Inter-pass Annealing Research Articles

Dynamic and Postdynamic Recrystallization Behavior of GWZ Magnesium Alloy Under Double‐Hit Hot Compression

Herein, dynamic and postdynamic recrystallization behaviors of GWZ magnesium are investigated. Toward this end, the single‐hit and double‐hit hot compression tests are conducted under strain rate of 0.001 s−1 at 400 °C. The prestrains of 0.1 and 0.5 are considered to investigate the effect of interpass time (5–300 s) on the compressive strength level. At the low strain level of 0.1, the contribution of Hall–Petch effect is considerable due to the occurrence of static recrystallization. In addition, the rare earth texture component is eliminated during interpass annealing. This causes increasing the strength of the material during second pass of hot compression. In contrast, at higher imposed strain, the strength level decreases with increasing the interpass time of annealing. The high amount of strain is completely consumed and the remaining stored energy is not high enough to trigger the occurrence of static recrystallization. The occurrence of metadynamic recrystallization and subsequent growth are characterized. In addition, the texture does not change in respect of the intensity or numbers/types of components. Accordingly, the observed decreasing trend of the strength is justified relying on the occurrence grain growth.

Enhancing strength and ductility of low RE content Mg-Gd-Y-Zr alloy via a novel thermomechanical treatment based on multi-directional forging

The high strength of most Mg-RE alloys is frequently accomplished by high RE addition, but their high density and poor plasticity restrict their applications in the field of aerospace. Focused on this problem, a novel process of the thermomechanical treatment based on multi-directional forging is developed to improve the strength and ductility of low RE content Mg-6.5Gd-1.2Y-0.49Zr (wt%) alloy simultaneously. The mechanisms of high strength and good ductility are revealed by analyzing the deformation behavior, the dynamic recrystallization (DRX) behavior, the work hardening behavior and the strengthening mechanism. The bimodal grain structure, along with the fine grain size of ∼0.8 µm and the coarse grain size of ∼11.6 µm, is prepared after a 4-pass-MDF process, which is ascribed to DRX during MDF and recovery during the interpass annealing. After a peak aging treatment, the ultimate tensile strength reaches 378.4 MPa, the yield strength 324.3 MPa and the elongation to failure 19.0%, respectively. The enhanced strength is more related to the combined effect of aging precipitation of β′ phase and grain refinement, while the good ductility benefits from the ability of bimodal structure to coordinate deformation, the excellent work-hardening effect and the full activation of<c + a> slip.

A new microstructural refinement mechanism of ultra-large β grains in near-β titanium alloys during primary hot-working

In this work, the multi-step hot-working of a near-β titanium alloy, Ti-55511 alloy with centimeter-sized grains, was performed at 1170 °C. It was confirmed that the static/metadynamic recrystallization during interpass annealing was the primary mechanism responsible for the microstructural refinement. Meanwhile, a new breakdown mechanism of the ultra-large β grains with 〈001〉 preferential orientation was observed during deformation, which was essentially different from the traditional discontinuous or continuous dynamic recrystallization. That is, new grain boundaries and even boundary networks were produced interconnecting the corners/protrusions of the 〈001〉 -orientated grains through a low-angle grain boundary merging mechanism, which progressively severed the large β grains into smaller ones. Such a breakdown mechanism may be an important complement for microstructural refinement during primary hot-working, which allowed the breakdown of the ultra-large β grains under fairly low strain level.

Effect of inter-pass annealing on the deformation microstructure of Ti-48Al-2Cr-2Nb alloy

TiAl alloy is a potential candidate structural material for lightweight and high temperature in aerospace, and multi-pass thermo-mechanical treatment, where dynamic recrystallization (DRX), static recrystallization (SRX), and post-dynamic recrystallization (PDRX) have significant effects on microstructure evolution, is commonly applied for microstructure control due to its poor plasticity. Accordingly, the complex recrystallization behaviors of Ti-48Al-2Cr-2Nb alloy were investigated by hot simulation compression experiments. The results of metallographic statistics showed that microstructure inhomogeneity of uniaxial compression could not be improved by changing the temperature and strain rate to adjust the DRX degree. Additionally, discontinuous dynamic recrystallization (DDRX) characterized by the formation of twining boundaries was a dominant DRX mechanism and generated at grain boundaries. Meanwhile, continuous dynamic recrystallization (CDRX) was formed within the grains and at grain boundaries, accompanied by the relatively high distortion at the initial stage. Residual deformed microstructure owing to the incomplete DRX could be eliminated by SRX and PDRX during the inter-pass annealing. A method based on the grain size difference induced by various recrystallization mechanisms was proposed to reveal that grain growth resulting from PDRX played a primary role and SRX was impaired with the increasing strain. The interaction effect of DRX, SRX, and PDRX during the multi-pass deformation could achieve the uniform microstructure, which is consistent with a near-isothermal multi-pass multidirectional canned forging process designed in the paper. Eventually, room-temperature tensile tests demonstrated that the mechanical properties of wrought billet were distinctly enhanced.

