Hot Plane Strain Compression Tests Research Articles

Development of a Computer Code for the Interpretation of Results of Hot Plane Strain Compression Tests.

In the present work an investigation of constitutive equations is made to obtain good fit to experimental data and ensure proper extrapolation out of experimental range of plane strain compression tests to low and high values of Zener–Hollomon parameter, which are required in finite element modelling. A computer program for optimisation of rheological parameters of material was developed. Optimisation using experimental data for two ultra low carbon steels was made and the results are compared with measured stress–strain curves. The obtained constitutive equations were used in the finite element model and comparison between experimental and computed displacement–force curves is made. The influence of inhomogeneity of strain rate, strain and temperatures distribution during compression tests on flow curves is illustrated.

Modelling of microstructure evolution in hot deformation

This paper reviews various approaches to the modelling of microstructure evolution in hot deformation, for the purpose of predicting the flow stress during deformation or for predicting the subsequent annealing behaviour. Two contrasting approaches are discussed, and illustrated for the example of hot plane–strain compression testing of Al–Mg alloy. These approaches are (i) physically based state variable models, in which the microstructure and property evolution is modelled explicitly; and (ii) advanced statistical methods, for linking processing conditions empirically to properties, or to annealing rate and final microstructure. The state variable models illustrate some general features of microstructure modelling and the level of experimental work that goes with it. Of particular importance are the accuracy of the data used to calibrate or validate a model, the implications that this makes on the volume of data needed, and the viable level of detail in the model that can realistically be verified. Various sensitivity analyses will be used to illustrate the need for a balanced view of model and experiment if a credible predictive capability is to emerge. The statistical methods provide no physical insight, but, nonetheless, warrant further consideration for hot–deformation problems. They potentially provide a means to optimize time–consuming experimental work, and may provide useful predictive capabilities for industry rather sooner than can be expected from complex physically based modelling.

Interpretation of hot plane strain compression testing of aluminium specimens

Plane strain compression (PSC) testing is now generally accepted as one of the most reliable methods for the generation of flow stress data and microstructural investigation of thermomechanical processing. It has been known for some time that extremely reproducible measurements may be made across different laboratories if a standardised procedure is used. However, particular care must be taken with both the experimental procedure and the interpretation of the measured force–displacement data. The present paper reports investigations that have built on previous work and looked further at the effects of spread of the specimen and friction. In deriving reliable flow stress data, the importance of tool and specimen geometry and consideration of the effects of lubrication and friction are clearly demonstrated. Furthermore, the paper demonstrates the current status of the work by presenting the algorithms behind new software that has been developed for interpretation of raw force–displacement data in a logical and consistent way.

Nucleation and growth of recrystallisation in Al–1Mg during thermomechanical processing

A procedure based on quantitative optical microscopy has been used to investigate nucleation and growth of recrystallisation in an Al–1Mg alloy. Hot rolling and plane strain compression tests were employed for this purpose, providing strains within the range 0·2–2·2 and strain rates within the range 2·5–4·9 s-1, while annealing was carried out at 400°C in all cases. The nucleation kinetics is not site saturated, particularly at low strains, the major discrepancy being observed early during recrystallisaton. However, in practice site saturation is a reasonable approximation. During recrystallisation, the average growth rate decreases continuously. Although concurrent recovery in the unrecrystallised fraction is expected to lead to a decrease in growth rate, the magnitude of the reduction is such that a non-uniform distribution in stored energy is expected to play a significant role.

Constitutive equations for high temperature flow stress of aluminium alloys

Constitutive equations for the relationship between stress, strain, strain rate, and temperature are an essential input for modelling thermomechanical processing. Hot plane strain compression testing was used to deform commercial purity aluminium, Al-1Mn, and Al-1Mg alloys at temperatures of 300, 400, and 500°C and equivalent strain rates of 0.25, 2.5, and 25 s−1, to an equivalent strain of 2. Flow stress data obtained from these tests were analysed using the Sah et al. and hyperbolic sine forms of relationships. Values of constants in the constitutive equations were obtained and were shown to provide an accurate description of the experimental stress-strain curves, including the effect of temperature rise due to deformational heating and the effect of changing strain rate. The application and limitations of the relationships are discussed.

Constitutive equations for high temperature flow stress of aluminium alloys

Constitutive equations for the relationship between stress, strain, strain rate, and temperature are an essential input for modelling thermomechanical processing. Hot plane strain compression testing was used to deform commercial purity aluminium, Al-1Mn, and Al-1Mg alloys at temperatures of 300, 400, and 500°C and equivalent strain rates of 0.25, 2.5, and 25 s−1, to an equivalent strain of 2. Flow stress data obtained from these tests were analysed using the Sah et al. and hyperbolic sine forms of relationships. Values of constants in the constitutive equations were obtained and were shown to provide an accurate description of the experimental stress-strain curves, including the effect of temperature rise due to deformational heating and the effect of changing strain rate. The application and limitations of the relationships are discussed.

