Fabrication of noncircular multicore microtubes by superplastic dieless drawing process

A superplastic dieless drawing process that requires no dies or tools is applied to the drawing of a Zn–22Al superplastic alloy for noncircular microtubes such as square, rectangular and noncircular multi core tubes having square inner and rectangular outer cross-sections. In this study, the effects of heating condition, such as heating length and the use or nonuse of cooling device, on deformation behavior are investigated. As a result, a square microtube with 0.58 mm side and a rectangular microtube of 0.75 mm × 1.3 mm were fabricated after 3-pass superplastic dieless drawing. In addition, the fundamental deformation behavior of noncircular tubes combined with square and rectangular tubes during the dieless drawing process has been clarified experimentally. The cross-sectional shape of the noncircular tubes after the superplastic dieless drawing process tends to be maintained on the basis of the similarity law in case of a wide heating length compared with a narrow heating length. Furthermore, a noncircular microtube, which has inner square tubes with a 335 μm side, and an outer rectangular tube of 533 μm × 923 μm were fabricated successfully after a 4-pass superplastic dieless drawing process. Consequently, it was found that the superplastic dieless drawing is effective for the fabrication of noncircular multicore microtubes.     

Experimental and FEM study on sinking of miniature inner grooved copper tube

Miniature inner grooved copper tubes (mIGCT) which have an outer diameter less than 6 mm are in demand for the production of heat pipes. In this work, it is proposed to manufacture such tubes by a multi-stage tube sinking process with an initial mIGCT having an outer diameter of 6 mm. A FEM simulation approach is used to analyze stress, strain and damage distribution for the proposed process. For comparison, a smooth copper tube is also used in the simulation study. Furthermore, experiments are conducted to investigate plastic deformation of the grooves and teeth of the tube. Results show that the maximum stress and strain are occurred at the grooves area, while the maximum damage is located at the top of the teeth. The ratio of groove width to tooth width (β) is reduced after each drawing pass. Bonding, folding and segmenting, which represent potential flaws, have also been observed in the multi-stage tube sinking process, and are discussed.     

Magnetic pulse cladding of aluminum alloy on mild steel tube

Bi-metal tubes, which combine the advantageous properties of two different metals, are desirable in industries where corrosion resistance is important. A new cladding method named magnetic pulse cladding (MPC) was used to form bi-metal tubes. A cladding of mild steel tube by aluminum alloy (AA3003) was achieved. The effect of the geometry of the field shaper on cladding quality was investigated as well as other main process parameters, such as, feeding size, radial gap and discharge voltage. The mechanical property was evaluated by compression-shear test and a maximum strength of 79.2 MPa and an average of 29.7 MPa were attained to by the following process settings: profiled field shaper, feeding size of 12 mm, radial gap of 2.0 mm and discharge voltage of 15 kV. OM and SEM images show a smooth integral interface and a small wavy one. EDS mapping reveals the interfacial diffusion zone up to 50-μm wide. The results show that the proposed MPC process is able to form sound cladding bonds and could be applicable to a tubular clad component with a high axial length.     

Microstructure and tensile behavior of a friction stir processed magnesium alloy

In this work the effect of multi-pass friction stir processing (FSP) followed by warm pressing on an as-extruded ZK60 Mg plate was investigated. The microstructure, texture and resulting mechanical properties are reported here. Multi-pass FSP to partial depths on the top and bottom plate surfaces produced a novel, layered structure with three distinct microstructural zones associated with stirred, transition and core regions. In the stirred zone, FSP, followed by pressing at 200 °C, created a 0.8 μm ultrafine grain size which accounts for ～55 vol.% of the material. The transition region (～10 vol.%), showed extensively sheared coarse grains distributed in a matrix of finer grains. However, the core region (～35 vol.%) showed extensive twinning inside coarse grains in an overall bimodal microstructure reminiscent of extrusion. The processed Mg with a strong basal texture exhibited high yield strength (>300 MPa) and retention of adequate tensile ductility (>10%). The enhancement in mechanical properties of processed Mg is found to be highly influenced by the layered microstructure: UFG grained stirred zone, finer precipitates and strong basal texture.     

