Calculations were made to radius of curvature of barrel assuming that the curvature of the barrel followed the shape of a circular arc. The radius of curvature of the aluminum cylinders measured physically was found to confirm close to the values calculated with the experimental data, even at higher temperature. Furthermore, different stresses such as hoop stress, hydrostatic stress and effective stress were found out for different temperatures using the dimensions of the deformed samples.
Because the cross-sectional area of the specimen changes closely along its height, and is maximum at the center, it is difficult to obtain stress-strain curves in compression. The shape of the sample after compression depends largely on the aspect ratio (h/d) of the sample. Some of the researchers predicted the yield stress of the material by some algorithms like fuzzy logic, neural network, genetic algorithm etc.
The proposed constitutive equation was also reconfirmed by comparing the geometric deformation of the samples and numerical simulation of the compression test. A new geometric shape factor (GSF) has been established based on the dimensions of the deformed samples. 19], investigated the effect of friction during the formation of cold disturbances by taking different friction conditions and concluded that the bulge produced is the lowest for molybdenum disulfide, regardless of the aspect ratio.
20], also carried out an analysis of the effect of friction and concluded that the final shape of the workpiece after the shaking process can be divided into two geometries, namely a cylinder part and a frustum of a cone, and the new hoop strain slope is found to increase. correspondingly with increasing aspect ratio.
KN models by INSTRON are ideal for high capacity tension, compression, bending and shear testing. Adjustable stainless steel latches keep the oven halves locked together during use, but can then be easily opened once the test is complete. The oven is available with optional heavy-duty brackets or mounts that can be mounted on a wide range of test systems.
The resistance wire is wound on the recrystallized aluminum tube in three independent zones to form the furnace element. This three-zone format allows the user to adjust the temperature gradient of the oven, creating a uniform central zone. Experimental setup for high temperature compression test Experimental setup for high temperature compression test.
It is structurally composed of planes of polycyclic carbon atoms that are hexagonal in orientation. In an oxidative atmosphere, graphite is continuously effective at high temperatures up to 450ºC and can withstand much higher temperature peaks. Graphite powder and machine oil in proper proportion were mixed well to form the lubricant for the test.
Samples of the required dimensions (figure 3.6) are cut from the aluminum alloy rod and the facing operation is performed to make both ends parallel. Specimens with lubricant coating on both ends are placed between the upper and lower plates of the installation. Extreme care was taken to place the axis of the cylindrical sample in concentricity with the axis of the ram.
The oven is now closed and the sample is heated to the desired temperature at a rate of approximately 6⁰C/min. For each temperature test, six samples of the same dimensions were taken and deformed to different strain levels. The loads used during each deformation were automatically recorded by the BLUEHILL software included with the UTM machine.
After each test, the following parameters were measured: (i) the height of the deformed sample (hf); (ii) the contact diameter of the sample (dc); (iii) the bulging diameter (db); and (iv) barrel radius (R). The cylinder radius at each deformation was measured using CALIPER PRO software built into an optical microscope and the rest of the dimensions were measured using a digital caliper.
- Upset Forging
- Modes of Deformation
- Upset Forging at High Temperature
- Effects of Temperature
- Effects of Rate of Deformation
- Aluminum alloy
- Alloy designation
- Applications
A compression test, in which the sample is subjected to a compressive load, provides information useful for these processes. Due to the friction between the sample and the dies, the cylindrical shape of the sample bulges, this effect is called a barrel. The actual stress-strain curves obtained from compressive and tensile tests on ductile materials coincide with each other, which is not the case for brittle materials, which are generally stronger and more extensible in compression than in tension.
However, the influence of temperature is best described in connection with the rate of deformation. The above equation indicates for a constant crosshead speed that the true strain rate will increase as the specimen length decreases. So to make the true strain rate constant, the crosshead speed should decrease with decrease in sample height.
Commercial purity of aluminum is 99.5 to 99.79%, but pure aluminum is too soft to be of structural value. Wrought aluminum is identified by a four-digit number that identifies the alloying elements, with the first digit indicating the major alloying elements.
