Metal Flow Behavior Research Articles

Study of Hot Deformation Behavior Using Phenomenological Based Constitutive Model for Austenitic Stainless Steel 316

A reliable and accurate prediction of flow behavior of metals considering the coupled effects of strain, strain rate and temperature is crucial in understanding the workability of the metal and optimizing process parameters for hot forming process.In this study, two phenomenological based constitutive models namely; modified Fields-Backofen (m-FB) and Khan-Huang-Liang (KHL) have been developed for flow stress prediction of Austenitic Stainless Steel (ASS) 316. Uniaxial tensile tests have been performed in the temperature ranges of 150°C– 600°C and strain rate ranges of 0.0001– 0.01s-1 for analyzing the deformation behavior. The prediction capability of these models have been verified with experimental data using various statistical measures such as correlation coefficient (R), average absolute error (Δ) and its standard deviation (S). Based on the analysis of statistical measures revealed Khan-Huang-Liang (KHL) model has good agreement with experimental flow stress data.

Analysis of Metal Flow Behavior and Residual Stress Formation of Complex Functional Profiles under High‐Speed Cold Roll‐Beating

To obtain a good surface layer performance of the complex functional profile during the high‐speed cold roll‐beating forming process, this paper analyzed the metal plastic flow and residual stress‐formed mechanism by using a theoretical model of the metal flow and residual stress generation. By using simulation software, the cold roll‐beating forming process of a spline shaft was simulated and analyzed. The metal flow and residual stress formation law in the motion were researched. In a practical experiment, the changes in the grains in the spline tooth profile section and the residual stress distribution on the tooth profile were studied. A microcorrespondence relationship was established between the metal plastic flow and the residual stress generation. The conclusions indicate that the rate at which the metal flow decreases changes gradually at different metal layers. The residual stress value is directly related to the plastic flow difference. As the roll‐beating speed increases, the uneven degree of plastic deformation at the workpiece surface increases, and the residual stress in the tooth profile is generally greater. At the same roll‐beating speed, the rate change trend of the metal flow decreases gradually from the surface to the inner layer and from the dedendum to the addendum. The residual stress distribution on the surface of the tooth profile decreases from the dedendum to the addendum. These findings provide a basis and guidance for the controlled use of residual stress, obtaining better surface layer quality in the high‐speed cold roll‐beating process of the complex functional profile.

Microstructure Evolution and Deformation Behavior of AZ31 Magnesium Alloy During Alternate Forward Extrusion

The paper presents a new extrusion method, alternate forward extrusion, in which the punch was replaced with double-split structures so as to achieve the grain refinement for material near the interface of double-split structures. The results showed that the unique loading mode made metal flow sequence and behavior significantly changed during alternate forward extrusion. The additional shear deformation produced by the double-split punch structures resulted in a refining effect on the microstructure of the blank, which was then further refined during flow through the die orifice owing to shear deformation. Compared with the conventional extrusion, the recrystallization process in the alternate forward extrusion process produced grains that were smaller and more homogeneous in size. The recrystallization process was more abundant, and the dislocation density was significantly increased. It can be concluded that the alternate forward extrusion process could achieve fine-grained strengthening, which provided technical support and scientific guidance for the engineering application of magnesium alloy extrusion forming technology.

A comparative study on Johnson-Cook and modified Johnson-Cook constitutive material model to predict the dynamic behavior laser additive manufacturing FeCr alloy

An accurate prediction of flow behavior of metals considering the combined effects of strain, strain rate and temperature is essential for understanding flow response of metals, and a key requirement for numerical modeling and simulation of machining. Thus, the effects of high strain rate and elevated temperature on the deformation behaviors of FeCr alloy specimen, obtained by laser additive manufacturing (LAM), were investigated by Split Hopkinson Pressure Bar tests with the strain rates of 1000–8000 s−1 and temperature range of 20–800 °C. Strain rate softening effect was found at high strain rate and discussed. Based on the experimental results, the Johnson-Cook model and modified Johnson-Cook model were established for the LAM FeCr alloy. The predictions of these constitutive models were compared using statistical measures like correlation coefficient and absolute average error. Analysis of statistical measures reveals that J-C model has more deviation from the experimental values. Whereas, the predictions of modified J-C model are very close to the experimental results, which is better model for predicting the dynamic behavior of the LAM FeCr alloy.