Periscope infrared thermography for local wall thinning in tubes

In this paper, a novel way to inspect local wall thinning in metal tubes with infrared thermography has been demonstrated. The first of its kind method utilizes a periscope-like reflector located deep inside a tube to mirror the radiated heat that flows across the tube thickness to an IR camera positioned outside the tube. Localized wall thinning was represented by partially drilled holes of different depths on a tube, of inner diameter 82 mm and outer diameter 91 mm. Feasibility studies were carried out through simulations using a finite element model. Experiments were performed to determine the thermal diffusivity of the material. Remaining thicknesses of the tubes in wall-thinned sections were found to be estimated reasonably well using measured time–temperature profiles obtained by flash method [Parker WJ, Jenkins RJ, Butler CP, Abbott GL. Flash method of determining thermal diffusivity, heat capacity, and thermal conductivity. J Appl Phys 1961;32:1679–84]. The good correlation found between model calculations and measurements vindicated the utility of a model-based approach to applications of pulsed IR thermography.     

Development of semi-dieless metal bellows forming process

A novel semi-dieless metal bellows forming process with local induction heating and axial compression without using any conventional dies is proposed. Firstly, the thickening of a tube is induced by local heating and axial compressive force. Secondly, the buckling of the tube occurs, producing a convoluted shape. The seamless tubes used are stainless steel SUS304 with an outer diameter of 5 mm and a thickness of 0.5 mm and 0.3 mm. The effects of compression ratio on the profiles of the bellows such as convolution height, pitch and thickness are investigated experimentally. It is found that convolution height can be controlled by compression ratio. Additionally, the mechanism of this process for fabrication of the metal bellows can be clarified by loading curve during processing. Furthermore, the validity of a two-step compression technique for improving convolution height and pitch is verified. The fundamental of the proposed technique can be confirmed as a basic key processing to fabricating metal bellows with various dimensions and small quantities.     

On a new concept of rotary draw bend-die adaptable for bending tubes with multiple outer diameters under non-mandrel condition

A traditional rotary draw bending die on the numerically controlled (NC) tube benders or other benders can bend tubes with only one kind of outer diameter. It is difficult for such a situation to meet the requirement of modern manufacturing with characters of much varieties and small batch. The present study proposed a new concept of rotary draw bending die called MDB-Die (Multiple-diameter Bending Die), on which tubes with different outer diameters within a definite range can be bent using the same die by only adjusting the pads inside the die set. Numerical and experimental approaches were employed to investigate the forming process of tubes with different outer diameters when bent on the MDB-Die, especially on the characters of force and elastic-plastic deformation of tube wall, and the effects of groove shapes and bending parameters on the cross-section distortion and wall thinning in the process. Analytical expressions in simple tube bending based on plastic theories given by Tang (2000) for calculating the magnitude of stresses, together with the wall thickness change, deviation of the neutral axis, and section flattening, were also used for comparison. The result proved that tubes with different outer diameters (from 18 to 25 mm in the study) can be bent successfully on MDB-Die without degrading the bending quality, i.e., the aspect ratios of section distortion of less than 5% and wall thinning of less than 7.8%.     

Formability testing of AZ31B magnesium alloy tube at elevated temperature

Magnesium alloy is a promising material for reaching the goal of weight reduction in automotive and aerospace industry. The mechanical properties of AZ31B seamless tube were tested by ring hoop tension test and compared with the results by axial tension test. Hydro-bulge test and newly proposed flatten-hydro-bulge test were also carried out to evaluate the formability of the tube. The effect of forming temperature and flattening displacement on the maximum expansion ratio and bursting pressure were measured and discussed. A tubular part with curved axis and different cross-sections was then manufactured successfully. It was found that the changing tendency of total elongation in hoop direction is quite different with that along axial direction. As temperature increased from RT. to 300 °C, two peaks of total elongation in hoop direction occurred at about 150 °C and 300 °C. In hydro-bulge test, the maximum expansion ratio increased as temperature increasing and reached the maximum value about 30% at 170 °C, then decreased quickly at higher temperature until 230 °C. In flatten-hydro-bulge test, flattening deformation has significant effect on the subsequent hydro-bulging process, and both maximum expansion ratio and bursting pressure decreased as flattening displacement increased. This should be taken into account during part design and process determining.     

