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Magnesium-zinc-calcium (Mg-Zn-Ca) alloys as a biomaterial have attracted much attention recently, owing to their excellent biocompatibility, similar mechanical properties to natural bone, and biodegradable properties. Despite the numerous advantages of MgZnCa alloys, the rapid degradation of magnesium proved challenging as the implant in unable to retain its structural integrity for a sufficient duration of time. For metallic glasses, the capability to produce a bulk sample that is sufficiently large for useful applications have been far less successful owing to challenging processing parameters that are required for rapid cooling. In this study, Mg65Zn30Ca5 melt-spun ribbons were produced using melt-spinning followed by spark plasma sintering under high pressure (60 MPa) at different temperatures (130–170 °C) to provide an insight into the consolidation, mechanical, and corrosion behavior. Microstructural interfaces were characterized using scanning electron microscopy while the thermal stability of the amorphous phase was characterized using differential scanning calorimetry and X-ray diffraction. Here, pellets with 10 mm diameter and 10 mm height with a complete amorphous structure were achieved at a sintering temperature of 150 °C with densification as high at ~98%. Sintering at higher temperatures, while achieving higher densification, resulted in the presence of nano-crystallites. The mechanical properties were characterized using microhardness and compression tests. The hardness values of the sintered products were relatively higher to those containing crystallite phases while the ultimate compressive strength increased with increasing sintering temperature. Bio-corrosion properties were characterized via electrochemical testing with PBS as the electrolyte at 37 °C. The corrosion results suggest that the sintered samples have a significantly improved corrosion resistance as compared to as-cast samples. More notably, SPS150 (samples sintered at 150 °C) exhibited the best corrosion resistance (35× compared to as-cast in the context of corrosion current density), owing to its single-phase amorphous nature. This study clearly shows the potential of spark plasma sintering in consolidating amorphous ribbons to near-full density bulk pellets with high corrosion resistance for bio-applications.

We present a combined experimental and numerical study that provides the understanding of deformation mechanisms and stresses in Mg AZ31B alloy across scales, from macro-scale (Type I) to micro- (inter-granular, Type II) and nano-scale (intra-granular, Type III). The combination of in situ synchrotron X-ray diffraction (XRD), in situ electron backscattered diffraction (EBSD) and crystal plasticity finite element (CPFE) modeling of crystal slip and twinning/detwinning was employed. The crystal rotation observed directly in the XRD and EBSD experiments revealed the onset and completion of the twinning/detwinning processes during in situ cyclic compression-tension loading. It also allowed reliable calibration of the key model parameters, in particular critical resolved shear stress (CRSS) of detwinning. The validation of the model was performed at distinct different scales corresponding to all stress types. Direct comparison with the data from the loading device provided the confirmation of the model validity in terms of correct description of the macroscopic stress-strain response (Type I stresses). The calibration led to the CRSS detwinning value of 23 MPa, and twinning of 46.5 MPa. At the inter-granular micro-scale (Type II stresses), the model satisfactorily predicted the transition between different plastic deformation modes (slip, twinning and detwinning), as confirmed by the comparison with peak intensities in XRD experiments. For the intra-granular (nano-scale) Type III stresses, it was concluded that the model was also valid at the level of statistical description (rather than local behavior). Namely, it has not been possible to predict correctly the real morphology of the twins observed in EBSD experiments.

In this work, an indirect extrusion process was designed to manufacture a stainless steel reinforced aluminium composite. The design utilized a feeding mandrel to prevent the deformation of the stainless steel wire and to facilitate the embedding of the wire within the aluminium alloy during extrusion. By using this method, a 12 mm diameter 6061 aluminium alloy embedded with a 2 mm diameter 304 stainless steel wire was successfully fabricated. Energy-dispersive X-ray spectroscopy revealed that aluminium and iron elements had diffused into the interfacial layer between the aluminium/stainless steel to induce some degree of metallurgical bonding. The yield strength, tensile strength, and elongation to failure of the extruded composite after solution and ageing heat treatment were 343.8 ± 5.5 MPa, 405.3 ± 5.1 MPa, and 18.0 ± 1.3%, respectively. As compared to extruded aluminium, yield strength, ultimate tensile strength, and elongation to failure were improved by 41.8%, 31.6%, and 6.3%, respectively. This result suggests that the composite material has better strength and toughness as compared to aluminium alone.

