Achieving high ductility and strength in magnesium alloy through cryogenic-hot forming

Magnesium alloys are the lightest structural alloys and have attracted substantial research attention in the past two decades. However, their mechanical properties, including ductility and strength, are limited after forming due to the formation of coarse grains and strong texture. This study proposes and proves a new cryogenic-hot forming process concept. Cryogenic deformation is imposed before the hot deformation. The effect of the cryogenic step has been compared with a conventional direct hot deformation process. The mechanical properties, microstructure,and texture of both the novel and conventional process routes have been compared. The cryogenic-hot deformed sample exhibits the highest ductility and fracture strength(ultimate tensile strength: 321 MPa, ductility: 21%) due to effective grain refinement and texture weakening by cryogenically formed twin-twin interaction induced recrystallisation. The proposed cryogenic-hot forming process can be a potential innovative manufacturing method for producing high-performance magnesium components.

Microstructure and mechanical properties of curved AZ31 magnesium alloy profiles produced by differential velocity sideways extrusion

Lightweight curved profiles are widely utilised in the transportation industry considering the increasing need for improving aerodynamic efficiency,aesthetics and cutting emissions.In this paper,curved AZ31 Mg alloy profiles were manufactured in one operation by a novel process,differential velocity sideways extrusion(DVSE),in which two opposed rams were used.Effects of extrusion temperature and velocity(strain rate) on curvature,microstructure,and mechanical properties of the formed profiles were examined.Profile curvature was found to be more readily controlled by the velocity ratio of the bottom ram v2to the top ram v1,whereas extrusion temperature(T=250,300,350℃)and extrusion velocity(v_(1)=0.1,1 mm/s) slightly affect curvature for a given velocity ratio.A homogeneous microstructure with equiaxed grains(~4.5 μm) resulted from dynamic recrystallisation(DRX),was observed after DVSE(v_(2)/v_(1)=1/2) at 300 ℃ and v_(1)=0.1 mm/s,where the initial billet had an average grain size of ~25 um.Increasing extrusion temperature leads to grain growth(~5 μm) at 350 ℃ and v_(1)=0.1 mm/s.DRX is incomplete at the relatively low temperature of 250℃(v_(1)=0.1 mm/s),and higher strain rate with v1=1mm/s(T=300℃),resulting in inhomogeneous bi-modal necklace pattern grains ranging in size around 1-25 μm for the former and 2-20μm for the latter.Grain refinement is attributed to DRX during the severe plastic deformation(SPD) arising in DVSE,and initiates at the prior boundaries of coarse grains in a necklace-like manner.Compared with the billet,micro-hardness and ultimate tensile strength of the profiles have been enhanced,which is compatible with grain refinement.Also,an obvious increase in tensile ductility was found.However,yield strength slightly decreases except for the complete DRXed case(300℃,v_(1)=0.1 mm/s),where a slightly higher value was found,indicating strengthening by grain refinement is greater than softening caused by texture modification.The initial billet had a strong basal texture wherein the {0002} basal plane is oriented parallel to the extrusion direction(’hard’ orientation),while DVSE results in the profiles having weak basal textures and the {0002} basal plane oriented ~5-10° to the extrusion direction(i.e.towards the orientation for easier slip).This significantly modified texture contributes to the softening of the profiles in the extrusion direction,in which tensile tests were performed,and the related elongation improvement.

The evolution of coarse grains and its effects on weakened basal texture during annealing of a cold-rolled magnesium AZ31B alloy

The nucleation,grain growth of 34 coarse grains during annealing were tracked using a quasi-in-situ EBSD method.These 34 coarse grains had different orientations and most grains were non-basal orientated.No preferable grain growth or special types of grain boundaries were identified.Only 9 coarse grains nucleated from deformed grain boundaries due to initial large grain size and limited grain boundary volume fraction.The main nucleation site of 34 coarse grains was dislocation cells or subgrains in deformed grain interiors.Their recrystallisation behaviour can be illustrated by abnormal subgrain growth(As GG)rarely reported in Mg alloys.The coarse basal grains showed no growth advantage in terms of grain size or number over other non-basal grains,leading to a weak basal texture in AZ31B alloy.

Equal-Channel Angular Pressing as a New Processing to Control the Microstructure and Texture of Metallic Sheets

The extended band structures of as-cold-rolled high Cr steel sheets are recrystallisation-resistant, and tend to become aggregates of the so-called grain colonies as a partially recovered state after final annealing. Such band structures diminish formability and become origin of the so-called ridging. A novel processing will be shown here, which involves strain-path change by introducing one-pass ECAP prior to cold-rolling, and facilitates recrystallisation. Indeed, the recrystallisation temperature was reduced by 100℃, compared with cold-rolling alone imposing an equivalent strain. Grain- scale microshear bands introduced during one-pass ECAP perturbed the banded structures in post-ECAP cold-rolling and enhanced the recrystallisation at the final annealing.

Influence of the Accumulative Roll Bonding Process Severity on the Microstructure and Superplastic Behaviour of 7075 Al Alloy

The 7075 Al alloy was processed by accumulative roll bonding （ARB） at 350 ℃ using 2：1, 3：1 and 4：1 thickness reductions per pass （Rp） up to 8, 6 and 3 passes, respectively. Microstructural examinations of the processed samples revealed that ARB leads to a microstructure composed of equiaxed crystallites with a mean size generally lower than 500 nm. It was found that, due to both the stored energy through- out the processing and the particle pinning effect, the alloy is affected by discontinuous recrystallisation during the inter-pass heating stages, the precise microstructural evolution being dependent on Rp. Me- chanical testing of the ARBed samples revealed that the main active deformation mechanism in the ARBed samples in the temperature range from 250 to 350 ℃ at intermediate and high strain rates is grain bound- ary sliding, the superplastic properties being determined by both the microstructure after ARB and its thermal stability.

Multi-Pass Simulation of Heavy Plate Rolling Including Intermediate Forced Cooling

Thermomechanical Controlled Processing (TMCP) including accelerated cooling after the final hot rolling pass is a well-established technology,widely applied in HSLA steel plate production.However,there are still certain limitations,especially for thicker plate.The rolling schedule includes a long holding period (HP) after the roughing stage to allow the temperature to fall sufficiently for optimised TMCP during finishing.Intermediate Forced Cooling (IFC) applied during the HP can increase productivity by decreasing the required hold time,can restrict austenite grain growth,and can also improve the subsequent strain penetration in thick plate with further metallurgical benefits.Multi-pass plane strain compression (PSC) tests have been performed on the thermomechanical compression (TMC) machine at Sheffield University including different severities of IFC.Clearly it is impossible to simulate all aspects of the temperature and strain gradients present in thick plates in laboratory specimens,and most of the tests were conducted at temperatures and strains calculated by Finite Element modelling as relevant to specific positions through the plate thickness.However,some aspects of the gradients were addressed with tests using cold platens.The results have indeed shown that IFC can shorten the HP and reduce austenite grain growth and its variation across thick plate.