Flow Stress of Metals and Its Application in Metal Forming Analyses

1973 ◽  
Vol 95 (4) ◽  
pp. 1009-1019 ◽  
Author(s):  
T. Altan ◽  
F. W. Boulger
Keyword(s):  
Flow Stress ◽  
Strain Rate ◽  
Metal Forming ◽  
Cold Extrusion ◽  
Rate Dependency ◽  
Tool Steels ◽  
Forming Load ◽  
Closed Die Forging ◽  
Upset Forging ◽  
Flow Stress Data

Forming load and energy can be determined if the flow stress of the deforming, material is known at the temperature and strain-rate conditions existing during the process. In this study domestic and foreign melalforming articles were reviewed and the available flow stress data have been presented for selected carbon, stainless, and tool steels; aluminum, copper, and titanium alloys; magnesium, uranium, zircaloy, molybdenum, tungsten, tantalum, and niobium. Whenever possible, data are presented by calculating and tabulating coefficients K and n to express strain hardening (flow stress σ¯ = Kε¯n), and C and m to express strain-rate dependency (σ¯=Cε¯˙m). Examples are given to illustrate the use of flow-stress data with simple formulas in predicting pressures in upset forging, closed-die forging, and cold extrusion.

Investigation on Hardening and Softening Behavior of Steel after Rapid Strain Rate Changes

2016 ◽  
Vol 716 ◽  
pp. 121-128 ◽  
Author(s):  
Jens Dierdorf ◽  
Johannes Lohmar ◽  
Gerhard Hirt
Keyword(s):  
Flow Stress ◽  
Dynamic Recrystallization ◽  
Strain Rate ◽  
Metal Forming ◽  
Elevated Temperatures ◽  
Relaxation Behavior ◽  
Rate Change ◽  
Strain Rates ◽  
Flow Curves ◽  
Strain Rate Change

The design of industrial hot metal forming processes nowadays is mostly carried out using commercial Finite Element (FE) software codes. For precise FE simulations, reliable material properties are a crucial factor. In bulk metal forming, the most important material property is the materials flow stress, which determines the form filling and the necessary forming forces. At elevated temperatures, the flow stress of steels is determined by strain hardening, dynamic recovery and partly by dynamic recrystallization, which is dependent on strain rate and temperature. To simulate hot forming processes, which are often characterized by rapidly changing strain rates and temperatures, the flow stress is typically derived from flow curves, determined at arbitrary constant temperatures and strain rates only via linear interpolation. Hence, the materials instant reaction and relaxation behavior caused by rapid strain rate changes is not captured during simulation. To investigate the relevance of the relaxation behavior for FE simulations, trails with abrupt strain rate change are laid out and the effect on the material flow stress is analyzed in this paper. Additionally, the microstructure evolution due to the strain rate change is investigated. For this purpose, cylinder compression tests of an industrial case hardening steel are conducted at elevated temperatures and different strain rates. To analyze the influence of rapid strain rate changes, changes by one power of ten are performed at a strain of 0.3. As a reference, flow curves of the same material are determined at the initial and final constant strain rate. To investigate the microstructure evolution, compression samples are quenched at different stages, before and after the strain rate change. The results show that the flow curves after the strain rate change tend to approximate the flow curves measured for the final strain rate. However, directly after the strain rate change significant differences between the assumed instant flow stress and the real material behavior can be observed. Furthermore, it can be shown that the state of dynamic recrystallization at the time of the strain rate change influences the material response and relaxation behavior resulting in different slopes of the investigated flow curves after the strain rate change.

Development of Processing Map for Superplastic Deformation of Ti-6Al-4V Alloy

2018 ◽  
Author(s):  
Mohd Abdul Wahed ◽  
Amit Kumar Gupta ◽  
Nitin Ramesh Kotkunde ◽  
Swadesh Kumar Singh
Keyword(s):  
Flow Stress ◽  
Strain Rate ◽  
Superplastic Deformation ◽  
Processing Map ◽  
Flow Instability ◽  
Hot Working ◽  
Uniaxial Tensile ◽  
Temperature Range ◽  
Instability Map ◽  
Flow Stress Data

