The mechanical and thermal behavior of machining chips during compaction and consolidation processes that occur in the friction extrusion and friction consolidation processes were studied and analyzed. Experiments were carried out to provide opportunities to measure and extract stress, strain and thermal response information on machining chip specimens during and/or after compaction and consolidation tests. Numerical simulations were carried out to predict the mechanical and thermal behavior of chips material in the validation experiments.
Regarding the mechanical behavior, the compaction process of AA 6061 machining chips with three different chip lengths was studied. Compaction tests and uniaxial compression tests with loading and unloading were carried out to understand the mechanical behavior of the chips during compaction and the compacted disks after compaction.
1. Test results show that quasi-static loading rate has a small effect on the mechanical behavior of the chips during compaction, and that chips lengths have an effect on the compaction process inside the chamber due to occupied volume but once they are compacted to the same level, this effect vanishes. Mechanical properties of Poisson’s ratio, Young’s modulus and the flow stress were extracted from the compaction and uniaxial compression tests and were found to depend on the relative density. Chips length has no effect on the stress
– strain curve during uniaxial tests of compacted disks when the compacted disks are at the same relative density level.
2. A porous elastic-plastic material model, in which the flow stress, the Poisson’s ratio and the Young’s modulus are function of the relative density, was developed to model the mechanical behavior of the chips during the compaction process and the compacted disks after compaction. Finite elements simulations were carried out to simulate the compaction tests. Simulation predictions of the axial stress-strain curves with loading and unloading agree well with experimental data.
3. A validation for the developed mathematical model was performed using diametrical compression tests for the compacted disks at three different levels of relative density. The predicted and measured load-displacement curves were found to have good agreement with experimental curves.
4. An enhanced method to experimentally calculate the Poisson’s ratio as a function of relative density through different correlations using the theory of thick walled cylinder was proposed. The porous elastic-plastic material model was used to model the mechanical behavior of the chips during the compaction process. Simulation predictions of the axial stress-strain curves, as well as a validation for the mathematical model were also performed and the predicted and measured data were found to have a better agreement with experimental data. The new method provides a realistic measure for Poisson’s ratio in the elastic range.
Regarding the thermal behavior, the heat generation and heat transfer process in the friction consolidation process was studied. Several consolidation experiments were conducted to measure temperature field of the chips material during the consolidation process.
1. Compacted and consolidated disks at different levels of relative density were made for thermal properties measurements. Based on these tests and measurements, thermal properties, such as thermal conductivity and specific heat, were represented as a function of relative density and temperature. 2. A mathematical and a full geometrical model representation was developed to
simulate the temperature field of the chips in the consolidation process using the extracted thermal properties. Simulation predictions were able to regenerate the temperature field in the consolidation process.
One important recommendation to be consider is the initial relative density. During the experimental part of this study, an important factor that has a direct effect on the stress – strain curves of the chips inside the chamber and compacted disks was the initial relative density. Literature on this specific effect was not found due to the fact that researchers interested in the behavior of the end product which is at relative density of 0.8 and higher that’s when this factor tend to vanish, but since the current study provides an understanding of the material behavior during and after compaction process, initial relative density is important to be noticed.
It is also recommended for future work to use a different mathematical model that governs the mechanical behavior of the chips material since the current model is the first
to be develop. Based on mechanical tests available, other models can be developed to better understand chips behavior.
Flow based models for the thermal behavior is also recommended. Combining temperature field studies along with material flow during the friction consolidation process will provide more information and understanding about the process.
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