Correlation between stacking fault energy and deformation microstructure in high-interstitial-alloyed austenitic steels

The correlation between stacking fault energy (SFE) and deformation microstructure of high-interstitial-alloyed austenitic Fe–18Cr–10Mn–(N or N + C) alloys was investigated. As the content of the interstitial elements increased, the deformation microstructure changed in a sequence strain-induced martensitic transformation, mixture of martensite and twin, and finally deformation twin. The SFE, playing an important role in the transition of deformation microstructure, was evaluated by the Rietveld whole-profile fitting combined with the double-Voigt size–strain analysis for neutron diffraction profiles of tensile-strained bulk samples. At fixed N + C content, the ratio of mean-squared strain to stacking fault probability remained constant regardless of the accumulated strain, whereas the ratio gradually increased with increasing N + C content. Almost linear dependence of measured SFE on N + C content could be established. According to the SFE, deformation bands exhibited distinct substructures, and their particular intersecting behavior resulted in the formation of different types of products (secondary ε martensite, α′ martensite and secondary twin) at the intersecting regions.     

Bulk metallic glass tube casting

Tubular bulk metallic glass specimens were produced, using a custom-built combined arc-melting tilt-casting furnace. Zr55Cu30Al10Ni5 tubes with outer diameter of 25 mm and 0.8–3 mm wall thicknesses were cast, with both tilt and suction casting to ensure mold filling. Tilt casting was found to fill one side of the tube mold first, with the rest of the tube circumference filled subsequently by suction casting. Optimized casting parameters were required to fully fill the mold and ensure glass formation. Too small melt mass and too low arc power filled the mold only partially. However, too large melt mass and higher arc power which lead to the best mold filling also lead to partial crystallization. Variations in processing parameters were explored, until a glassy ring with 1.8 mm thickness was produced. Different sections of the as-cast ring were investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and instrumented indentation to ensure amorphous microstructure. Atomic force microscopy (AFM) was used to compare the surface qualities of the first- and last-filled sections. These measurements confirmed the glassy structure of the cast ring, and that, the tilt cast tube section consistently showed better surface quality than the suction cast section. Optimized casting parameters are required to fully realize the potential of directly manufacturing complex shapes out of high-purity bulk metallic glasses by tilt casting.     

Plasticity of indium antimonide between −176 °C and 400 °C under hydrostatic pressure. Part II: Microscopic aspects of the deformation

Indium antimonide (InSb) has been plastically deformed over a wide temperature range, from 400 down to ?176 °C (see the companion paper: Kedjar B, Thilly L, Demenet JL, Rabier J. Acta Mater 2009) and transmission electron microscopy was used to characterize the deformation microstructures. In the ductile regime, i.e. T > T_tr1 ≈ 150 °C, the crystal deforms via the nucleation and motion of perfect dislocations belonging to the glide set. In the brittle domain, i.e. for T < T_tr1 ≈ 150 °C, two regimes are observed: for T_tr2 ≈ 20 °C < T < T_tr1 ≈ 150 °C, the crystal deformation takes place via the nucleation and glide of dissociated perfect dislocations or only leading partials, while for T < T_tr2 ≈ 20 °C, the crystal deformation proceeds via the nucleation and motion of perfect dislocations belonging to the shuffle set. In view of these observations, the brittle-to-ductile transition (at T_tr1) is confirmed to correspond to a change in the dislocation nature in the glide set, from partial-mediated plasticity at low temperature to perfect-mediated plasticity at high temperature. Another transition is observed at T_tr2 and corresponds to the glide-to-shuffle transition which is observed experimentally for the first time in a III–V compound semiconductor.     

Transition from homogeneous-like to shear-band deformation in nanolayered crystalline Cu/amorphous Cu–Zr micropillars: Intrinsic vs. extrinsic size effect

The microcompression method was used to investigate the compressive plastic flow behavior of nanolayered crystalline/amorphous (C/A) Cu/Cu–Zr micropillars within wide ranges of intrinsic layer thicknesses (h ～ 5–150 nm) and extrinsic sample sizes (350–1425 nm) with the goal of revealing the intrinsic size effect, extrinsic size effect and their interplay on the plastic deformation behavior. The nanolayered C/A micropillars exhibited deformation behaviors of strain-hardening followed by strain-softening that were dependent on the thickness of the layers. At h ? 10 nm, the strain-softening is related to shear deformation that is caused by fractures in the amorphous layers. At h > 10 nm, however, the strain-softening is related to the reduction in dislocation density caused by dislocation absorption. Correspondingly, the deformation mode of the C/A micropillars transitioned from homogeneous-like to shear band type as h decreased to the critical value of ～10 nm, which is indicative of a significant intrinsic size effect. The extrinsic size effect on the plastic deformation also became remarkable when h was less than ～10 nm, and the interplay between the intrinsic and extrinsic size effects leads to an ultrahigh strength of ～4.8 GPa in the C/A micropillars, which is close to the ideal strength of Cu and considerably greater than the ideal strength of the amorphous phase. The underlying strengthening mechanism was discussed, and the transition in deformation mode was quantitatively described by considering the strength discrepancy between the two constituent crystalline and amorphous layers at different length scales.     

