Predicting the Rheological Behavior of AISI D2 Semi-Solid Steel by Plastic Instability Approach
1Department of Mining and Metallurgical Engineering, Amirkabir University of Technology, Tehran, Iran
Determining tool steels properties, especially deformation and flow behavior in the semi-solid zone, is an essential parameter for thixoforming process. The main objective of this work is an analysis microstructure and flow behavior determining of AISI D2 tool steel in the combined uniaxial constant load compression and electrical resistance heating in the semi-solid state. The as-received material used for experiments, were in the hot rolled-annealed condition. The globular solid particles suspended in liquid phase, were obtained by the joule heating of samples. The experiments were carried out for 0.2 to 0.3 liquid phase fraction under 160N uniaxial constant load in the specimens with 10 mm height and the same Diameter value. Microstructural evaluation before and after heating has also been presented. Structural and elemental analysis was performed with the help of optical microscopy and scanning electron microscopy. The microhardness test has been carried out on the solid particles of quenched samples. The hardness values were changed from 317HV0.1 to 463HV0.1 by increasing the liquid phase. Finally, a new approach based on plastic instability and power law equation was used for flow behavior detection. The results showed that combination joule heating and plastic instability approach is compatible for flow behavior detection of AISI D2 steels in the semi-solid state.
At a glance: Figures
Keywords: rheological behavior, plastic instability, tool steel, semi-solid state, joule heating
American Journal of Materials Engineering and Technology, 2013 1 (3),
Received March 10, 2013; Revised March 26, 2013; Accepted September 01, 2013Copyright © 2014 Science and Education Publishing. All Rights Reserved.
Cite this article:
- kalaki, A., and M. ketabchi. "Predicting the Rheological Behavior of AISI D2 Semi-Solid Steel by Plastic Instability Approach." American Journal of Materials Engineering and Technology 1.3 (2013): 41-45.
- kalaki, A. , & ketabchi, M. (2013). Predicting the Rheological Behavior of AISI D2 Semi-Solid Steel by Plastic Instability Approach. American Journal of Materials Engineering and Technology, 1(3), 41-45.
- kalaki, A., and M. ketabchi. "Predicting the Rheological Behavior of AISI D2 Semi-Solid Steel by Plastic Instability Approach." American Journal of Materials Engineering and Technology 1, no. 3 (2013): 41-45.
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Semi-solid forming (SSF) technology, developed at Massachusetts Institute of Technology during the 1970s by Spenser and co-workers , has received considerable interest in recent years. The SSF technique offers advantages, such as reduce ﬂow stress during shearing in comparison of traditional metal processing methods, namely casting, forging, and powder metallurgy . Essential requirement of SSF is the thixotropic slurry with non-dendritic morphologies of the solid phase . The SSF is a very potential near net shape technology suitable for producing all kinds of metals and alloys in the mushy state . In steel thixoforming, it is necessary to conduct basic flow behavior analysis on suitability of specific grade of steels. Furthermore, role of constant temperature in the upset forging between parallel plate at constant load is high important for accuracy of data. Two or more constitutive equations describe the semi-solid material reaction to applied loads and its macroscopic behavior. Semi-solid flow stress is affected by the morphologies of microstructure, which dominates fluidity of liquid phase in melting zone and mode of deformation of solid grains [4, 5, 6, 7].
Difficulties of performing semi-solid tests equipments and instrument is a restriction of this process [8, 9, 10]. Cooling during transport of the test specimen to the upsetting chamber cannot be avoided and is a drawback of conventional experiment setup. These drawback deals with specimen overheating during heating up to semi solid zone and so that the temperature of the test specimen is slightly above the experimental temperature. On the other hand, increasing soaking time in ferrous metals and alloys produces unfavorable microstructure and limits thixoforming process. In order to protect the samples from increased soaking time and being over heated, the electrical resistance heating (direct resistant joule heating) in high alloy steels such as tool steels is suitable situation for rheological investigation, especially in compression test by constant load . However, the resistant joule heating processes produce non-uniform distribution of temperature inside heated materials; especially in lateral section of the samples. The Figure 1 shows comparison between heating strategy of conventional and joule heating systems.
One of the most important methods which decrease semi-solid processing steps is partially remelting of alloys and steel in melting range . As a result, decreasing heating time in globular microstructure preparation is very important. For this reason, combination of joule heating [8, 9] and direct partial remelting  can be used for rheological and microstructural evolution of steel slurry.
2. Material and Methods
In this research, the AISI D2 tool steel with chemical composition is mentioned in table 1 was used for experimental analysis. The ∅ 10mm × 10mm cylindrical samples were joule heated under 160N constant load to understanding flow behavior.