Ajuste de las curvas tensión-deformación a alta temperatura de un acero microaleado Nb-Ti

A series of hot plane strain compression tests have been conducted on a Nb-Ti microalloyed steel in a Servotest machine on samples of approx. 10 mm in thickness. Two sets of tests at strain rates of 5 s-1 and 15 s-1 were used and monitored the stress-strain behavior at deformation temperatures varying from 850 to 1,025 °C. The experimental data were corrected for friction, deviation from plane strain compression (PSC) conditions and adiabatic rise of temperature, to give the isothermal equivalent tensile stress-strain curves. Thereafter these curves have been modeled by two types of equations depending on the softening mechanism operating during the deformation process and calculated the value of the coefficients in the equation by the application of a Taylor series around an initial point close to the solution. Finally, by the application of the least squared method, the previous values are compared to the experimental ones in order to determine an optimized value of the coefficients in the equations which model the stress-strain behaviour of the steel.

Hot Plane Strain Compression Testing of Aluminum Alloys

Abstract Plane strain compression is a versatile laboratory testing method for simulating industrial hot working operations such as plate and strip rolling. The deformation can be closely controlled to the required conditions of temperature and strain rate, and high strains can be achieved without instability. However, for accurate determination of flow stress, care must be taken with experimental procedure and the interpretation of the measured force-displacement data. This paper reports the results of work carried out in three laboratories on samples of the same cast of Al-1% Mn alloy. In particular, the effects of spread and friction are analyzed as a function of initial specimen geometry. When consistent procedures are used it is shown that excellent reproducibility of flow stress data is obtained between laboratories. In deriving constitutive relationships, the importance of considering the effects of lubricant films, of deformational heating, and of heat transfer are clearly demonstrated.

Modelling the plane strain compression test to obtain constitutive equations of aluminium alloys

Hot plane strain compression tests on 1050, 1198, 3003 and 3004 aluminium alloys have been conducted. Based on these experiments and on a set of internal type constitutive equations for hot working, the values of the parameters in the constitutive functions are determined. The constitutive equations proposed here, with the constitutive functions and material parameters associated, accurately reproduce the basic tests. The procedure used to fit the material parameters is improved, in comparison with classical slip line analysis, by using a finite element modelling of the plane strain compression test. It is demonstrated that accurate plane strain or three-dimensional large strain finite element analysis can be used to correct the friction and lateral spread effects. Furthermore, it is demonstrated from comparison with the experimental observations that microstructural parameters can be accurately determined from numerical modelling. The constitutive equations and finite element procedure proposed here can be useful for obtaining an improved analysis of hot rolling of aluminium alloys.

Confirming mathematical metalworking models by comparison with microstructures

AbstractReference microstructures produced from hot plane strain compression tests have been compared with as extruded microstructures for a powdered aluminium 2014 alloy. This comparison enables an independent check to be made of a mathematical model of the changes in billet temperature during extrusion, which are difficult to monitor experimentally. Powder product microstructures that are more uniform than those produced from equivalent solid feedstock have been attributed to a narrower process temperature range even though the same range of billet temperatures was used. Within this range microstructural changes have been related to those in reference samples produced under closely controlled test conditions.MST/2004

Hot plane strain compression testing of partially consolidated powder compacts: H. Shi and A. Greasley (University of Sheffield, UK), Powder Metallurgy, Vol 35, No 4, 1992, 269–274

Hot plane strain compression testing of partially consolidated powder compacts: H. Shi and A. Greasley (University of Sheffield, UK), Powder Metallurgy, Vol 35, No 4, 1992, 269–274

Hot Plane Strain Compression Testing of Partially Consolidated Powder Compacts

Hot plane strain compression testing is shown to be more suitable than hot torsion testing in providing flow stress data for partially consolidated aluminium 2014 powder which forms the matrix of many advanced composites. It is demonstrated that data provided by this technique are comparable to hot torsion data by the use of a solid control alloy. Activation energies and constitutive equation constants are derived from the data for use in analytical process models. It is proposed that this method of testing provides an extremely useful way of monitoring the flow behaviour of powder based materials that are not amenable to other test methods

Influence of changing strain rate on microstructure during hot deformation

Hot plane strain compression tests have been carried out on a ferritic stainless steel, commercial purity aluminium and an aluminium 1% magnesium alloy under conditions of constant strain rate and with strain rate changed in a controlled manner from one constant value to another. At constant strain rate, subgrain size and dislocation density inside subgrains are related to flow stress in the manner observed previously on other materials. During and after changes in strain rate, subgrain size and dislocation density are not uniquely related to flow stress. Significant strain intervals after a strain rate change are required to establish the new equilibrium subgrain size. For the alloys, these strain intervals are larger when strain rate is increased than when it is decreased. The opposite is found for the commercial aluminium. This difference in behaviour is explained In terms of the relative rates of glide and climb of dislocations.