Optimal gating system design for investment casting of 17-4PH stainless steel enclosed impeller by numerical simulation and experimental verification

The impellers of centrifugal pumps are highly susceptible to the latent damage of corrosion and cavitation after long periods of transporting chemical fluids. To enhance the structural integrity and effective lifespan of impellers, this study applied mold flow analysis to the design of gating systems for 17-4PH stainless steel enclosed impellers. Our objective was to eliminate shrinkage and porous defects common in investment casting. We adopted various bottom, side, and top pouring systems with different pouring parameters to examine the behavior of the molten metal flow and solidification in the mold cavity. We designed a pressurized gating system with specific gating ratio to achieve a stable flow velocity at in-gates. Physical sensors preset in the interior of the cavity were also used to detect thermodynamic behavior and analyze phase changes during casting simulations. The probability of shrinkage defect formation was assessed using the retained melt modulus (RMM) and the Niyama criterion. Experiments and nondestructive inspections show that optimizing the design of the gating system prevented surface shrinkage and interior defects. The improvements also reduced post-processing time and costs, increased yields, and enhanced casting quality.

Influence of extrusion speed on the seam weld quality in the porthole die extrusion of AZ31 magnesium alloy tube

In the present work, an accurate 3D-FE model for simulating the extrusion process of AZ31 magnesium alloy tube was established based on the DEFORM 3D software package. The metal flow behavior and formation process of weld seam in the porthole die were revealed. The evolutions of temperature, velocity, and effective stress during the whole extrusion cycle were investigated. The K welding criteria was utilized to evaluate the influence of extrusion speed on the seam weld quality. Extrusion experiments were carried out at the extrusion speed ranging from 0.25 to 4 mm/s on an 800-t extrusion press. The simulation results show that the temperature distribution in the workpiece was not homogeneous. As the extrusion process proceeded, the greater the temperature gradient of workpiece was. The maximum temperature appeared at the bearing region. The dead metal zones existed at the corner between the container and the die face and between the bottom and the sidewall of welding chamber. The effective stress near the bearing and welding chamber was maximum, followed by the inlet ports of porthole die, and the minimum value was located in the container. As extrusion speed increased, the temperature, welding pressure, and effective stress on the welding plane increased simultaneously. The calculated K value decreased rapidly when extrusion speed increased from 0.5 to 2 mm/s and then reduced slowly when larger than 2 mm/s. Extrusion speed had a negative influence on the seam weld quality. The criteria exhibited a good predicting capability as compared with the experimental results. The optimum extrusion speed was about 0.5 mm/s for the extrusion of AZ31 magnesium alloy tube at the billet temperature of 400 °C on the 800-t extrusion press.

PERKEMBANGAN CACAT RONGGA PADA PROSES EKSTRUSI

Cavity defect occurs in an extruded product when the remaining billet reaches a certain thickness. This defect can create serious problem in the quality of products because it produces waste material and requires an extra operation for finishing. The requirement of wasting a certain portion of the product becomes undesirable especially for a relatively short, headed product. Since the occurrence of this defect is troublesome in industrial practice, it is important to study the conditions leading to the cavity formation. By using this prediction, it may be possible to stop the process before the cavity starts to develop. In this work, a new measuring technique to monitor the development of cavity defect in forward extrusion was studied. This technique uses electrical contact principle, which detects the contact between metal pins and the leading face of the billet. When the cavity develops, a void or gap between the punch and the billet interface develops, and breaks the electrical current of the circuit. The experiment of extrusion process was undertaken in parallel with a finite element analysis (FEA) using ABAQUS software. It was shown that at a certain distance the cavity defect was revealed. In this work, the cavity formation was investigated by a variation of die reduction. The influence of coefficient of friction, punch speed, and billet thickness was also observed. This study concentrated on improving the understanding of the behavior of metal flow and the development mechanism of cavity formation with the aim of providing general guidance for the extrusion industry in minimizing the cavity defect formation of the extruded product.

