Hot Coextrusion Research Articles

Co-extrusion of a Mg/Al composite billet: A computational study validated by experiments

A coupled finite element analysis in conjunction with experiments was used to investigate hot co-extrusion of a novel composite Mg AZ80/Al 1100 billet assembly. To validate the computational model, results from both radiography, and metallography were used.It is demonstrated that the billet configuration proposed in this study results in a uniform composite rod, with no indication of cracks, tears, wavy interface surfaces or voids. The computational model is first validated by the experiments and then used to thoroughly study the thermo-mechanical fields that develop during the co-extrusion process. The interplay of the thermal and mechanical fields during the process is examined for different initial temperatures, die angle and ram velocity. It is demonstrated that using the proposed billet assembly, constant compressive radial stress develops across the magnesium core, leading to sound-proportional flow conditions.

Study on Fracture of the Fibrous Monolith Cemented Carbide

A fibrous monolith cemented carbide with WC-6%Co as cell and WC-20%Co as cell boundaries was produced through hot co-extrusion process in this paper. The density, hardness, transverse rupture strength (TRS) and fracture toughness (KIc) of the fibrous monolith cemented carbide were tested, and the fracture and crack propagation were observed by metalloscope and Scanning Electron Microscope. The result shows fibrous monolith structure design could effectively improve the TRS and the KIc of WC/Co cemented carbide without a significant decrease of hardness. It is the reason of high transverse rupture strength and fracture toughness that WC-20%Co can absorb more fracture energy in order to slow, prevent, or deflect crack propagation, proved by metalloscope and scanning electron microscope.

Microstructure and Mechanical Properties of Sintered Fibrous Monolith Cemented Carbide

A fibrous monolith cemented carbide with WC-6Co as cell and WC-20Co as cell boundaries was produced through hot co-extrusion process in this paper. The density, hardness, bending strength and fracture toughness of the fibrous monolith cemented carbide were tested, and the fracture and crack propagation were observed by metalloscope and SEM. The results showed that the bending strength and fracture toughness of the fibrous monolith cemented carbides was remarkably improved 71.91% and 45.7% respectively, while the hardness was slightly decreased 1% compared with WC-6Co composites. It is the reason that the tougher shell WC-20Co with higher bending strength and fracture toughness can absorb more fracture energy, which can slow down and prevent the crack propagating from brittle core WC-6Co.

Improved ductility in ZM21 magnesium–aluminium macrocomposite produced by co-extrusion

ZM21 magnesium alloy/CP aluminium macrocomposite containing millimetre scale aluminium wire rod reinforcement was fabricated through direct hot co-extrusion. The composite was extruded at 450 °C with three different extrusion ratios (ER) such as 4:1, 9:1 and 16:1 with constant ram speed. The macrocomposite was characterised by microscopy and mechanical testing (tensile and hardness). The characterization proved that the structural integrity between ZM21 Mg alloy and aluminium was good with appreciable grain refinement in ZM21 Mg alloy in all extrusion ratios. Macrocomposite obtained through 16:1 ER exhibited better tensile and yield strength with significant improvement in ductility. The reinforced aluminium in ZM21 Mg alloy significantly contributes the strength and ductility which was confirmed by micrographs and fractographs.

Enhancing the Performance of Magnesium Alloy AZ31 by Integration with Millimeter Length Scale Aluminium-based Cores

New bimetal magnesium/aluminium macrocomposites containing millimeter-scale Al-based core reinforcement were fabricated using solidification processing followed by hot coextrusion. Two approaches were attempted for enhancing the performance of AZ31: (a) AZ31 shell integration with pure Al core (pure Al core approach) and (b) AZ31 shell integration with AA5052 core (AA5052 core approach), in that order. In the AA5052 core approach eventually, macrointerface width was increased by 2 orders of magnitude (compared to that in the macrocomposite obtained using the pure Al core approach), while stiffness was increased (+36%), 0.2%YS was unchanged and UTS was increased (+14%) (compared to monolithic AZ31). The evolution of the Mg-Al macrointerface and its effect on microstructure and mechanical properties is investigated in this article.