Investigation and Modelling of Magnesium Alloy Grain Size during Hot Strip Rolling with Inter-Pass Annealing

Investigation and Modelling of Magnesium Alloy Grain Size during Hot Strip Rolling with Inter-Pass Annealing

Microstructure Evolution and Meta-Dynamic Recrystallization Behavior of Ti-45Al-8.5Nb-0.2W-0.2B-0.3Y Alloy during Inter-Pass Annealing after Deformation

The effect of processing parameters on meta-dynamic recrystallization (MDRX) and the microstructural characteristics evolution of TiAl alloy with high Nb containing during annealing period were studied through double-pass hot compression tests. The results indicated that the occurrence of MDRX had a strengthening effect to flow behavior of the present alloy, resulting in grain equalization. And the high temperature and large strain were beneficial for the occurrence of MDRX, resulting in the increase of recrystallization grains. For the present alloy, MDRX rate is low, hence, enough annealing time is needed for MDRX during inter-pass annealing. Besides, dislocation density is consumed with the occurrence of MDRX.

Metadynamic recrystallization behavior of β-solidified TiAl alloy during post-annealing after hot deformation

The metadynamic recrystallization (MDRX) of β-solidified TiAl alloy and its effects on microstructural homogeneity were investigated in the present work. Based on the comprehensive microstructure characterizations in terms of scanning electron microscope in back-scattered electron (BSE) mode and electron back scattered diffraction (EBSD) technique, it is found that the occurrence of MDRX during the post-annealing process after hot compression leads to the decomposition of lamellar colonies, grain equalization and reduction of geometrically necessary dislocation. The MDRX in present alloy during interpass annealing process has a hardening effect on the subsequent flow behavior, while it is restricted due to the phase transformation γ to α when annealing at 1200 °C. Moreover, the occurrence of MDRX can reduce the microstructure heterogeneity among various areas induced by the different strains and stain rates, thus, to result in the microstructural homogenization. Through investigating the MDRX behavior, the present work supports important information for optimizing forged microstructure of TiAl alloys.

Effect of Ag, In and AgIn Alloying Additions on Microstructure and Texture of Mg-3Al-1Zn Alloy during Multi-Pass Warm Rolling

In the present investigation the rolling response, microstructure and texture evolution of four Mg alloys during multi-pass warm rolling were evaluated. The nominal composition of the base alloy (alloy-1) was Mg-3Al-1Zn. The alloy-2, 3 and 4 were developed by separate additions of non-rare earth elements Ag and In, and a master alloy 85Ag15In (wt.%) to make target compositions Mg-3Al-1Zn-0.5x, (x = Ag ,In, AgIn). Samples from all four alloys were subjected to multi-pass warm rolling at 300 °C to accumulative reductions of 50, 75 and 90% with 8 minutes inter-pass annealing. For all four alloys, crack free sheets of less than 1 mm thickness were produced successfully with true strain corresponding to 90% reduction. The as-cast microstructures revealed second phase particles at grain boundaries and grains interiors for all alloys. A slight scatter in the size of the deformed grains was observed for alloy-1, 2 and 3 after rolling reductions of 50, 75 and 90%. However, a sustained decrease in grain size with increasing the rolling reductions was only observed in alloy-4, despite inter-pass annealing. XRD macro-texture results of alloy-2 and 3 presented very strong basal texture showing almost concentric contours around normal direction (ND). Such strong sheet texture is attributed to a preferential alignment of basal planes parallel to the sheet surface. On the other hand alloy-1 and alloy-4 revealed a weaker texture with basal poles spread more towards transverse direction (TD) as compared to rolling direction (RD) and may be due to the activation of some &lt;c+a&gt; non-basal slip and twinning in addition to basal slip under the same processing parameters.

