ABSTRACT: In order to realize the numerical simulation of continuous extrusion of H65 brass alloy and to determine its hot forming process parameters rationally, the flow stress characteristics of H65 brass alloy under hot deformation condition were studied by using Gleeble-1500 thermal simulation experimental machine. The results show that the dynamic recrystallization of H65 brass alloy occurs during hot compression deformation, and the flow stress behavior of H65 brass alloy during hot compression deformation can be well described by the exponential form of Arrhenius equation The activation energy q of Hot deformation is obtained in the interval, and the constitutive equation of H65 brass alloy is established in subsection.

Key words: H65 Brass Alloy; flow stress; constitutive equation; thermal simulator; classification of continuous extrusion: TG146. TG113. 25 document ID: A ARTICLE ID: 1007-2012(2008)06-0113-05

Introduction

At present, in the domestic Non-ferrous metal processing industry, most manufacturers use horizontal continuous casting method to produce brass rods. However, the production of brass wire by continuous extrusion is relatively rare. The continuous extrusion process is simple, can save energy, reduce cost, and can be formed at one time, which makes it has great potential in brass wire production. However, due to the high deformation temperature, high deformation resistance and high temperature oxidation of brass alloy, the continuous extrusion technology of brass alloy needs to be further studied in plastic deformation mechanism and process parameters.

The flow stress in hot deformation of metal is one of the basic properties of material at high temperature, which is not only affected by the deformation temperature, the degree of deformation, the strain rate and the chemical composition of alloy, but also the comprehensive reflection of the microstructure evolution in the deformed body. The accurate numerical value or expression of flow stress is the key to improve the accuracy of theoretical calculation, both in the development of reasonable hot working process and in the modern plastic working mechanics represented by plastic finite element method, in recent years, the research in this field has been very active both at home and abroad. Zhang Honggang et AL analyzed the flow stress of KFC copper alloy during hot compression deformation, Zhou Xiaohua and Liu Ruiqing studied the flow stress of several copper alloys at high temperature, but the flow stress of H65 brass alloy was seldom studied. In this paper, Gleeble-1500 thermal simulator and continuous extrusion process are used to make the process plan. The deformation temperature is 100 °C ~ 800 °C and the strain rate is 0. 01s-1 ~ 1s-1, isothermal hot compression tests were carried out on H65 brass alloy, and the relationship between hot compression deformation flow stress and deformation degree, strain rate and deformation temperature was analyzed, the constitutive equation is established to provide a reference for the rational formulation of hot deformation process of brass alloy, and to provide accurate data or mathematical model for further analysis of finite element numerical simulation.

1. Experimental methods

Gleeble-1500 thermal simulation machine was used to carry out the isothermal compression experiment of cylinder. The compression temperature is 100 °C ~ 800 °C and the strain rate is 0. 01s-1 ~ 1. 0s-1; Total compressive strain is 0. Five. The heating rate was 200 °C/MIN and the holding time was 3 min. The experimental scheme is as follows: temperature 100 °C ~ 800 °C, temperature interval 100 °C, strain rate 0. 01s-1,000. 10s-1 and 1. 0s-1. The chemical composition of H65 brass alloy is shown in Table 1.

2. Results and analysis

Fig. 1 shows that the h 65 brass alloy has a strain rate of 0.5% at 100 °C ~ 800 °C. 01s-1 ~ 1. 0s-1 flow stress curve under hot compression. 1. It can be seen from Fig. 1 that the flow stress of H65 brass alloy changes with the deformation degree of the specimen. H65 brass alloy is brittle at low temperature (100 °C, 200 °C) , so it has high strain rate (0. 1s-1,1. 0s-1) , the specimen was fractured. The fracture morphology of H65 brass is shown in Fig. 2. There is no obvious dimple on the fracture surface, but fiber structure and cracks along the fiber structure during compression. At the beginning of compression deformation, the flow stress increases rapidly with the increase of strain, and presents the trend of work hardening, especially at the lower deformation temperature (300 °c) , the flow stress always shows an obvious upward trend, and then with the increase of strain, the flow stress tends to smooth or even decrease, but the decrease is not obvious, that is, the softening is not obvious. 2) at the constant strain rate, the flow stress decreases with the increase of deformation temperature, but at the higher temperature of the same strain rate (400 °C ~ 600 °C) , the flow stress curve appears peak value, and then tends to smooth. This is because under the same deformation condition, work hardening occurs with the increase of deformation, dynamic recrystallization softening occurs after a certain deformation variable, and the flow stress reaches the maximum when the softening rate is in balance with the hardening rate

After fully dynamic recrystallization, the grain structure and flow stress do not change with the deformation, that is to say, it enters the steady-state deformation stage. When the temperature is between 700 °C and 800 °C, the flow stress curve tends to smooth without peak value. This is because with the increase of temperature, the kinetic energy of metal atoms and the amplitude of atomic vibration of brass alloy increase, so that more dislocations are activated and the slip systems are increased, thus improving the compatibility of the grains of brass alloy and increasing the plasticity of the material, decrease in alloy strength [9]. 3) when the strain rate is low (0. 01s-1) , the strain rate is small, the material appears the instantaneous yield, the softening is obvious, then along with the strain rate increases, the processing hardening takes the leading position, causes its flow stress to increase, this kind of situation only occurs when the strain rate is small, this indicated that the flow softening under the big strain rate, does not have the sufficient time to produce the full softening, but when the strain rate is low, the material has the sufficient time to carry on the recrystallization in the forming process.