Simultaneously enhanced tensile and compressive response of AZ31–NanoAl2O3–AA5052 macrocomposite

New bimetal AZ31–Al2O3/AA5052 macrocomposite comprising (a) Al2O3 nanoparticle-reinforced magnesium alloy AZ31 shell and (b) aluminum alloy AA5052 millimeter-scale core reinforcement was fabricated using solidification processing followed by hot coextrusion. Microstructural characterization revealed more rounded intermetallic particle of decreased size, reasonable Al2O3 nanoparticle distribution, and non-dominant (0 0 0 2) texture in the longitudinal and transverse directions in the AZ31–Al2O3 nanocomposite shell. Interdiffusion of Mg and Al across the core–shell macrointerface into each other was also significant. Compared to monolithic AZ31, the AZ31–Al2O3 shell exhibited significantly higher hardness (+33%). In tension, the presence of Al2O3 nanoparticles (in the AZ31 shell) and AA5052 core significantly increased stiffness (+39%), yield strength (0.2% TYS) (+9%), ultimate strength (UTS) (+19%), average failure strain (+7%), and work of fracture (WOF) (+27%) of AZ31. In compression, the presence of Al2O3 nanoparticles (in the AZ31 shell) and AA5052 core significantly increased yield strength (0.2% CYS) (+58%), ultimate strength (UCS) (+4%), average failure strain (+11%), and WOF (+49%) of AZ31. The effect of joint presence of (a) Al2O3 nanoparticles (in the AZ31 shell) and (b) AA5052 millimeter-scale core on tensile and compressive properties of AZ31 is investigated in this article.

Adding carbon nanotubes and integrating with AA5052 aluminium alloy core to simultaneously enhance stiffness, strength and failure strain of AZ31 magnesium alloy

New bimetal AZ31-CNT/AA5052 macrocomposite comprising: (a) carbon nanotube (CNT) reinforced magnesium alloy AZ31 shell and (b) aluminium alloy AA5052 millimeter-scale core reinforcement was fabricated using solidification processing followed by hot coextrusion. Microstructural characterisation revealed more rounded intermetallic particle of decreased size, reasonable CNT distribution, and dominant (1 0 −1 1) texture in the longitudinal and transverse directions in the AZ31-CNT nanocomposite shell. Interdiffusion of Mg and Al across the core-shell macrointerface into each other was also significant. Compared to monolithic AZ31, the AZ31-CNT shell had significantly higher hardness (+30%). In tension, the presence of CNT (in the AZ31 shell) and AA5052 core significantly increased stiffness (+39%), ultimate strength (+13%), failure strain (+17%) and work of fracture (+27%) of AZ31, while yield strength (−2%) was marginally decreased. In compression, the presence of CNT (in the AZ31 shell) and AA5052 core significantly increased yield strength (+35%), failure strain (+42%) and work of fracture (+70%) of AZ31, while ultimate strength (+1%) was marginally increased. The effect of joint presence of: (a) CNT (in the AZ31 shell) and (b) AA5052 millimeter-scale core on tensile and compressive properties of AZ31 is investigated in this paper.

Enhancement of compressive strength and failure strain in AZ31 magnesium alloy

New bimetal AZ31/AA5052 macrocomposite containing millimeter-scale aluminium alloy core reinforcement was fabricated using solidification processing followed by hot coextrusion. Microstructural characterization revealed decreased intermetallic particle spacing, Mg texture change and significant interfacial interdiffusion of Mg and Al into each other. Compressive testing revealed that presence of AA5052 core increased compressive yield strength (0.2% CYS) (+51%), ultimate compressive strength (UCS) (+4%), average failure strain (+18%) and work of fracture (WOF) (+50%) of AZ31. The effect of presence of mm-scale AA5052 core on the compressive properties of the bimetal macrocomposite is investigated in this paper.

Micro-Engineering the Stressed Macro-Interface and Enhancing the Performance of Mg/Al Bimetal Macrocomposites

New bimetal magnesium/aluminium macrocomposites containing millimeter-scale Al based core reinforcement were fabricated using solidification processing followed by hot coextrusion. The initial macrocomposite consisted of a combination of pure Mg shell and pure Al core. Some problems encountered with the macrocomposite were Mg and Al grain coarsening, an inadequate Mg-Al interface (macrointerface) and consequent reduction in strength, compared to monolithic Mg. To rectify these problems, three approaches were taken in the following order primarily to widen (strengthen) the Mg-Al interface: (a) pouring of pure Al at 900°C (higher temperature approach), (b) pure Mg shell substitution with AZ31 shell (single substitution approach) and (c) pure Mg shell and pure Al core substitution with AZ31 shell and AA5052 core, respectively (double substitution approach). The evolution (strengthening) of the Mg-Al interface and its effect on microstructure and mechanical properties in each macrocomposite is investigated in this paper.