Narrow channels or ribs can be commonly found in sheet metal products. These narrow with high length-to-width aspect ratio channels are often integrated into sheet metal panels for functional features such as stiffeners or guide rails of various topographies, i.e. straight line or composite of curves. This work focused on the development of Single Point Incremental Forming (SPIF) as a cost-efficient forming alternative to stamping or embossing of these features in short-series production. These channels present unique forming challenges, i.e. effects of channel spline geometry, varying cross section, end geometries on part formability, and geometrical accuracy. SPIF of SS316L 0.5-mm-thick narrow channels of three different geometries with a maximum width of 20 mm and its length-to-width aspect ratio larger than 5, i.e. (1) straight channel, (2) taper channel, and (3) curvature channel, was investigated. Based on the experimental results, 5 mm dia. carbide tool and 0.1 mm step-down size were recommended to optimize SPIF of these narrow channels. The effects of the different part features and their interactions with SPIF forming parameters on part formability and deviations are presented and analyzed for further development of die-less narrow channel forming for sheet metal industry.

The uniaxial tensile flow behavior of an AZ31 magnesium alloy was examined after processing by severe plastic deformation (SPD) and short time post-annealing. The SPD process of constrained groove pressing (CGP) was performed to three cycles at temperatures of 503–453 K. After CGP, the alloy was further annealed at 473 K for a short duration of 3 min. By using this procedure, a bimodal fine-grained microstructure with an average grain size of 3.5 μm was produced. In the room temperature tensile tests, the yield strength and elongation to failure were improved by 34% and 11%, respectively as compared with the initial material before SPD. The tensile deformation behavior and deformation mechanism of the alloy were further examined at moderately high temperatures from 373 to 523 K under initial strain rates from 1 × 10 −3 to 1.0 s −1 . Examination of the microstructures near the fractured tips indicate that dynamic recrystallization could occur in the fine-grained alloy at a relatively low temperature of 423 K and the maximum strain rate sensitivity of 0.14 was measured at 523 K. The apparent activation energy tested at 423–473 K with initial strain rates of 1 × 10 −3 -1.0 s −1 was estimated to be 123.6 kJ mol −1 , which is close to the value for lattice self-diffusion of magnesium. The results suggest that climb-controlled dislocation creep is the dominant deformation process associated with lattice diffusion. Despite the relatively fine-grained microstructure, superplastic-like ductility was not observed because of the low thermal stability of the microstructure at elevated temperatures, and is attributed to the high stored strain energy in the microstructure imparted by the CGP. Nevertheless, enhanced ductility was measured at 523 K and 1 × 10 −3 s −1 where a maximum elongation to failure was 100 ± 2.5%, and dynamic recovery was observed as the main restoration process.

The introduction of an overload or underload within a constant amplitude loading fatigue test leads to a retardation or acceleration of the Fatigue Crack Growth Rate (FCGR). The understanding of the causes of these effects is essential in the context of variable amplitude fatigue loading, since in principle any loading history can be represented as a sequence of overloads and underloads. In the case of overload, along with some other minor causes, the residual stress changes at the crack tip and crack closure behind the tip can be thought of as the main factors that affect the fatigue crack growth rate. Whilst this has been recognised and accepted for many decades, controversy persists regarding the relative significance and presence of these two effects, and consensus is yet to emerge. The effect of crack closure, when the baseline loading ratio is high enough, can be inhibited so that the main cause of retardation becomes doubtless the residual stress present ahead the crack tip. In the present paper we report our attempt to deconvolve the contributions of crack closure and residual stress on crack retardation following an overload. To accomplish this task we analyse the results of fatigue tests run at two baseline load ratios, namely R=0.1 and R=0.7. At the load ratio of R=0.7 the crack closure effect is not operative, as confirmed by Digital Image Correlation analysis of the crack flanks close to the tip, and post mortem fractographic analysis of crack surfaces. Therefore, for R=0.7 the compressive residual stress region created by the overload ahead of the crack tip is the sole mechanism causing crack retardation. Therefore, for R=0.7 the focus must be placed entirely on the strain field around the crack tip. To this end, line profiles along the crack bisector of elastic strain in the crack opening direction were collected at several stages of crack propagation past the overload using in situ Synchrotron X-ray Powder Diffraction (SXRPD) technique. By performing comparison between the two loading conditions (R=0.7 and R=0.1), information was extracted regarding the role of residual stress alone, and then, by subtracting this effect for the R=0.1 sample, for crack closure alone. To enable this analysis, we propose a introducing the concept of equivalent effective stress intensity factor range, ∆ K eq , eff proposed by Walker. Afterwards, the SIF range reduction ratio, β , which represents the “knock down” factor with respect to the steady state growth was assessed. It is in terms of these newly introduced parameters that the magnitude and extent of the overload-induced crack growth rate retardation can be plotted, fitted and decomposed into closure and residual stress effects, respectively. It is concluded that although the residual stress effect is present at all values of the load ratio R, its effect is relatively short-lived, whilst the closure effect that is dominant at low values of R causes longer range retardation.