A processing map plays a major role in indicating safe and failure regions of a process conducted in a hot working regime. It also shows the response of a material, by indicating changes in the microstructural evolution through temperature. In the present study, a processing map has been developed depending on the flow stress data of Ti-6Al-4V alloy sheet in a strain rate range of 10−2 /s to 10−4 /s and over a temperature range of 700°C to 900°C in order to identify the presence of superplasticity region. The flow stress data have been acquired on the basis of temperature, strain and strain rate by conducting hot uniaxial tensile tests. Based on this, a power dissipation map is obtained to show the percentage of efficiency, as it is directly related to the amount of internal entropy produced. In addition, an instability map is also obtained, as it identifies the flow instability that are to be avoided during hot working process. Finally, a processing map has been established by overlaying instability map on efficiency map. The results clearly reveal that the superplastic deformation occurs within a temperature range of 750°C to 900°C at a strain rate of 10−4 /s, without any flow instability in this region.

A Study of the Effect of Extrusion Parameters on the Properties of Extruded Zr-2.5Nb Pressure Tubes

2012 ◽  
Vol 585 ◽  
pp. 371-375
Author(s):  
Nachiket Keskar ◽  
Vikrant Raizada ◽  
Vaibhaw Kumar ◽  
Sanjay Kumar Jha ◽  
Dinesh Srivastava ◽  
...  
Keyword(s):  
Flow Stress ◽  
Strain Rate ◽  
Extrusion Process ◽  
Phase Region ◽  
Work Piece ◽  
Two Phase ◽  
Strain And Strain Rate ◽  
Transus Temperature ◽  
Flow Stress Data ◽  
Temperature Strain

The hot extrusion of Zr-2.5Nb alloy has been simulated using an FEM based software and validated by the experimental measurements during real extrusion process. The flow stress data for Zr-2.5Nb alloy at different temperatures, strain and strain rates were generated by hot compression testing on Gleeble Machine. The flow stress data, obtained both below β transus temperature in two phase α+β region as well as above β transus temperature in β phase region, were directly used in the FEM simulation of extrusion process, eliminating the need for any assumptions regarding material plastic flow law. Two important extrusion parameters, viz. included die angle and reduction ratio, were varied and for each set of conditions, temperature, strain and strain rate at different nodal points of the tube at different stages of extrusion were calculated. The extrusion parameters were optimized to obtain minimum variation in the force, temperature, strain and strain rate in the extruded product. A fair agreement has been found between the measured values of the applied ram force and the temperature profile at the die landing area in a 3780 ton horizontal extrusion press and those obtained through simulations. The effect of the friction and heat transfer coefficient between the die chamber and work piece was also investigated in this study.

Determination of Flow Stress Data for Practical Metal Forming Analysis

CIRP Annals ◽  
1981 ◽  
Vol 30 (1) ◽  
pp. 129-134 ◽  
Author(s):  
T. Altan ◽  
S.L. Semiatin ◽  
G.D. Lahoti ◽  
T. Altan
Keyword(s):  
Flow Stress ◽  
Metal Forming ◽  
Forming Analysis ◽  
Flow Stress Data

A coupled thermo-elastic-plastic model with a modified direct iteration method in a cold extrusion process

1993 ◽  
Vol 28 (2) ◽  
pp. 89-100
Author(s):  
Z C Lin ◽  
W C Pan
Keyword(s):  
Flow Stress ◽  
Strain Rate ◽  
Iteration Method ◽  
Strain Curve ◽  
Extrusion Process ◽  
Cold Extrusion ◽  
Convergence Criteria ◽  
Plastic Model ◽  
Elastic Plastic ◽  
Realistic Situation

A thermo-elastic-plastic model in conjunction with an experimental flow stress-strain curve, is applied to a cold extrusion process. The flow stress is considered as a function of strain, strain rate, and temperature to simulate a more realistic situation. A modified iteration method is proposed to a linear piecewise continuous model to increase the accuracy and avoid divergence. The strain is calculated by Taylor's series expansion and is associated with its corresponding convergence criteria; an available scheme for the calculation of the strain rate is thus involved. In addition, a least squared method is proposed to check loading or unloading situations. An extrusion experiment is then made and the simulated loading force compared with the experimental loading force is very satisfactory.

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