Gas forming of ultra-high strength steel hollow part using air filled into sealed tube and resistance heating

A new gas forming process of ultra-high strength steel hollow parts using air filled into sealed tubes and resistance heating was developed to omit the subsequent heat treatment. In this process, a sealed quenchable steel tube was rapidly resistance-heated to improve the formability. By applying die-quenching for holding at the bottom dead centre of a press, the formed part had very high strength, a hardness of 450 HV10 equivalents to a tensile strength of 1500 MPa. In addition, the dimensional accuracy of the formed part was improved by the increase in internal pressure for heating and compression of air filled into the sealed tube. To increase the hardness, the formed tube was cooled with air blowing during holding at the bottom dead centre and the corner of the die was optimised as to be in contact with the tube. The oxidation on the outer surface of the formed part was prevented by forming in a case filled with CO₂ gas.     

Press for hydrostatic extrusion with back-pressure and the properties of thus extruded materials

A press for hydrostatic extrusion within the extrusion pressure range up to 2 GPa with back-pressure up to 0.7 GPa was designed and constructed. The press is equipped with an integrated pressure intensifier, and a control and recording system which permits recording the process parameters, such as extrusion pressure, back-pressure and its stability, time and speed of the extrusion, and enables monitoring the process on-line. The double-layer high-pressure chamber and the monobloc back-pressure chamber were analyzed using the finite element method with allowance made for the self-strain-hardening effect known as autofrettage. The maximum permissible load imposed on chambers and the resulting balance pressure established in the case of the two chambers being accidentally connected were also evaluated. Several cold extrusion processes assisted with back-pressure from 400 MPa to 700 MPa were conducted, experimenting with low or non-ductile materials, such as the ZW3 magnesium alloy, GJL250 grey cast iron, GJS500 nodular cast iron, bismuth of 99.999% purity, and molybdenum of 99.9% purity. The bulk, non-defected products with diameters ranging from 4 to 7 mm were obtained. The use of back-pressure permitted the materials to be plastically deformed during a single cold operation with the percent deformation from 36% in grey cast iron to more than 80% in Bi. Thanks to the strain-hardening due to the severe plastic deformation, the materials acquired excellent properties (YS = 392 MPa in the magnesium alloy, σ_d0.2 = 709 MPa in molybdenum, σ_dM = 1140 MPa in grey cast iron, and σ_d0.2 = 643 MPa in nodular cast iron) impossible to achieve by classical plastic deformation processes. The hardness of the materials was also increased adequately, and the refinement of their microstructure resulted in an increase of ductility. These advantageous results obtained by using the press indicate that hydrostatic extrusion with back-pressure has a great applicative potential.     

Plastic deformation mechanism of crystalline polymer materials in the equal channel angular extrusion process

The plastic deformation mechanism of polymer materials was observed during the equal channel angular extrusion process with polypropylene as crystal polymer. The variations of the crystalline morphology, microstructure, and microhardness were discussed during the process. The extent of plastic deformation increased from the top surface to the bottom surface, and the maximum molecular orientation increased from R = 1 near the top surface to R = 2.2 near the bottom surface. The plastic deformation at the surface was small, especially in the range of 400 μm distance from the top surface, without definite change of the crystalline structure. The plastic deformation was obvious when the sample was pressed into the outer corner of die. The spherulitic structure extended to the ellipsoidal shape along the 45° direction of the diagonal line because of the shear strain. The plastic deformation led to the destruction of spherulites near the bottom surface. However, the spherulites were not destroyed at center part, so their refinement as metallic material could not be expected. The variation of internal structure and material orientation increased along the direction of the maximum molecular orientation.     

Shear strength of CMT brazed lap joints between aluminum and zinc-coated steel

Higher shear strength and fusion line failure were measured in CMT brazed lap joint of aluminum alloy 6061 and zinc coated steels with high strength (DP600) or thick plate (1.2 mm). Lower shear strength and interface failure were observed only if aluminum was brazed with low strength (270 MPa) and thin steel sheet (0.7 mm). A numerical model was developed for the prediction of shear strength and failure modes of the CMT lap joints. The maximum principle stress and deformation energy at the interface layer of the CMT joints were adopted as failure criteria for interface failure prediction. The equivalent plastic strain in the weld metal, HAZ and base metal of aluminum side of the CMT brazed joints was used as a criterion for failure prediction occurred on the fusion line. The shear strength of CMT joints and the two failure modes can be accurately estimated by the developed numerical model.     