Specimens size before and after electrical resistance heating (joule heating) up to 1300°C was shown in Figure 2. The six samples were heated by direct resistance Joule heating. Alternating current at 50Hz with a voltage less than 3V and 16KA flows throughout the steel samples. The level of current has been regulated by a K-type thermocouple on the surface of the samples at the middle of their heights. Two springs in back of samples supplied uniaxial constant load. The resistance heating machine used in this investigation is presented in Figure 3. It is important to consider different layer of temperature on rheological properties and chemical segregations . According to Sołek et.al experiments  during compression test, different zone were produced due to non-uniform distribution of strains and temperature in the sample.
For detection of non-uniformity distribution, the samples have been sectioned and microstructural changes were studied.
3. Theory and Calculation
Firstly, microstructure evaluation was showed a composite cylinder consisted of two layers produce, and these two zones were used for plastic instability calculation. Secondly, two zones were considered in joule heating samples: (I) a semi-solid slurry cylinder inserted in (II) a solid shell as illustrated in Figure 4. Thirdly, composite cylinder plastic instability was calculated as a function of strain and strain rate.
Flow behavior of solid shell and semi-solid (slurry) cylinder, described by power law equations. In constant volume of samples, plastic instability equation can be written by equation (1);
Where the Vl and Vs are volume of liquid and solid slurry, respectively, L is the length of sample, and flow stress values are represented by (σl) in semi-solid and (σs) in solid part. Relationship between flow stress and strain rate is σs=Aέn and σl=Bέm.
Using flow stress and stress rate relationship, Equation (1) was rewritten as follows:
Where A, n are two parameters of flow behavior on the nearest zone to solidus temperature and B and m are flow behavior parameters of semi-solid part in AISI D2 steel. Ε and έ are strain and strain rate which are measured in compression test. As a result, by substitution of ε, έ, A and n in equation (2), it is possible to calculate B and m as a flow behavior parameters of semi-solid part. Considering plateau form of stress/strain rate curve of AISI D2 steel in hot deformation in nearest zone to semi-solid range, especially at 1275°C, A and n were calculated from Maciol experiment results to be 10MPa and 0 . Substituting the calculated values in equation (2), B and m values were calculated. The sequences of the plastic instability approach in this work are listed below:
1. Hot compression test in nearest point to solidus temperature
2. Sufficient joule heated samples by constant load carry out
3. Strain, strain rate of joule heated samples, calculation
4. Microstructure analysis to calculation solid shell thickness
5. Plastic instability equation rewritten by composite cylinder consideration
6. Calculated flow behavior parameters of slurry
4. Results and Discussion
The shear stress and viscosity vs. shear rate curves of AISI D2 steel which illustrated in Figure 5 was obtained from plastic instability approach in 1300°C.
On the other hand, microstructures of these six samples in slurry regions are shown in Figure 6. By increasing the contact time, the grains became globular and the shape was rounded. In short contact times, dendritic structures are broken and grain size is decreased and globalized. The liquid phase volume fraction in samples varied from 0.2 for sample No.1 and 0.3 for sample No.6. As it is illustrated in Figure 6, the globular microstructure produced in a short time which is a very suitable condition for thixoforming process. The SEM microstructures of the samples were shown in Figure 7. The laminated carbides were surrounded by grains as a ledeburite structure. Cr and Fe carbides were shown in layers form of ledeburite. The chemical composition variations around the grains were analyzed with point analysis SEM, see Figure 8. As a result of these point analysis, carbide type in the laminated ledeburite was known M7C3 type. The two middle point (a) and (b) EDX analysis result is shown in Figure 9(a) and Figure 9(b). It is observed that SEM and OM microstructure of joule heated samples are similar to microstructure obtained from other conventional heating process. As a result of increased liquid phase and soluble carbon content of liquid and solid regions, micro hardness of the samples has been varied from 317HV0.1 to 463HV0.1 for samples No. 1 to 6, as presented in Figure 10.
When 10mm×10mm samples are joule heated under constant load, the grain boundary carbides dissolved and internal resistance to deformation dropped rapidly after a few seconds. The grain boundary liquation produced equiaxed grains which is good for thixoforming. Average viscosity and shear rate of material were calculated by Power law relationship as a rheological behavior of AISI D2 steels in 1300°C. (See Figure 11).
As it is shown in this figure the resistance to deformation is decreased by increasing in shear rate and it means that shear thinning be occurs in this situation. Finally, the result of this shear thinning is related to decreasing the flow stress less than one MPa in 4 second.
In this article, AISI D2 tool steel flow behavior was studied under semi-solid state by using resistance heating machine. Results show the joule heating has a good potential for semi-solid rheological investigations. The short time of semi-solid microstructure preparation led to suitable microstructure and the material under thixoforming was protected from hot degradation. Therefore hardness value measurements and plastic instability of given layers obtained from microstructural analysis can be considered as criteria to investigate the rheological properties of AISI D2 semi-solid steel. Finally, a new approach is developed on microstructural analysis and plastic instability to evaluation the rheological parameter of joule heated AISI D2 steel samples in semi-solid state.
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