Study on the welding quality in the porthole die extrusion process of aluminum alloy profiles

As a typical plastic deformation process with high productivity, porthole die extrusion process has been widely used to produce a variety of aluminium alloy profiles. For the profiles extruded with porthole dies, the welding quality of longitudinal weld seams is the key factor to determine their performance. In this study, different types of welding defects from millimetre scale to nanometre scale were found during porthole die extrusion by varying extrusion die structure and process parameters. Furthermore, the formation mechanisms of these welding defects were studied from both the macro and micro levels. Finally, the factors influencing the welding quality of extruded profiles were summarized. It was found that the welding quality is determined by metal flow behaviour, solid state bonding process and microstructural evolution. The formation of the un-bonded macro-defects in the extruded profile is attributed to improper metal flow behaviour, while the formation of the micro-voids on the bonding interface is caused by the insufficient solid state bonding. In addition, the metal in the welding zone usually has the particular microstructure different from the other zones, since it experiences the special thermal and mechanical history.

Finite element analysis of die offsetin two-turn incremental ECAP

Severe plastic deformation (SPD) is the process used to produce ultra fine grain(UFG), metals are characterised by improved mechanical properties which make them suitable for advanced structural applications. Apart from various SPD techniques, two turn incremental equal channel angular pressing (ECAP), S-shape channel discussed in this paper to double the effective strain in a single press. Unfortunately, processed UFG metals are rarely used in industrial applications due to lack of advancement in ECAP technique. Analysis of ECAP technique by changing die geometry and process parameter helps to produce defect free UFG metals. ECAP channel continuously pressed a very long billet in which repetitions accumulate large plastic strain. Two turn ECAP is useful to increase the productivity. However, any changes in channel angle, corner angle as well as offset length are cause for variation in mechanical properties of metals. In order to check the die suitability of two-turn ECAP with variation of effective strain, punch load, a FEA simulation is carried out and the suitable tool geometry and process kinematics are established. Die channel sat two different offset length are used to study the flow behaviour of metal through 900 channel angel. The mode of material flow is the same as in the well established classical L-shape ECAP process following route C pass, while continuous character and improved productivity suggest that new S-shape ECAP process might be suitable for grain refinement of metals on an industrial scale.

Modeling of mass flow behavior of hot rolled low alloy steel based on combined Johnson-Cook and Zerilli-Armstrong model

Accuracy and reliability of numerical simulation of hot rolling processes are dependent on a suitable material model, which describes metal flow behavior. In the present study, Gleeble hot compression tests were carried out at high temperatures up to 1300 °C and varying strain rates for a medium carbon micro-alloyed steel. Based on experimental results, a Johnson-Cook model (JC) and a Zerilli-Armstrong (ZA) model were developed and exhibited limitation in characterizing complex viscoplastic behavior. A combined JC and ZA model was introduced and calibrated through investigation of strain hardening, and the coupled effect of temperature and strain rate. Results showed that the combined JC and ZA model demonstrated better agreement with experimental data. An explicit subroutine of the proposed material model was coded and implemented into a finite element model simulating the industrial hot rolling. The simulated rolling torque was in good agreement with experimental data. Plastic strain and stress distributions were recorded to investigate nonlinear mass flow behavior of the steel bar. Results showed that the maximum equivalent plastic strain occurred at 45° and 135° areas of the cross section. Stress increased with decreasing temperature, and the corresponding rolling torque was also increased. Due to the extent of plastic deformation, rolling speed had limited influence on the internal stress of the bar, but the relative rolling torque was increased due to strain rate hardening.

Modeling of steady state hot flow behavior of API-X70 microalloyed steel using genetic algorithm and design of experiments

API-X70 microalloyed steel is one of the most conventional materials that has been used to produce the pipelines used in oil and gas industry. This steel is produced by thermo mechanical processing (TMP). Prediction of steady state hot flow behavior of metals during TMP, for design of its forming process is of great importance. In this research, flow curves of API-X70 were obtained using hot torsion test at temperature range of 950–1150°C and strain rates of 0.001–3s−1. Genetic algorithm (GA) was used to find parameters of steady state stress semi-empirical model in the way that minimizing the difference between experimental data and model output. The optimal combination of GA parameters were chosen by Taguchi design of experiments(DOE) method in order to increase efficiency of GA. Accuracy of developed model to predict flow stress in steady state region was evaluated through statistical methods. Results showed a good agreement between developed model and experimental data with R2=0.99 and this model can predict steady state flow stress well.