In this work, the effect of fine grain sizes on the mechanical and bio-corrosion properties of AZ31 magnesium alloy was studied. Bio-corrosion refers to the accelerated degradation of metal within the human body. Fine-grained (~1.5 µm) AZ31 was obtained through Severe Plastic Deformation (SPD) via three cycles of Constrained Groove-Pressing (CGP) under elevated temperature. The effects of CGP and post-annealing (at 473 K for 15 and 30 min) on bio-corrosion were preliminarily investigated by potentiodynamic polarization measurements and constant immersion tests. Results obtained show that the as-processed samples with annealing exhibited improvements in yield strength and ductility while the bio-corrosion rate in Hank’s solution remains fairly similar during the early stage. ASTAR (Agency for Sci., Tech. and Research, S’pore) Published version

Fatigue Crack Growth Rate (FCGR) is altered by a single anomalous load exceeding cyclic maximum (Overload) or compressive load below cyclic minimum (Underload). The authors study fatigue crack acceleration due to a single compressive Underload using residual stress mapping (by synchrotron XRD) and crack closure analysis (by DIC). The relative influence and duration of these two principal causes of FCGR alteration are revealed. Validated FEA model is used for parametric analysis of the effect of baseline cyclic loading ratio and magnitude of Underload on the cyclic J-integral.

Diffraction data were collected using synchrotron X-ray scattering (sXRD) and electron back-scattered diffraction (EBSD) during in situ tensile-compressive deformation of Mg alloy AZ31B dogbone samples. The onset and evolution of twinning and detwinning were monitored based on intensity changes in sXRD 2D scattering patterns (which also provided average elastic strain values through the calculation of orientation-specific lattice spacing changes), and EBSD, that revealed the micro-scale grain morphology changes. The observations were interpreted and analysed with the help of crystal plasticity finite element modelling (CP-FEM), as reported in the published article (https://doi.org/10.1016/j.ijplas.2019.02.018). Keywords: Twinning-detwinning, Crystal plasticity, EBSD, Synchrotron XRD