Processing and properties of magnesium alloy micro-tubes for biodegradable vascular stents

Magnesium alloys are considered to be one of the most promising stent materials due to their good biological compatibility and biodegradable properties. However, the poor workability at room temperature restricts the processing of high-precision micro-tubes for the application as stent materials. In this study, Mg–Zn–Y–Nd alloy tube blank was firstly produced through hot extrusion. By multi-pass cold drawing combining with interpass annealing, the extruded tube blank can be processed into high-precision micro-tube with an outer diameter of about 2.0 mm and a wall thickness of about 0.15 mm. The drawn micro-tube after annealing exhibits improved mechanical properties with an ultimate tensile strength of about 298 MPa and a large breaking elongation of up to 20%. Meanwhile, the micro-tube shows good corrosion resistance with a trend of uniform corrosion in simulated body fluid solution. The investigation revealed that a significant improvement in the mechanical properties is mainly attributed to grain refinement and texture weaken, while the good biodegradable properties is related with the low density of crystallographic defects as well as fine and homogeneous microstructure. As a result, the successful processing of magnesium alloy micro-tube provides the precursor for laser engraving stents.

Kinetics of static recrystallization in the coarse-grained Fe–40 at.%Al–Zr–B alloy

The aim of works was to describe mathematically the kinetics of static recrystallization of the alloy type Fe–40at.%Al–Zr–B (with 24.6 Al–0.01 B–0.18 Zr–0.01 C in wt.%) with the coarse-grained structure. The microstructure of the laboratory castings made of this extremely brittle alloy was homogenized by hot rolling of the material in the protective capsules and by the long-term annealing at 1200°C. An initial microstructure with average grain size 0.77±0.27mm was obtained. Based on the isothermal plastic deformation tests and EBSD analysis, the static recrystallization kinetics of the prepared coarse-grained B2 iron aluminide after strain 0.20 was mathematically described. Recrystallized fraction depends on deformation/annealing temperature (900–1100°C) as well as on annealing time. The activation energy of static recrystallization was calculated as 255kJmol−1. Competition between dynamic recovery and static recrystallization was proved after strain 0.35 and annealing temperature 1100°C. Static recrystallization starts relatively easily in the studied alloy, but a very long-term annealing is quite necessary for the complete course of recrystallization. The mean size of recrystallized grains falls with the decreasing annealing temperature (0.47±0.15mm for temperature 1100°C, and 0.22±0.04mm for 900°C). Even at a temperature of 1200°C the annealing after deformation should last approx. 1min for obtaining the fully recrystallized microstructure. That is why the standard hot forming technologies should be combined by an interpass annealing in order to refine sufficiently the coarse grains.

Enhancement effect of inter-pass annealing during equal channel angular pressing on grain refinement and ductility of 9Cr1Mo steel

To obtain enhanced mechanical property in both the strength and the ductility, 9Cr1Mo steel (T91) was severely deformed by equal channel angular pressing (ECAP) combined with an additional inter-pass annealing. Tensile results show that the additional inter-pass annealing can significantly improve the ductility (i.e. 18% of the total elongation after four-pass extrusion with the inter-pass annealing) but slightly decrease the tensile strength comparing with the case without the inter-pass annealing (i.e. 10% of the total elongation after four-pass ECAP processing). The average grain size of the two passes ECAP-processed materials with the inter-pass annealing (~0.8µm) is smaller than that of the sample without inter-pass annealing (~2µm), and the fraction of the high angle grain boundaries in the samples with the inter-pass annealing (~40%) is higher than that of ~34% (two-pass ECAP) without the inter-pass annealing based on electron backscattering diffraction analysis. The crystallite size and dislocation density were evaluated by means of the modified Williamson-Hall plot based on X-ray diffraction analysis. The microstructural analysis indicates that the enhanced ductility of the ECAP processed and inter-pass annealed materials can be attributed to the relatively smaller grain sizes, larger crystallite sizes and lower dislocation densities.

Optimization of interpass annealing for a minimum recrystallized grain size and further grain refinement towards nanostructured AA6063 during equal channel angular pressing

Grain structure evolving during severe plastic deformation (SPD) and interpass annealing strongly influences grain refinement occurring during subsequent SPD. The aim of this research was to determine an optimum interpass annealing condition that could lead to the formation of a nanostructure of the AA6063 alloy during subsequent equal channel angular pressing (ECAP). Different interpass annealing temperatures and times as well as different prior ECAP passes were used to define an optimum annealing condition that led to a minimum grain size for further ECAP by route A. TEM confirmed the attainment of nano-scale grains with sizes around 500nm after six passes of ECAP. EBSD revealed the majority of grain boundaries being high-angle ones.