In situ TEM straining of single crystal Au films on polyimide: Change of deformation mechanisms at the nanoscale

In situ transmission electron microscopy straining experiments were performed on 40, 60, 80 and 160 nm thick single crystalline Au films on polyimide substrates. A transition in deformation mechanisms was observed with decreasing film thickness: the 160 nm thick film deforms predominantly by perfect dislocations while thinner films deform mainly by partial dislocations separated by stacking faults. In contrast to the 160 nm thick film, interfacial dislocation segments are rarely laid down by threading dislocations for the thinner films. At the late stages of deformation in the thicker Au films prior to fracture, dislocations start to glide on the (0 0 1) planes (cube-glide) near the interface with the polymer substrate. The impact of size-dependent dislocation mechanisms on thin film plasticity is addressed.     

Microstructure evolution of 6061 O Al alloy during ultrasonic consolidation: An insight from electron backscatter diffraction

6061 O Al alloy foils were welded to form monolithic and SiC fibre-embedded samples using the ultrasonic consolidation (UC) process. Contact pressures of 135, 155 and 175 MPa were investigated at 20 kHz frequency, 50% of the oscillation amplitude, 34.5 mm s^?1 sonotrode velocity and 20 °C. Deformed microstructures were analysed using electron backscatter diffraction (EBSD). At all contact pressures deformation occurs by non-steady state dislocation glide. Dynamic recovery is active in the upper and lower foils. Friction at the welding interface, instantaneous internal temperatures (0.5–0.8 of the melting temperature, T_m), contact pressure and fast strain rates result in transient microstructures and grain size reduction by continuous dynamic recrystallization (CDRX) within the bonding zone. Bonding occurs by local grain boundary migration, which allows diffusion and atom interlocking across the contact between two clean surfaces. Textures weaken with increasing contact pressure due to increased strain hardening and different grain rotation rates. High contact pressures enhance dynamic recovery and CDRX. Deformation around the fibre is intense within 50 μm and extends to 450 μm from it.     

Deformation behavior of Zr55.9Cu18.6Ta8Al7.5Ni10 bulk metallic glass matrix composite in the supercooled liquid region

A Zr_55.9Cu_18.6Ta₈Al_7.5Ni₁₀ bulk metallic glass (BMG) composite with an amorphous matrix reinforced by micro-scale particles of Ta-rich solid solution was prepared by copper-mold casting. Isothermal compression tests of the BMG composite were carried out in the range from glass transition temperature (∼673 K) to onset crystallization temperature (∼769 K) determined by differential scanning calorimetry (DSC). The compressive deformation behavior of the BMG composite in the supercooled region was investigated at strain rates ranging from 1 × 10⁻³ s⁻¹ to 8 × 10⁻² s⁻¹. It was found that both the strain rate and test temperature significantly affect the stress–strain behavior of the BMG composite in the supercooled liquid region. The alloy exhibited Newtonian behavior at low strain rates but became non-Newtonian at high strain rates. The largest compressive strain of 0.8 was achieved at a strain rate of 1 × 10⁻³ s⁻¹ at 713 K. The strain rate change method was employed to obtain the strain rate sensitivity (m). The deformation mechanism was discussed in terms of the transition state theory based on the free volume.     

A modified die for equal channel angular pressing

Tensile stress occurs in the vicinity of upper surface of the specimen in the severe plastic deformation zone, which increases the cracking and fracture tendency of the specimen and impedes the further ECAP processing. In this paper, the conventional ECAP die (Ψ = 16° and Φ = 90°) was modified to eliminate the tensile stress and enhance the compressive stress in the severe plastic deformation zone, therefore reducing the cracking and fracture tendency of the specimen. Finite element analysis demonstrated that the stress state changes from tensile to strongly compressive when using the modified die. A modified die was made and employed to extrude the commercially pure aluminum to verify its effectiveness experimentally. The billet was successfully extruded for 20 passes without obvious surface defects with the modified die, compared to 13–14 passes at most for the conventional die. Consequently, much more fine and uniform microstructure was obtained with the average grain size of 200–300 nm, while the average grain size is ～500 nm in the case of using the conventional die.