In this work, a thermomechanical treatment using severe plastic deformation (SPD) via a modified constrained groove pressing (CGP) technique and short duration post-annealing was introduced to improve the mechanical properties of AZ31 magnesium alloy plate. This method is effective for producing fine-grained microstructure in thin magnesium plates and also enhances the mechanical strength without sacrificing the fatigue and corrosion properties. In this process, the magnesium plate is subjected to repeated deformation at elevated temperature using a specified pressing sequence designed to accelerate the grain refinement process. This technique has unique advantages over other principal SPD techniques such as Equal Channel Angular Pressing (ECAP) and High-Pressure Torsion (HPT). Firstly, it circumvents the difficulties faced in ECAP and HPT such as buckling and size limitation, respectively. Secondly, it is possible to implement the pressing procedures into rolling process which will benefit continuous processing of long magnesium strip. Thirdly, CGP was demonstrated as an effective grain refinement process for improving the microstructure of squeeze-casted AZ31 alloy. This opens up to the possibility of producing fine-grained magnesium plate with improved properties by using lower cost magnesium preform prepared by casting. Investigation of the deformation and temperature sequences in CGP process revealed interesting dependency of these conditions on strain, texture development and microstructure. Finite element (FE) simulation showed that the total strain imparted in the material was affected by the constraints imposed on the material by the die just before shear deformation and also by the compressive and stretching of the material during die filling. Experimental studies showed that decreasing processing temperature limit static recrystallization, recovery and grain growth and resulted in the development of micro and sub-micron grains. As such, effective choice of deformation and temperature sequences could positively influence the total strain, texture development and grain refinement efficiency. Electron backscatter diffraction (EBSD) analysis of the microstructure evolution during modified CGP revealed that dislocation boundaries of initially low angle grain boundaries (LAGB) were developed in the grain interiors and at the vicinity of the grain boundaries. This evidence suggests that mechanisms of both rotational dynamic recrystallization (rDRX) and continuous dynamic recrystallization (cDRX) were responsible for the efficient grain refinement and was unique to this process. The microstructure stability in the strain-induced fine and ultrafine grain boundaries prepared by modified CGP was investigated by annealing and interpreted by grain growth kinetic equations. From this study, it was revealed that despite the lower microstructure stability, as indicated by the low activation energy, the average grain sizes can be maintained considerably smaller than the parent material at shorter annealing. The hot tensile behaviour and deformation mechanism of the modified CGP processed alloy with short annealing was investigated at elevated temperatures and under various strain rates. From the room temperature tensile tests, the yield strength and elongation to failure were significantly improved by 34% and 11%, respectively. The results of the strain rate sensitivity and activation energy suggested that climb-controlled dislocation creep is the dominant deformation process associated with lattice diffusion. Enhanced ductility was measured at 523 K and 1×10-3 s−1 where a maximum elongation to failure of 100±2.5% was obtained, and dynamic recovery was observed as the main restoration process. From the biodegradation studies, it was established that the average corrosion rate was marginally higher after modified CGP. This reduction in corrosion resistance can be attributed to the non-uniformity of the microstructure; some grains contain high dislocation and some grains were almost without dislocation. This introduced strain energy difference and stress differences which worsen the corrosion properties despite having a higher area of grain boundaries. It was further established from the study of the fatigue crack growth rate that the CGP process does not significantly alter the fatigue performance. The above studies suggest that CGP enhances the yield strength of magnesium alloy without significantly affecting the fatigue and corrosion performance and that combining CGP treatment with coating technologies are viable approaches to improve the performances of magnesium material for biomedical implants. New insights into the microstructure evolution and mechanical properties of severe plastically deformed AZ31 magnesium alloy through modified CGP have been gained through this study. From an application point of view, this study has demonstrated very clearly the potential of the CGP process for preparing fine-grained AZ31 plate with enhanced mechanical strength and improved ductility by adopting the appropriate heat treatment strategy. The process can be made continuous through adaptation of its design into rolling process. It is reasonably believed that this thermomechanical processing route can be established for a wide range of commercial application in the foreseeable future, particularly for improving the mechanical properties of biodegradable magnesium alloy. Doctor of Philosophy (MAE)

In the last decade or so, severe plastic deformation (SPD) has been shown to be an effective means for the production of fine grained microstructure in magnesium alloys at relatively low temperatures while altering the material texture to improve its formability. SPD of sheet geometry is particularly attractive in order to encourage wider use of magnesium alloys for lightweight applications. In this study, the microstructure evolution of magnesium alloy AZ31 sheet in the groove pressing process was examined at different processing conditions. The average grain size of the material was reduced from 13.3 μm to 1.9 μm after 4 cycles of deformation under the condition of progressive decreasing temperature and specimen rotation between cycles. The final microstructure was observed to be homogenous in both the transverse and rolling directions consisting of fine grains of 0.6-1 μm and a small fraction of coarser grains of 3-5 μm. The average hardness of the material was increased from 62 to 91 Hv with a uniform distribution along the thickness direction. This study has demonstrated that groove pressing is a highly effective method for production of fine grained microstructures in magnesium sheet using the technique of progressive decreasing temperature while alternate specimen rotation is necessary to prevent premature failure of the material.