Toughening mechanisms in Mg/Al macrocomposites: texture and interfacial mechanical interlocking

In this study, two types of new bimetal magnesium/aluminium (Mg/Al) macrocomposites with millimetre-scale Al core reinforcement were fabricated using casting coupled with hot coextrusion. The first macrocomposite was fabricated with an unthreaded Al core while the second one had a threaded Al core. Microstructural characterization revealed Mg texture change and interdiffusion of Mg and Al into each other across the interface in an unbalanced manner. Stress at the bimetal interface was attributed to solid solution formation, thermal expansion mismatch, unbalanced Kirkendall strain, lattice misfit strain and strain localization effects. Results revealed that the presence of the Al core leads to a decrease in strength but an increase in failure strain and toughness (work of fracture) of Mg. The threaded Al core reduced 0.2%YS and UTS of Mg by 17% and 15%, respectively, and enhanced the failure strain and toughness of Mg by 102% and 73%, respectively. This was 1.4 and 1.6 times less the reduction, and about 6 and 7 times more the increment, when compared with the corresponding changes of 0.2%YS, UTS, failure strain and toughness of the unthreaded macrocomposite, respectively. The toughening mechanisms in bimetal Mg/Al macrocomposites are investigated in this paper.

Improving Compressive Failure Strain and Work of Fracture of Magnesium by Integrating it with Millimeter Length Scale Aluminum

Solidification processing followed by hot coextrusion was used to fabricate a bimetal magnesium/aluminum macrocomposite containing millimeter-scale Al core reinforcement. Microstructural characterization revealed limited interfacial interdiffusion of both metals into each other. Compressive testing revealed that the presence of Al core does not affect 0.2%YS, marginally decreases ultimate compressive strength (UCS), and significantly increases failure strain (115%) and work of fracture (55%) of Mg. The effect of the presence of mm-scale Al core on the compressive properties of the bimetal macrocomposite is investigated in this article.

Heat-treating below recrystallization temperature to enhance compressive failure strain and work of fracture of magnesium

New bimetal magnesium/aluminium macrocomposite containing millimeter-scale Al core reinforcement was fabricated using solidification processing followed by hot coextrusion. Microstructural characterisation revealed increased grain size, Mg texture change and unbalanced interfacial interdiffusion of Mg and Al into each other. Stress at the bimetal interface was attributed to solid solution formation, thermal expansion mismatch, unbalanced Kirkendall strain, lattice misfit strain, and strain localization effects, these being interface localized strengthening phenomena. Compressive testing revealed that presence of Al core decreased 0.2% YS (−23%) and ultimate compressive strength (UCS) (−11%), but significantly increased failure strain (+134%) and work of fracture (+60%) of Mg in the as-extruded macrocomposite. Also, interfacial relaxation during heat treating significantly increased failure strain (+17%) and work of fracture (+17%) of Mg/Al macrocomposite without compromising 0.2% YS and UCS. The effects of presence of millimeter-scale Al core as well as interfacial relaxation on the compressive properties of the bimetal macrocomposite are investigated in this paper.

Solidification processed Mg/Al bimetal macrocomposite: Microstructure and mechanical properties

Abstract A new bimetal magnesium/aluminium (Mg/Al) macrocomposite containing mm-scale Al core reinforcement was fabricated via casting and hot coextrusion. Characterization revealed fairly uniform Al volume fraction along the extruded rod length attributable to mechanical interlocking between Mg shell and Al core. Major defects were absent and Mg–Al interfacial integrity was good. Thermal stability of the macrocomposite was marginally improved when compared to pure Mg. Results revealed that the presence of the Al core leads to a decrease in strength of Mg, but an improvement in stiffness as well as significant increase in failure strain (144%) and work of fracture (73%) of Mg. An attempt is made in the present study to investigate the effect of presence of mm-scale Al core on the microstructure and mechanical properties of the bimetal Mg/Al macrocomposite.