Influences of temperature and grain size on the material deformability in microforming process

This paper investigated the influences of temperature and grain size on the deformability of pure copper in micro compression process. Based on the dislocation theory, a constitutive model was proposed taking into account the influences of forming temperature, Hall-Petch relationship and surface layer model. Vacuum heat treatment was employed to obtain various grain sizes of cylindrical workpieces, and then laser heating method was applied to heat workpieces during microforming process. Finite element (FE) simulation was also performed, with simulated values agreed well with the experimental results in terms of metal flow stress. Both the FE simulated and experimental results indicate that forming temperature and grain size have a significant influence on the accuracy of the produced product shape and metal flow behaviour in microforming due to the inhomogeneity within the deformed material. The mechanical behaviour of the material is found to be more sensitive to forming temperature when the workpieces are constituted of fine grains.

Finite element analysis of die angle at constant offset length in two-turn ECAP

Severe plastic deformation (SPD) is the process used to produce smaller grain in the range of 1-pm. These materials have been classified as equiaxed ultrafine grain (UFG) metals. These metals are characterised by high defect density and improved mechanical properties which make them suitable for advanced structural applications. From all available SPD techniques, two turn incremental equal channel angular pressing (ECAP), S-shape channel discussed in this paper to double the effective strain in single press. Analysis of ECAP technique by changing die geometry and process parameter helps to produce defect free UFG metals. ECAP channel continuously pressed a very long billet in which repetitions accumulate large plastic strain. Two turn ECAP is useful to increase the productivity. However, any changes in channel angle, corner angle as well as offset length are cause for variation in mechanical properties of metals. In order to check the die suitability of two-turn ECAP with variation of die angle a finite element analysis (FEA) simulation is carried out and the suitable tool geometry with process kinematics are established. ECAP channels at two different channel angles are used to study the flow behaviour of metal through 15mm offset length. The mode of material flow is the same as in the well-established classical L-shape ECAP process following route C pass, while continuous character and improved productivity suggest that new S-shape ECAP process might be suitable for grain refinement of metals as well as minimization of deformation load on an industrial scale.

3-dimensional numerical analysis of friction stir welding of copper and aluminum

A time dependent Eulerian thermal/material flow model of friction stir welding was developed and applied to the dissimilar joining of pure copper and aluminum 1050-H16 alloy to investigate the maximum penetration of base metals. Thermal and material flow analysis was done with the assumed velocity field in the stir zone and considering a thermal source of energy obtained from the both Coulomb type of friction and the loss of shear stress in a non-Newtonian viscous behavior of metal flow. The developed model was used to estimate temperature gradient and penetration of material under three different conditions of tool offset and compared with the experimental results. The model shows that the penetration of the base metals is closely related to tool offset. In all of the cases, the metal fixed in the advancing side is copper. Nevertheless, when considering tool offset in the copper side and also when considering tool offset in the aluminum side, penetrating metals are copper and aluminum, respectively. Also, the model shows that the maximum temperature achieved in the base metals significantly depends on the tool offset.

Investigation on metal flow and forming load of bi-metal gear hot forging process

Aiming at the lightweight of gears, this article presents the hot forging process of a bi-metal gear fabricated by two metals of steel (tooth ring material) and aluminum alloy (core material). The process was studied by the finite element (FE) simulation and forming experiment. The FE model of the bi-metal gear forging process was established based on the symmetry of the gear and the structure of forging die by using the software Deform-3D. The hot forging experiment of the bi-metal gear was carried out to verify the accuracy of the FE simulation by the comparison of the metal flow behavior. Metal flow of three stages, namely upsetting, gap and tooth filling, and corner filling was analyzed in the hot forging process. Effects of three aspects including the gap between the core and ring, the height difference between the core and ring, and the ring thickness on the metal flow and forming load were investigated. The results of numerical simulation showed that different gaps and different height differences between the core and ring influence the relative flow of two metals and different ring thicknesses significantly influence the structure of the bi-metal gear; large gap and small height difference between the core and ring and small ring thickness can reduce the forming load.