In this study, an AZ31 magnesium alloy plate was processed by constrained groove pressing (CGP) under three deformation cycles at temperatures from 503 to 448 K. The process resulted in a homogeneous fine grain microstructure with an average grain size of 1.8 μm. The as-processed microstructure contained a high fraction of low-angle grain boundaries (LAGB) of subgrains and dislocation boundaries that remained in the structure due to incomplete dynamic recovery and recrystallization. The material's yield strength was found to have increased from 175 to 242 MPa and with a significant weakening of its initial basal texture. The microstructure stability of the CGP-processed material was further investigated by isothermal annealing at temperature from 473 to 623 K and for different time. Abnormal grain growth was observed at 623 K, and this was associated with an increased in nonbasal grains at the expense of basal grains. The effect of annealing temperature and time on the grain growth kinetics was interpreted by using the grain growth equation, Dn+D0n=kt, and Arrhenius equation, k=k0 exp (−(Q/RT)). The activation energy (Q) was estimated to be 27.8 kJ/mol which was significantly lower than the activation energy for lattice self-diffusion (QL = 135 kJ/mol) and grain boundary diffusion (Qgb = 92 kJ/mol) in pure magnesium. The result shows that grain growth is rapid but average grain size still remained smaller than the as-received material, especially at the shorter annealing time.

In the present study, the phenomenon of diametric growth in the forward flow forming process has been studied through finite element simulation and experimental investigation. Implicit integration scheme was employed in the simulation to achieve good accuracy for diametric growth prediction. The simulation results were in good agreement with the experiments for both types of materials considered, the aluminum alloy AA6061 and stainless steel SS304L. The residual stresses in the flow-formed parts were measured using x-ray diffraction to validate the model as well as provide the explanation for the diametric growth behavior. Based on the numerical and experimental results, an empirical function was proposed here to describe the amount of diametric growth in the flow-formed parts, which can be used as a predictive tool for dimension control in the flow forming process. © 2013 Springer-Verlag London.

Conventional superplastic forming has been applied in automotive and aerospace industries for a few decades. Recently, superplastic forming combined with mechanical pre-forming process has been reported to be capable of forming non-superplastic AA5083 at 400 °C to a surface expansion of 200 % [1]. In this paper, finite element modeling (FEM) was used to develop the combined forming process by using the non-superplastic material AA5083-O. The simulation follows the experimental sequence and was divided into two phases (mechanical pre-forming and superplastic forming). A conventional creep equation based on tensile test data was adopted as a material model for the simulation. The pressure cycle and forming time was simulated according to the actual process route. The thickness distributions obtained from simulation validated the capability of the model to be used for this case. The influence of different parameters, such as holder force, friction, and punch depth was investigated by comparing the final sheet thickness and level of material draw-in. It was found that the punch depth played a significant effect on the uniformity of thickness distribution, from which a more uniform formed part can be obtained by using the punch with higher depth during mechanical pre-forming phase.

There are growing demands from the precision engineering industries for components that are net or near net shape while striving to reduce material wastage during the manufacturing process. This study investigates the spin formability of liquid forged Al-6061 pre-forms into near net shape cylindrical components with high aspect ratios. The liquid forging process offers very significant materials savings by providing pre-forms for the spin forming process. The end products are near net shape components with very good mechanical properties. Results from this study indicate that liquid forging is able to supply Al-6061 pre-forms, otherwise machined from solid bars, which are sufficiently formable for spin forming. The microstructure of a spin formed liquid forged pre-form is also indiscernible from that of a fully machined pre-form from extruded bar stock. As such, spin forming of liquid forged pre-forms has the potential to reduce material wastage and hence production costs.

The effect of forming temperature and feed rate on the flaring of AZ31 magnesium cups was investigated in the warm flow-forming process. Flaring is a common phenomenon in flow-forming process whereby the diameter of the work-piece is observed to increase gradually along the flow-formed path. In this study, forward flow-forming process was performed on a cup-shaped work-piece by using two diametrically opposite rollers (approach angle of 20) at a constant thickness reduction. To study the effects on flaring, experiments were performed at feed rates of 0.2mm/rev, 0.4mm/rev, and 0.8mm/rev and at forming temperatures between 50~300°C. Experimental results indicated that feed rate was the most significant parameter influencing flaring while temperature mainly improve the formability of the material. It is believed that a higher feed rate resulted in an increase in metal flow in the circumferential direction because of higher mean circumferential strain and hence increases flaring. Flaring was also observed to be most severe at forming temperature of 300°C and feed rate of 0.8 mm/rev due to the excessive metal flow in the cirucmferential direction of the test-piece. This study clarifies the relationship between feed rate and the extend of flaring and that forming temperature has little influence on flaring from temperature range of 50°C to 250°C.

