texture or the usual C{001}<110>which is the strongest shear texture component of f.c.c.metals with high stacking fault energy.By using comparison experiment with relatively low purity aluminum,the formation mechanism of this texture was confirmed at the experimental level.(4)At room temperature t-HPS,the Pb/Sn combined samples realized the physical transition of the outer spliced surfaces of the undeformed dissimilar materials to the inner phase boundaries of the laminated composites.The interfaces obtained complete metallurgical bonding of atomic scale.After 8 turns of t-HPS,the average layer thickness is less than 20 ?m.Compared to other methods such as ARB,the t-HPS process shows great potential for synthesizing laminated composites with high efficiency in a single step process having the additional advantage of preventing any contamination through contact of the interfaces with the environment.(5)4N Al(99.99%wt.)/5N Al(99.999%wt.)combined samples were processed by t-HPS with 25 turns at room temperature.Fine grains with uniform distribution were obtained.The "extra" grain refinement of 5N Al was happened when processed together with the 4N Al.According to the model calculation,a network formed by 4N Al involved boundaries(with relatively low mobility)during t-HPS had a "pinning" effect to 5N Al grain boundaries so that the grain structures were stabilized.This effect lowered the limit of grain refinement of 5N Al.(6)Cubic ?'-Al2Cu intermediate phase which is different from 0 phase in the equilibrium phase diagram of the Al-Cu or the square ?'phase precipitated during the aging process of the Al-Cu alloy was produced at Al/Cu interfaces after five turns of t-HPS at room temperature using Al/Cu combined samples.This is the first report of cubic ?' in experiments.The calculation shows that the strain driving effects in the process of t-HPS can increase the atomic mobility by about 10 orders of magnitude.Thus,the kinetic conditions(atomic migration)of the generation of Cubic ?' phase,which also need a relatively low solid-reaction temperature(<150? for example)in thermodynamics,are satisfied by room temperature t-HPS.