On hot upset forging
1], showed that the maximum stress level of aluminum alloy 7150 decreases with increasing deformation temperature and decreasing strain rate. Deformed structures exhibit elongated grains with indentations developed at the grain boundaries Dynamic recovery and recrystallization are the main reasons for flow softening at low Z value 2], found that, in hot deformation of stainless steel, work hardening can be neglected and the constitutive equation can only be taken as a function of strain rate.
3], found out that the true stress-strain curves of Al-Mg-Si-Cu aluminum alloy show a peak stress at a small strain, after which yield stress decreases monotonically until high strain. 4], concluded that the yield stress of 2197 Al-Li alloy decreases with the increase of deformation temperature and increases with the increase of deformation rate. The peak yield stress during high temperature deformation can be represented by Z parameter in a hyperbolic sine function.
5], revealed that the stress-strain curves of the AZ91 magnesium alloy can be explained in terms of microstructure evolution, with emphasis on the eutectic phase and dynamic precipitation phenomena occurring at low and high deformation temperatures. 6], found that Fe-25Al alloy can be deformed without cracking at temperatures above 873 K. The results show that Ti addition tends to reduce the strain rate sensitivity and activation energy for flow deformation of Fe-25Al alloy.
7], showed that the apparent activation energy for deformation of TC11 titanium alloy increases with the increase of deformation temperature gradually and decreases with the increase of strain rate. 8], found that heat treatments of Fe-Al alloys at temperatures below 600°C have no major influence on the deformation behavior. 10], adopted ANN and genetic algorithm to optimize the flow stress of 304 stainless steel under cold and hot compression.
11], found that aluminum alloy compression curves obtained using the barrel correction factor method and the Bridgman rework technique (no barrel allowed during the test) were found to have close values, even at higher temperatures. The new constitutive equation breaks down under machining conditions due to the adiabatic temperature rise during high strain rate deformations. 13], adopted Hill's general method to calculate the yield stress of a cylindrical specimen of AA6063 aluminum alloy under simple uniaxial compression and also to consider the effect of friction at the specimen-sample interface.
On cold upset forging
Some other lubricants used at high temperature are molybdenum disulfide grease base, Teflon sheet, Molycote spray, carbon foil and so on.
Experimental Setup
- INSTRON SATEC KN 600 Specifications
- Furnace
- Furnace Specifications
- Hydraulic Power Supply
These three zone resistance wire wound furnaces are of split construction to facilitate quick and easy filling of a pre-assembled sample train. The box is constructed of stainless steel with aluminum and hardened insulation plate end plates.
Graphite as a lubricant
Procedure
- Specimen Preparation
- High temperature compression test in INSTRON
Samples are held at the test temperature for 2-3 minutes after reaching the desired temperature for uniform heating.
Specifications of Optical Microscope
Flow Curve
Non linear least square regression analysis
The nonlinear least squares regression analysis fits the experimental data well, as the R2 values (Table 1) lie around 0.8. From the regression analysis, we obtained the strength coefficient K and the strain hardening exponent n for different temperatures. 3 Variation of n with respect to temperature Figures 4.5 and 4.6 show the variation of the strength coefficient K and the strain hardening exponent n with respect to temperature.
The non-linear regression analysis results in a total power equation that accounts for voltage and temperature as follows.
Radius of Curvature of the Barrel
So taking the barrel arc after a circular arc even at higher temperature is also justified.
Determination of Various Stresses
11 Variation of different stresses for different temperature, (a)-(f): 30°C-300°C Figure 4.13 shows the different stresses acting on the sample during the experiment at different temperatures. The axial stress is negative as it is compressive in nature, true as effective stress.
The consideration of the shape of the barrel, which follows as a circular arc, also applies at elevated temperature. The value of the radius increases with an increase in the test temperature and decreases with an increase in the amount of deformation. 3] Zhang Hui et.al, Hot deformation of a new Al-Mg-Si-Cu aluminum alloy during pressing at elevated temperatures, Journal of Materials characterization.
Flow stress optimization for 304 stainless steel under cold and hot compression by artificial neural network and genetic algorithm, materials and design. Constitutive equations for predicting high temperature flow stress in a Ti-modified austenitic stainless steel, Journal of Materials science and engineering A. Deformation behavior of Aa6063 aluminum alloy after removal of friction effect under hot working conditions, Acta Metall.