Superplastic forming has been considered as an attractive process in the automotive and aerospace industries. However, the disadvantages of slow forming rate, high-temperature requirement, poor thickness distribution, and expensive base material have hindered its widespread use for high production volume. In this paper, the non-superplastic grade of 5083 aluminum alloy (AA5083) sheets with thickness of 3 mm was employed in a superplastic-like forming process, which is a combination of drawing (mechanical pre-forming) and superplastic forming (blow forming). Experimental trials were conducted to verify the possibility of improving the forming rate and lowering the process temperature. The blank was firstly pre-formed during the mechanical pre-forming phase. As a result, some part of material along the flange area was introduced inside the deformation cavity in advance of the blow forming phase. Secondly, argon gas was applied on the sheet, which would be deformed to come into contact with the inner die surface at the end of pressure cycle. It took only 8 min for the blow forming phase, and the process achieved an almost fully formed part at 400°C. The minimum thickness occurred at the inward corners, and the maximum thinning of the formed part was 54%. Grain growth and cavitation were found from the microstructure observations.

High cost and poor room temperature formability of magnesium alloy sheet are the key factors that limit its application as a feedstock material for press forming. Production of Mg plates by squeeze casting with further processing by severe plastic deformation (SPD) is a potential method to reduce cost and improve formability. In this study, AZ31 Mg plate of dimension 96×96×4 mm was successfully produced by squeeze casting, using a novel melt transfer technique, at a forging force and speed of 180 Ton and 200 mm/sec respectively. The effect of severe plastic deformation (SPD) using groove pressing on the mechanical properties of squeeze-casted Mg plate after partial homogenization was subsequently investigated. Observation of the microstructure after two cycles of groove pressing, under decreasing temperature from 543K to 493K, shows a significant grain refinement from 39 to 4.7 µm. The Vickers hardness increased by approximately 25% from 56 to 74.1 which suggests an improvement in mechanical strength as a...

Processing of wrought magnesium alloy sheet by severe plastic deformation (SPD) for improving its formability is attractive so as to encourage the wider applications of the alloy. In this study, SPD by means of groove pressing (GP) was carried out on AZ31B-O magnesium sheet at different deformation paths and temperatures in order to investigate its effects on microstructure, textures, and mechanical properties. GP using an orthogonal pressing at every cycle and at a progressively decreasing temperature was found to be effectively for manufacturing fine-grained microstructures with an average grain diameter of 1.9 μm. The final microstructures were homogenous in both the transverse direction (TD) and rolling direction (RD) and consisting of fine grains of 0.6–1 μm with a small fraction of coarser grains of 3–5 μm. The increase in yield stress (YS), ultimate tensile strength (UTS), and tensile elongation after annealing was 12%, −2.9%, and 25.6%, respectively, in the RD. A good balance between fine-grained microstructure and ductility was obtained by the pressing at a constant processing temperature of 473 K. In this pressing path, average grain diameter was 3.8 μm and the increased in YS, UTS, and tensile elongation before annealing was 21.9%, 9.1%, and 19.8%, respectively, in the RD. It was shown that the texture modification combined with fine-grained microstructure contributed to the overall improvement in ductility.

High energy 2D X-ray powder diffraction experiments are widely used for lattice strain measurement. The 2D to 1D conversion of diffraction patterns is a necessary step used to prepare the data for full pattern refinement, but is inefficient when only peak centre position information is required for lattice strain evaluation. The multi-step conversion process is likely to lead to increased errors associated with the 'caking' (radial binning) or fitting procedures. A new method is proposed here that relies on direct Digital Image Correlation analysis of 2D X-ray powder diffraction patterns (XRD-DIC, for short). As an example of using XRD-DIC, residual strain values along the central line in a Mg AZ31B alloy bar after 3-point bending are calculated by using both XRD-DIC and the conventional 'caking' with fitting procedures. Comparison of the results for strain values in different azimuthal angles demonstrates excellent agreement between the two methods. The principal strains and directions are calculated using multiple direction strain data, leading to full in-plane strain evaluation. It is therefore concluded that XRD-DIC provides a reliable and robust method for strain evaluation from 2D powder diffraction data. The XRD-DIC approach simplifies the analysis process by skipping 2D to 1D conversion, and opens new possibilities for robust 2D powder diffraction data analysis for full in-plane strain evaluation.