Microstructure and mechanical properties of carbon steel A210–superalloy Sanicro 28 bimetallic tubes

The viability by hot co-extrusion of a new bimetallic tube: carbon steel A-210-G°A1/iron based superalloy Sanicro 28 (UNS N08028) both by hot torsion tests and by diffusion bonding experiments using hipping has been verified. An excellent metallurgical bond was obtained after the industrial hot co-extrusion process. Both the interdiffusion of the elements across the interface, and the microstructure have been analysed by optical microscopy, SEM, TEM and EBSD (electron backscattered diffraction). On the Sanicro 28 side a profuse precipitation of Cr/Mo carbides was found in the region close to the interface. In the hipped specimens and in the heat-treated bimetallic tubes, as result of the nickel and chromium diffusion from the superalloy to the carbon steel, austenite and martensite sub-bands were observed parallel to the interface on the steel side. The optimum heat treatment performed on the bimetallic tubes consisted of an austenitising-solution treatment at 1100 °C, and a stabilisation annealing at 900 °C, followed by air cooling.

Microstructure and mechanical properties of low alloy steel T11–austenitic stainless steel 347H bimetallic tubes

A low alloy steel (T11) has been bonded to an austenitic stainless steel 347H by hot coextrusion under industrial conditions. The final product was a seamless bimetallic tube with 347H cladding the exterior for corrosion resistance in severely corrosive environments at high temperatures. The microstructures of the coextruded bonding have been compared to those obtained in the laboratory, after diffusion bonding experiments, using hot isostatic pressing (hipping). In all cases both the interdiffusion of the different elements across the interface and the microstructure have been analysed by optical microscopy, SEM, and TEM. On the 347H side a profuse precipitation, mainly of NbC, was found in a region near the interface. Only in the hipped specimens, as result of nickel and chromium diffusion from the stainless steel to the T11 steel, a martensite band was observed parallel to the interface. The heat treatment performed on the bimetallic tubes, to obtain the optimum combination of mechanical properties and corrosion resistance, consisted of austenitisation between 1050–1100°C, water quenching, and a stabilisation treat ment at 850–900°C, followed by slow cooling.

Review of corrosion resistant co-extruded tube development for power boilers

Conditions encountered in operating a high temperature plant often impose conflicting demands for optimum strength and long term corrosion resistance. The use of conventional single walled tubes could force undesirable modifications to tube parameters or ameliations of the operating conditions. The answer to this problem could be the use of a double walled tube which is metallurgically bonded to ensure stability in the high temperature environment. A standard alloy can be selected for mechanical strength which is then protected from aggressive corrosion/oxidation by cladding with a dissimilar alloy. Hot co-extrusion is used to produce this type of tube. The design and manufacturing aspects of various combinations of carbon steel, stainless, and alloy steels are described. Particular attention is given to the substantial service experience of using some 250 km of co-extruded tubes, for furnace walls and superheater/reheater replacements in large pulverized coal fired boilers in the UK. The evidence obtained indicates that in areas of extreme fireside attack, the use of this type of tube can double the period between planned outages. A brief review is made of other examples in the energy and chemical process industries where this type of tube is used.

A Method of Hot Coextruding Complex Shapes through Round Dies

A round die hot coextrusion technique used to prepare rods having complex cross sections was developed and successfully applied to the fabrication of powder metallurgy parts with and without external cladding. The procedure involves assembling and extruding steel billets of round cross section which contain an axially located assembly of powder metallurgy and sheet metal parts which is an enlarged and shortened mockup of the desired part. After extrusion, the steel parts are removed and a fully dense rod having the desired shape and composition remains. Rods having several compositions in metallurgically joined zones along their lengths can be produced by this method. For example, a Y- shaped rod with a Ti-25 vol.% Eu/sub 2/O/sub 3/ dispersion at one end, a Ti10.7 vol.% B/sup 10/ dispersion midsection, and a pure Ti length at the other end was successfully produced. It showed dimensional uniformity in keeping with usual tolerances for reactor control components and was clad with a fully bonded, 0.005- inch thick layer of titanium. Data on the physical properties, corrosion performance, thermal cycling resistance, and irradiation damage resistance of parts produced by this technique are presented. These data indicate that the method can be successfully used to produce reactormore » components which are very difficult to manufacture by other techniques. (auth)« less