Modelling microstructure evolution during equal channel angular pressing of magnesium alloys using cellular automata finite element method

Equal channel angular pressing (ECAP) is one of the most popular methods of obtaining ultrafine grained (UFG) metals. However, only relatively short billets can be processed by ECAP due to force limitation. A solution to this problem could be recently developed incremental variant of the process, so called I-ECAP. Since I-ECAP can deal with continuous billets, it can be widely used in industrial practice. Recently, many researchers have put an effort to obtain UFG magnesium alloys which, due to their low density, are very promising materials for weight and energy saving applications. It was reported that microstructure refinement during ECAP is controlled by dynamic recrystallization and the final mean grain size is dependent mainly on processing temperature. In this work, cellular automata finite element (CAFE) method was used to investigate microstructure evolution during four passes of ECAP and its incremental variant I-ECAP. The cellular automata space dynamics is determined by transition rules, whose parameters are strain, strain rate and temperature obtained from FE simulation. An internal state variable model describes total dislocation density evolution and transfers this information to the CA space. The developed CAFE model calculates the mean grain size and generates a digital microstructure prediction after processing, which could be useful to estimate mechanical properties of the produced UFG metal. Fitting and verification of the model was done using the experimental results obtained from I-ECAP of an AZ31B magnesium alloy and the data derived from literature. The CAFE simulation results were verified for the temperature range 200-250 °C and strain rate 0.01-0.5 s-1; good agreement with experimental data was achieved

Determination of friction factor by ring compression testing and FE analysis

The goal of this study was to examine performance of various lubricants for aluminium alloy AA5083. Conventional ring compression tests were conducted at 200 °C. Samples were compressed to 50% of the initial height with a constant ram velocity 0.5 mm/s using a servo-controlled hydraulic press. The optimization procedure was implemented in self-developed software to identify friction factors from experiments. The application launches remotely finite element (FE) simulations of ring compression with a changing friction factor until a difference between experiment and numerical prediction of the internal diameter of the sample is smaller than 0.5%. FE simulations were run using Forge3 commercial software. The obtained friction factor quantitatively describes performance of a lubricant and can be used as an input parameter in FE simulation of other processes. It was shown that application of calcium aluminate conversion coating as pre-lubrication surface treatment reduced friction factor from 0.28 to 0.18 for MoS2 paste. It was also revealed that commercially available graphite-based lubricant with an addition of calcium fluoride applied on conversion coating of calcium aluminate had even lower friction factor of 0.1

In situ analysis of the influence of twinning on the strain hardening rate and fracture mechanism in AZ31B magnesium alloy

The influence of twinning on the strain hardening rate and fracture mechanism in AZ31B magnesium alloy was studied in this work by in situ microstructural analysis during tensile testing in a chamber of scanning electron microscope. Three types of samples used in this study were obtained by (1) extrusion (as-supplied), (2) I-ECAP and (3) I-ECAP followed by side upsetting. Microstructures, textures and mechanical properties were examined after each processing step. An analytical equation was used to describe flow stress curves of the samples which exhibited various modes of deformation (1) only by slip, (2) dominated by tensile twinning followed by slip and (3) dominated by contraction twinning followed by slip. It was shown that tensile twinning increases strain hardening rate, while the opposite is observed for contraction twinning. The effective Schmid factors for slip in volumes deformed by tensile and contraction twinning were determined in this work using modelling approach as 0.215 and 0.45, respectively. Contraction twinning was also revealed to be responsible for an earlier fracture of the extruded sample subjected to tension, since microcracking was shown explicitly to be initiated within twins

The origin of fracture in the I-ECAP of AZ31B magnesium alloy

Magnesium alloys are very promising materials for weight-saving structural applications due to their low density, comparing to other metals and alloys currently used. However, they usually suffer from a limited formability at room temperature and low strength. In order to overcome those issues, processes of severe plastic deformation (SPD) can be utilized to improve mechanical properties, but processing parameters need to be selected with care to avoid fracture, very often observed for those alloys during forming. In the current work, the AZ31B magnesium alloy was subjected to SPD by incremental equal-channel angular pressing (I-ECAP) at temperatures varying from 398 K to 525 K (125 °C to 250 °C) to determine the window of allowable processing parameters. The effects of initial grain size and billet rotation scheme on the occurrence of fracture during I-ECAP were investigated. The initial grain size ranged from 1.5 to 40 µm and the I-ECAP routes tested were A, BC, and C. Microstructures of the processed billets were characterized before and after I-ECAP. It was found that a fine-grained and homogenous microstructure was required to avoid fracture at low temperatures. Strain localization arising from a stress relaxation within recrystallized regions, namely twins and fine-grained zones, was shown to be responsible for the generation of microcracks. Based on the I-ECAP experiments and available literature data for ECAP, a power law between the initial grain size and processing conditions, described by a Zener–Hollomon parameter, has been proposed. Finally, processing by various routes at 473 K (200 °C) revealed that route A was less prone to fracture than routes BC and C