Conventional superplastic forming (SPF) is normally conducted at a slow forming rate (10−3 s−1 or slower) and high temperatures (typically 773 K or higher for aluminum alloys), which is not attractive for wider applications. Recently, SPF has revived as an attractive process in the automotive and aerospace industries, especially for forming of excellent precision, large, and complex-shaped work pieces. A hybrid superplastic forming process has exploited the usage of superplastic forming for faster times, lower temperatures, and also for use by non-superplastic materials by designing a process that combines stamping and gas blow forming in one operation to establish an energy-saving technology. With recent developments in these areas, the light-weight, yet inexpensive aluminum alloys are promising candidates for green manufacturing. In comparison with the conventional SPF process, the forming time has been shortened (typically from 30 min to 8 min), and the forming temperature has been lowered down (from 773 K to 673 K). Furthermore, as a greener manufacturing process, it uses the non-superplastic grade materials, which is more compatible with existing manufacturing processes with less material preparation time and expense cost.

Conventional superplastic forming (SPF) has been modified to increase the productivity and reduce some of the drawbacks, such as high forming temperature and high percentage thinning, to suit the automotive industries. One of the modifications was to combine between the conventional SPF and the use of a mechanical preformed blank to form the non‐superplastic grade aluminum alloy (AA5083‐O). The requirement of high temperature usually results in microstructural defects during forming process. In this paper, finite element modeling was adopted to investigate the superplastic‐like forming process using the non‐isothermal heating system. In the simulation, two phases (mechanical pre‐forming and gas blow for ming) of the process were conducted under different temperatures, where the material was mechanically drawn into the die cavity at 200° C in the first phase, and it formed with gas pressure applied at a global temperature increasing from 400° C to 500° C. Because of the non‐isothermal heating of material, ...

Within the class of lightweight metallic materials, magnesium alloys are gaining popularity mainly thanks to abundant supply and high specific strength. A weakness of Mg alloys is their poor formability at room temperature. For this reason, in recent years thermo-mechanical treatments have often been sought that would improve this aspect. One particular way towards the combination of better formability (ductility) and strength is through grain refinement by means of Constrained Groove Pressing (CGP). The use of a pair of matching dice having groove-like geometry, coupled with controlled process temperature, allows promoting microstructural refinement and achieving sub-micron grain size in the processed plates. The purpose of the present study was to characterise the resulting fine-grained material in plate form in terms its fatigue fracture behaviour. Compact tension samples were machined and subjected to cyclic tensile loading to monitor fracture propagation and extract their Fatigue Crack Growth Rate (FCGR) behaviour as a function of the applied crack tip Stress Intensity Factor range. Firstly, as a reference sample an as-received material plate was tested at the loading ratio R=0.1. Subsequently, CGP-processed fine-grained material was tested at R=0.1 and R=0.7, and subjected to anomalous load that included overload (50% increase in the maximum load) and, in the specific case of R=0.1, underload (reversal of the sign of minimum load from tension to compression). These studies provided the critically needed input for the development of new approaches to the evaluation of the apparent fracture resistance of the material processed by CGP under variable amplitude loading (i.e. overload and underload) essential for accurate fatigue life prediction for a broad variety of applications. By combining the Walker model accounting for the mean stress effect and a modified Wheeler model for crack growth retardation due to the application of a single overload, a new predictive approach was formulated.

The bulk forming process of sheet metal has a high potential for the near-net–shape forming of metal components with thin wall thickness. However, the main critical challenges in this process are the high forming pressure and the difficulties in the control of metal flow in lateral or radial direction which are caused by the compression of thin workpiece in its thickness direction. In the present study, the multi-stage cold forging process was introduced to address the current challenges in the forming of thin-walled metal components with multiple wall thicknesses. The case studies were conducted through FE simulation and experiments and the results showed that this process is effective to reduce the forming load and improve the radial metal flow in the forming of two kinds of thin aluminium alloy components with multi thicknesses.