High ductility bulk nanostructured Al–Mg binary alloy processed by equal channel angular pressing and inter-pass annealing

Bulk nanostructured (NS) metallic materials usually have high strength but extremely low ductility. In this study, we successfully prepared a bulk NS binary Al–7 Mg alloy possessing both high strength and high uniform ductility, approaching ∼600 MPa and ∼14%, respectively, by utilizing room temperature equal channel angular pressing (ECAP) combined with inter-pass annealing. The mechanism that governs the high ductility in the alloy and especially the role of the high Mg solute concentration was explored in terms of microstructure analysis.

Forming of magnesium alloy microtubes in the fabrication of biodegradable stents

Magnesium alloys have, in recent years, been recognized as highly promising biodegradable materials, especially for vascular stent applications. Forming of magnesium alloys into high-precision thin-wall tubes has however presented a technological barrier in the fabrication of vascular stents, because of the poor workability of magnesium at room temperature. In the present study, the forming processes, i.e., hot indirect extrusion and multi-pass cold drawing were used to fabricate seamless microtubes of a magnesium alloy. The magnesium alloy ZM21 was selected as a representative biomaterial for biodegradable stent applications. Microtubes with an outside diameter of 2.9mm and a wall thickness of 0.2mm were successfully produced at the fourth pass of cold drawing without inter-pass annealing. Dimensional evaluation showed that multi-pass cold drawing was effective in correcting dimensional non-uniformity arising from hot indirect extrusion. Examinations of the microstructures of microtubes revealed the generation of a large number of twins as a result of accumulated work hardening at the third and fourth passes of cold drawing, corresponding to the significantly raised forming forces. The work demonstrated the viability of the forming process route selected for the fabrication of biodegradable magnesium alloy microtubes.

Through-Process Modeling of Microstructure Development during Multi-Pass Hot Deformation Including Partial Recrystallization

During reversible break-down hot rolling, recrystallization can take place during inter-pass annealing and even after the final pass, because the material is kept at high temperature throughout the process. A partially recrystallized microstructure is quite often obtained during inter-pass annealing and is the starting state for the subsequent rolling pass. This is in particular the case when fine particles are already present or have just precipitated in the microstructure. The particles can pin the moving grain boundaries in the hot deformed material with low energy stored during deformation. To transfer the partly recrystallized microstructure from the recrystallization model to the deformation model for simulation of the subsequent pass, a new link method has been developed for multi-pass through-process modeling. This method allows a direct data flow between the deformation model GIA-3IVM+ and the recrystallization model CORe. This new model framework was used to simulate the microstructure evolution during break-down hot rolling of precipitates containing alloy AA6016. The predicted microstructure agrees well with experimental observations.

Influence of Mg Content, Grain Size and Strain Rate on Mechanical Properties and DSA Behavior of Al-Mg Alloys Processed by ECAP and Annealing

Ultrafine-grained (UFG) binary Al-xMg (x=1, 5 and 7 wt %) alloys were processed by equal channel angular pressing (ECAP) at room temperature via route Bc combined with inter-pass annealing. The effects of Mg content, grain size and strain rate on mechanical properties and dynamic strain aging (DSA) behaviour of the Al-Mg alloys upon tensile testing at room temperature were studied. An increase in Mg content from 5 to 7 wt % leads to a pronounced increase in strength and uniform elongation in both the as-homogenized and as-ECAP Al-Mg alloys. Thereby, the Al-7Mg alloy, either prior to or after ECAP processing, possess significantly higher strength and comparable or even higher uniform elongation than the more dilute Al-Mg alloys. However, the as-ECAP Al-Mg alloys exhibit significantly higher strength but little work hardening and hence rather limited uniform elongation. In general, decreasing grain size leads to significant increase in strength while dramatic decrease in ductility. Moreover, DSA serration amplitudes increase with reducing grain size in the micrometer range. However, the UFG Al-Mg alloys exhibit much less DSA effect than the micrometer scaled grain size counterparts, i.e. probably due to the high dislocation densities and special grain boundary features in the UFG materials. Also, the Al-Mg alloys, especially those with a UFG structure, exhibit higher strength and ductility at lower strain rate than at higher strain rate, due mainly to enhanced DSA effect and hence work hardening at a lower strain rate.