Analysis of longitudinal weld seam defects and investigation of solid-state bonding criteria in porthole die extrusion process of aluminum alloy profiles

In porthole die extrusion process of aluminum alloy profiles, the formation of longitudinal weld seams (L-seams) is inevitable. Analysis of L-seam defects and investigation of solid-state bonding criteria are important issues for practical production. In this study, a set of modular porthole extrusion dies with different depths of welding chambers were designed and manufactured. The profiles extruded with different depths of welding chambers were obtained by performing extrusion experiments.The welding quality of extruded profiles was characterized by means of microstructure observation, tensile test and fracture analysis. The true stress-strain curves of homogenized AA6063 aluminum alloy with various strain rates and deformation temperatures were obtained by means of isothermal hot compression test. Three-dimensional transient numerical simulation models of the porthole die extrusion processes were established. Flow behaviors of aluminum alloy in porthole die extrusion process were investigated, and the formation processes of L-seams and their defects were revealed. The solid-state bonding processes of metal particles were traced and their welding paths were determined. Finally, based on the plastic deformation and diffusion mechanisms for closure behaviors of micro voids on contact interfaces, a new dimensionless solid-state bonding criterion related to stress triaxiality, effective strain rate, temperature and contact time was proposed. The bonding quality of L-seams of extruded profiles without any macro defect was predicted successfully by using this new criterion. In addition, it was found that the formation of a macro hole in the profile extruded with a shallow welding chamber is attributed to metal flow behavior, and does not relate to solid-state bonding process.

A Nonlinear Viscous Model for Sn-Whisker Growth

Based on the mechanism of the grain boundary fluid flow, a nonlinear viscous model for the growth of Sn-whiskers is proposed. This model consists of two units, one with a stress exponent of one and one with a stress exponent of n −1. By letting one of the constants be zero in the model, the constitutive relationship reduces to a linear flow relation or a power–law relation, representing the flow behavior of various metals. Closed-form solutions for the growth behavior of a whisker are derived, which can be used to predict the whisker growth and the stress evolution.

Influence of shielding gas on undercutting formation in gas metal arc welding

Two sets of vision-based sensing systems were established to investigate the behavior of droplet transfer, arc shape and molten metal flow by varying shielding gas mixtures. Orthogonal experiments and analysis of variance indicated that the CO2 content in shielding gas was the major welding parameter compared with welding current and arc voltage, which affected the size of undercutting defects. Results demonstrated that the suppression of undercutting defects was mainly caused by the reduction in backward flow velocity of molten metal in weld pool due to the lowering in droplet impact and arc force when CO2 content increased from 10% to 100%. Non-dimensional fitting method was used to establish the relationships between welding parameters and undercutting defects. It showed that appropriate pulse welding current could suppress undercutting and spatter with high CO2 content in shielding gas.

Low pressure sand casting of ultrasonically degassed AlSi7Mg0.3 alloy: Modelling and experimental validation of mould filling

The effect of combining smooth mould cavity filling and ultrasonic degassing by using a low pressure sand casting process with US melt treatment on the microstructure and mechanical properties of AlSi7Mg0.3 alloy was investigated. Computational fluid dynamics (CFD) modelling and experimental validation was used to study the liquid metal flow behaviour and oxide film defect distribution for different filling rates, using two pressurization curves (p–t). The effect of ultrasonic degassing on the resultant microstructure (morphology and presence of porosities and inclusions) was studied for 650 and 700°C melt degassing temperatures. Results show that ultrasonic degassing and mould filling velocities below 0.5m/s improve the alloy microstructure and lead to high mechanical properties.

Effect of pass schedule and groove design on the metal deformation of 38MnVS6 in the initial passes of hot rolling

The deformation behaviour of a hot rolled micro-alloyed steel bar of grade 38MnVS6 was examined using an FEM model during the initial passes in a blooming mill, as a function of three different pass schedules, roll groove depth, collar taper angle and corner radius. The simulations predicted the effective strain penetration, load, torque, fish tail billet end shapes, and metal flow behaviour at a chosen temperature, mill rpm and draft. The model predictions were validated for typical groove geometry and a typical pass schedule. Lower collar taper angle, lower corner radius and higher depth of groove in hot rolling enabled achievement of higher strain penetration, higher mill load and lower fish tail formation. The present study establishes the capability of the model to improve the internal quality of the rolled billet as measured by effective strain which was corroborated to the rolled bar macrostructure and microstructure. The model enables yield improvement by the choice of draft to minimise fish tail losses. The surface quality is improved by the ability to avoid fin formation that occurs at certain conditions of rolling. Thus, the groove geometry, roll pass schedule and rolling mill parameters and temperature can be optimised for best product quality and yield.