The role of microstructure and texture in controlling mechanical properties of AZ31B magnesium alloy processed by I-ECAP

Abstract Mechanical properties of AZ31B magnesium alloy were modified in this work by various processing routes of incremental equal channel angular pressing (I-ECAP) followed by heat treatment. Possible strategies for improving ductility and strength of the alloy were investigated. Processing by routes A and BC showed that texture plays predominant role in controlling mechanical properties at room temperature. Four passes of I-ECAP by route C followed by annealing enhanced ductility up to 0.35 of true strain. It was found that tensile twinning was important in accommodating strain during tensile testing, which resulted in a very good hardening behaviour. The yield strength was improved to 300 MPa by refining grain size to 0.8 µm in I-ECAP at 150 °C. The obtained structure and properties were shown to be stable up to 150 °C. True strain at fracture was increased to 0.2 after annealing at 150 °C without lowering strength

The effect of initial grain size on formability of AZ31B magnesium alloy during I-ECAP

The goal of this work was to investigate formability of AZ31B magnesium alloy during incremental equal channel angular pressing (I-ECAP). Square billets were processed using different routes of I-ECAP at temperatures varying from 125 °C to 250 °C. The billets were obtained from commercially available coarse-grained, hot-extruded rod and fine-grained, hot-rolled plate. A strong influence of the initial microstructure on processing temperature was reported. Fine-grained samples were successfully processed at 200 °C, while coarse-grained ones must have been heated up to 250 °C to avoid fracture. A gradual temperature decrease with subsequent passes allowed successful pressing at 150 °C. Processing using various routes of I-ECAP showed that a billet rotation before the last pass had strong influence on mechanical properties. The results of experiments were plotted on the diagram of allowable processing temperature for AZ31B. It was found that the relation between the minimum temperature in I-ECAP and the initial grain size could be described by a logarithmic equation

FE simulation of magnesium alloy microstructure evolution in tension

Finite element (FE) simulation of microstructure evolution was performed in the current work. The flow stress curve for FE simulation was obtained from tensile test whichwas carried out at room temperature. Samples were machined from a rolled sheet of AZ31B magnesium alloy. Simplified micro scale models were developed in order to study the influence of the round inclusion and twin-like inclusion on the material fracture behaviour. It was shown that fracture initiation point is dependent on the yield stress of the inclusion. Finally, polycrystalline model including ten grains of similar sizes was developed. The triple junction points were recognized as sites of fracture initiation

Improving mechanical properties of magnesium alloy by incremental equal angular pressing (ECAP)

This presentation looks at improving mechanical properties of magnesium alloy by incremental equal angular pressing (ECAP

Microstructure evolution in AZ31B magnesium alloy subjected to tension

Incremental Equal Channel Angular Pressing (I-ECAP) is an incremental severe plastic deformation (SPD) process, which can be used to process continuous bars, plates and sheets. In the current study, commercially available AZ31B magnesium alloy bars were subjected to four passes of I-ECAP using route BC; the process was conducted at 250 °C since fracture was reported at lower temperatures. The obtained fine-grained bars were subjected to side upsetting at 200 °C to produce sheets. The significant height reduction, from 10 mm to 2 mm, was obtained without fracture. Then, flat tensile samples were machined from as-received coarse-grained bars and fine-grained, strongly textured sheets. The samples were polished and etched to enable observation of microstructure evolution during testing. Tensile testing was conducted in the SEM chamber at room temperature with constant tool velocity. It was shown that earlier fracture of coarse-grained samples, comparing to fine-grained ones, can be attributed to twinning as twin-size voids were observed in the fracture zone. Avoidance of twinning was identified as the most important factor for ductility enhancement of fine-grained and strongly textured sheets