3. ESTABLISHMENT OF CONSTITUTIVE EQUATION 

3. 1 effect of strain rate on flow stress

It can be seen from Fig. 3 that the steady flow stress of the alloy increases with the increase of strain rate, but not in proportion. The high temperature flow stress under any strain or steady state is strongly dependent on the deformation temperature t and strain rate during thermal deformation of metal materials, the high temperature deformation of H65 brass alloy is usually described by a different mathematical modelAs a result, the thermal activation energy of a material can be defined as a temperature-independent constant, reflecting the difficulty of thermal deformation, it is an important mechanical property parameter in the process of hot deformation Equation (1) can well describe the conventional hot working deformation of copper alloy, such as compression, torsion, extrusion, etc. . Therefore, the Logarithm of (1) on both sides of the equation is arranged into the equation ln = mln + mqrtn-ln a (2) as a strain rate sensitive index, m = 1/n,

Let B = MQRTLN A, when t is constant, m = LN T (3) , because its stress-strain curve peaks at 400 °c ー600 °C and tends to level at 700 °C ー800 °c, the flow stress at 400 °C ー600 °c takes its peak stress, the steady-state stress is taken at 700 °C ~ 800 °C, and the flow stress and strain rate of brass alloy at different deformation temperatures are substituted by formula (3) , and the corresponding ln-ln relationship curves are drawn, as shown in Fig. 4. Fig. 4 shows that the logarithmic relationship between steady state flow stress and strain rate of H65 brass alloy during high temperature deformation satisfies a linear relationship. The M and B values are obtained by linear regression. As can be seen from Fig. 5, the M value decreases with the increase of temperature, indicating that the deformation resistance of the alloy is less sensitive to the strain rate.

In order to improve the accuracy of finite element simulation of H65 brass alloy, the average slope M = 0. 143, N = 6. M = 0 at 993,700 °C ~ 800 °C. 0653, N = 15. And the regression correlation Coefficient of each straight line is over 0. 99. It can be concluded that the relationship between flow stress and strain rate of H65 brass alloy during compression deformation at high temperature satisfies the form of exponential relationship.

3.2 effect of temperature on flow stress

Changes in temperature directly affect the magnitude of flow stress (see figure 3) . The change of temperature may also cause the dynamic recrystallization and dynamic recovery of the alloy during deformation. LN = QNRT + LN N-LN AN (4) from formula (1) shows the linear relationship between LN and 1/t. The curves of ln-1/T are shown in Fig. 6. The analysis shows that the curves have different slopes in different temperature ranges, which is different from the results in [13-14] , k = 3290 at 400 °C ー600 °C, k = 2810 at 700 °C ー800 °C, and Q = Nr LN 1/t (5) under constant strain rate

4. METALLOGRAPHIC observation and calculation show that different activation energy is obtained in different temperature range, which is related to the change of phase and relative content of phase during high temperature deformation. Fig. 7 shows the microstructure of H65 brass at different deformation temperatures. It can be observed that dynamic recrystallization begins at 400 °C and ends at 600 °C. after dynamic recrystallization, the grains are refined, the grain boundaries are visible (see figures 7A and 7B) . When the temperature reaches 700 °C, the microstructure is obviously different, and the needle-like phase (see Fig. 7C) starts to be precipitated from the phase. With the increase of the temperature, the phase content increases and distributes uniformly in needle-like shape (see Fig. 7d) . The characteristic of the curve is that the peak value appears at 400 °C ~ 600 °C and then tends to be stable, while the peak value does not appear at 700 °C ~ 800 °C, the curve tends to level and the flow stress value decreases. It is reasonable to explain the theory that there are different activation energies of deformation in different temperature ranges. At the same time, the rationality and accuracy of piecewise constitutive equation are fully proved. The accuracy of finite element simulation is improved by building the material model in stages.

5. Conclusion 1) at the constant strain rate, the flow stress decreases with the increase of deformation temperature, but at the higher temperature of the same strain rate (400 °C ~ 600 °C) , the true Stress–strain curve has an obvious peak value, when the temperature is higher than 700 °C, the true Stress–strain curve tends to level, and the deformation mechanism is dynamic recrystallization softening. 2) the exponential form of Arrhenius equation can be used to describe the flow stress behavior of H65 brass alloy during high temperature deformation. 99. 3) the stress sensitivity index decreases with the increase of temperature, and different activation energies of deformation are obtained in different temperature range, from 400 °C to 300 °C

Q = 191188J/mol at 600 °C and 357599j/mol at 700 °C ー800 °C. The activation energy is related to the change of relative content of phase and phase. 

Source: Chinanews.com, by Wang Yanhui