Department of Metallurgy, Indian Institute of Science

Bangalore, India


The input to generate a processing map is the experimental data of flow stress as a function of temperature, strain rate, and strain.  As the map generated will be only as good as the input data, it is important to obtain accurate, reliable and reproducible data.  The hot compression technique is a simple laboratory method that can be controlled to have a constant true strain rate, an isothermal chamber, a good temperature control with ability to measure the adiabatic temperature rise.  In view of this, a laboratory was established in 1986 with generous funding from the Department of Science and Technology, Government of India and consists of three sections – (1) Dartec Laboratory, (2) Analytical Modeling Laboratory (computer lab), and (3) Hot extrusion facility.  In addition to the processing science laboratory, mechanical testing facilities that support the effort include Instron testing facility and vacuum hot press.  In the following are given brief details of the facilities available in the processing science laboratory.


Dartec hot compression testing facility


Compression testing is done on a computer controlled servohydraulic testing machine custom built by Dartec, Stourbridge, UK, of 100 kN capacity and with actuator speeds that range from about 0.015 – 1500 mm per second. The machine is equipped with a control system that can give constant true strain rate by an exponential decay of the actuator speed with time as per the equation:

X(t) = - ho[1 – exp ( t)]

Where X(t) is the actuator position at any time ‘t’ ho is the initial sample height and  is the true strain rate.  The control system makes the actuator speed correction 150 times for the true strain rates in the range 0.0003 – 1 s-1, 138 times for the strain rate of 10 s-1 and 13 times for the strain rate of 100 s-1.  A photograph of the machine is shown below.


Dartec Hot Compression Testing Facility


The machine is equipped with two interchangeable split (clam shell) furnaces, one with kanthal heating elements used for testing at temperature up to 1000oC and the other with silicon carbide heating elements used for testing at temperatures up to 1250oC.  The temperature control is within about + 2oC and the adiabatic temperature is measured using a Nicolet transient recorder (oscilloscope).  The loading system has superalloy push rods and replaceable platens.  A photograph of the tooling set up is shown below:


Tooling Setup



Specimen geometry

Cylindrical specimens with the geometry shown below are generally used for hot compression testing.  The specimen height to diameter ratio (aspect ratio) is 1.5 and has chamfered edges to avoid fold over in the initial stages of compression.  The top and bottom faces of the specimen are machined parallel and have concentric grooves of 0.5 mm depth to retain lubricant.  The specimen has a 0.8 or 1.0 mm diameter hole reaching to its center for inserting a fine thermocouple to measure the actual specimen temperature as well as the adiabatic temperature rise during deformation


Specimen Geometry


A general test matrix

A matrix of 6 temperatures and 6 strain rates is generally selected for testing.  Test temperatures are chosen in the homologous scale of 0.6 –0.8 with 50oC gap between tests and a true strain rate of 0.001 – 100 s-1 with each test done at strain rate values one order of magnitude apart.  At each test temperature, about 10-15 minutes soaking time is allowed for equilibration of temperature before the test is started.  All tests were conducted until the specimen height reaches one half of its original height.  From the data acquisition system, the load-stroke data are obtained and the adiabatic temperature rise is recorded on the transient recorder.


Data Analysis

The load – stroke curves were converted using a computer program into true stress – true plastic strain curves first by subtracting the elastic portion of both the material and machine from the stroke values at each of the loads and then by using the standard equations:


s = P/Ao (1 – e)


e = - ln (ho/h)


where s is the true stress, P is the load, Ao is the original area of cross section of the specimen, e is the engineering strain (ho – h)/ho, e is the true plastic strain, ho is the original height of the specimen and h is the instantaneous height.


The flow stress data obtained at different temperatures, strain rates and strains are corrected for the adiabatic temperature rise and the m values are calculated from the strain rate dependence of flow stress.  The efficiency values and the values for the instability parameter are computed and plotted as contour maps.  Details of the computer programs that are used for obtaining the maps are given under “Programs”.


Hot Tensile Testing


The Dartec machine is also equipped with the facility for hot tensile testing up to a temperature of about 1100oC at nominal strain rates in the range 0.001 to 1 s-1.  Cylindrical specimens with a gage length of 25 mm and a gage diameter of 4 mm are used for this purpose.  The diameter of the specimen head is 8 mm and the radius of curvature joining the gage diameter to the head diameter is 2.5 mm.  The total lenth of the specimen is 50 mm.  The parameter of importance in hot workability is the hot ductility, although the flow stress is also measured in these tests.


Computer Facility

The computer laboratory is equipped with a microvax computer(being replaced), a Sun workstation, multimedia personal computers with all the pereferals including a HP scanner, laser printer, color printer and others.  Several standard commercial softwares are available on these computers.  A link with the supercomputer facility of the Institute is available in this laboratory.


Hot Extrusion Facility

This facility is about two decades old and consists of a 250 ton vertical hydraulic four column press with a constant ram speed of 3.2 mm/sec.  A hot extrusion set up was designed and built on this press.  The container can extrude billets of 50 mm diameter and 150 mm height and the container and die assembly can be heated externally to about 250oC.  Shear dies with different extrusion ratios are available with this facility. The billets are heated in an outside furnace and quickly loaded in the container for extrusion.  Since the press is slow, the extrusion temperatures are so far limited to about 600oC.  The facility is being replaced by a full fledged higher capacity extrusion press in the Department.