Achieve high ductility and strength in an Al–Mg alloy by severe plastic deformation combined with inter-pass annealing

Metallic materials processed by severe plastic deformation generally possess very high strength but a rather low uniform elongation, usually <5%. In the present Al–7Mg alloy processed by room temperature equal channel angular pressing combined with inter-pass annealing, impressive combination of high ductility (elongation~14.5%) and high strength (UTS~600MPa) was achieved. The high ductility was rationalized by an enhanced work hardening originated from the pronounced dynamic strain aging effect and a bimodal grain structure. The high strength was mainly due to a prominent grain refinement and high dislocation density.

Metadynamic recrystallization behavior of AZ61 magnesium alloy

Metadynamic recrystallization (MDRX) behavior of AZ61 magnesium alloy and its effects on flow behavior and microstructure evolution have been investigated in this study. Towards this end, a set of double-hit hot compression tests was conducted under strain rate of 0.1s−1 at 400°C. To differentiate the static and metadynamic recrystallization dominant strain regions, the first stage of deformation was carried out up to the different pre-strains with a constant inter-pass annealing time of 200s. The results indicated that the MDRX is predominant recrystallization mechanism where the pre-strains are higher than 0.35. Furthermore, to investigate the influence of MDRX on subsequent flow behavior and the related microstructure, an elaborated inter-pass annealing treatment was executed employing a range of inter-pass annealing time (2–500s). The results show that the progress of MDRX leads to an increase in the flow stress as well as the rate of work hardening encountered in the subsequent deformation. Additionally, the microstructural examinations confirm that the observed hardening phenomenon is a consequence of grain growth evolved from MDRX and its direct effect on the onset of dynamic recrystallization at the second stage of deformation.

Multipass cold drawing of magnesium alloy minitubes for biodegradable vascular stents

Magnesium alloys possess highly limited room-temperature formabilities. This presents a technological barrier to the fabrication of minitubes for biodegradable vascular stents. The research was aimed at developing precision forming technology to fabricate ZM21 magnesium alloy minitubes with a refined microstructure. A multipass cold drawing process with a moving mandrel was successfully developed to convert seamless hollow billets through five passes of cold drawing and an interpass annealing treatment into minitubes with an outside diameter of 2.9mm and a wall thickness of 0.217mm, ready for laser cutting into vascular stents. It was found that a cumulative reduction in cross-section area as much as 32% could be applied to the material without causing fracture. However, a further reduction in cross-section area required annealing at 300°C for 1h to change a twinned microstructure into a recrystallized grain structure and to regain formability. The interpass annealing treatment after the fourth pass led to a reduction in drawing force by 22%, in comparison with the drawing force at the fourth pass of drawing. The variations in the outside diameter and wall thickness of the minitubes could be kept within 5 and 12μm, respectively. Further research is directed toward improvements in dimensional precisions.

Effects of Different Modes of Hot Cross-Rolling in 7010 Aluminum Alloy: Part I. Evolution of Microstructure and Texture

The current study describes the evolution of microstructure and texture in an Al-Zn-Mg-Cu-Zr-based 7010 aluminum alloy during different modes of hot cross-rolling. Processing of materials involves three different types of cross-rolling. The development of texture in the one-step cross-rolled specimen can be described by a typical β-fiber having the maximum intensity near Copper (Cu) component. However, for the multi-step cross-rolled specimens, the as-rolled texture is mainly characterized by a strong rotated-Brass (Bs) component and a very weak rotated-cube component. Subsequent heat treatment leads to sharpening of the major texture component (i.e., rotated-Bs). Furthermore, the main texture components in all the specimens appear to be significantly rotated in a complex manner away from their ideal positions because of non-symmetric deformations in the two rolling directions. Detailed microstructural study indicates that dynamic recovery is the dominant restoration mechanism operating during the hot rolling. During subsequent heat treatment, static recovery dominates, while a combination of particle-stimulated nucleation (PSN) and strain-induced grain boundary migration (SIBM) causes partial recrystallization of the grain structure. The aforementioned restoration mechanisms play an important role in the development of texture components. The textural development in the current study could be attributed to the combined effects of (a) cross-rolling and inter-pass annealing that reduce the intensity of Cu component after each successive pass, (b) recrystallization resistance of Bs-oriented grains, (c) stability of Bs texture under cross-rolling, and (d) Zener pinning by Al3Zr dispersoids.