POSSIBILITIES OF THE HOT EXTRUSION IN ALUMINUM CHIPS PROCESSING

This article deals with the analysis and comparison of the conditions for solid state welding during extrusions by means of numerical simulation. Numerical simulations made on four different matrices, such as square die without inte grated extrusion and equal channel angular pressing (iECAP), porthole die whit one and four sq uare profile. Maximum volume and distribution of effective strain, strain rate and s hear stress was analyzing. The results show that during cold molding of chips there is mutual deform ations of chips, which are rearranged and mechanically crimped. In the first phase of pressin g occurs complete elimination of porosity. The square die with iECAP was extrusion force is 30 % higher than porthole die. Maximum effective strain was calculated for the porthole di whit four square profile.


Introduction
Currently producing more and more products from light weight non ferous materials mainly based on Al, Ti, Mg, Ni and Be, the manufacture of which there is a considerable amount of waste.This waste can be in the form of chips or scraps produced in the manufacturing process as die casting and plastic forming processes, especially metal machining.Most common recycling processes are remelting of chips or scraps.However, these processes are energy-intensive and inefficient, as these materials have a high affinity for oxygen.The shape and thickness of the particles depends on the method of working.Particles can be short from sawing or long and narrow from high speed machining, milling and turning operations [1][2][3].The hot extrusion process can be used to direct conversion of aluminum chips to final products.Several papers [4][5][6] reported that direct extrusion with a lower ratio of 12:1 is not sufficient to establish a coherent / fixed connection between the chips.Due to the implementation of a new technologies operation, such as SPD (severe plastic deformation), into forward extrusion process.They are in the molding process to ensure a high plastic strain by simple shear.To a continuous process forward fit is possible to implement a number of methods such as equal channel angular pressing (ECAP) [7][8][9][10][11][12][13][14], twist extrusion (TE) [15][16][17], or friction stir processing called as KOBO [18,19].Authors [6,20] [4][5][6]21].Authors [4] reported that for the welding of the chips two criterions must be fulfilled.First the oxide layers must be broken down to allow virgin metal-to-metal contact and the second cumulative value of the ratio of the mean stress to the flow stress must be greater than a constant value.High plastic deformations are achieved when pressing hollow moldings, using porthole die.This article deals with the analysis and comparison of the conditions for solid state welding during extrusions by means of numerical simulation.Numerical simulations made on four different matrices, such as square die without integrated extrusion and equal channel angular pressing (iECAP), porthole die whit one and four square profile.Maximum volume and distribution of effective strain, strain rate and shear stress was analyzing.

Finite element modelling
Numerical simulation of forward extrusion chips were carried out using three different extrusion dies (Fig. 1).The first one the square die (Fig. 1a) is conventionally used in industry for solidsections produckt, the second one the square die whit integrated equal channel angular pressing (iECAP) tool including the fourECAP steps (Fig. 1b) procedures using in severe plastic deformation (SPD), and the third one Fig. 1 c,d porthole die that is typically used to produce complex hollow and semi hollow profiles.Three dimensional finite element simulations of forward extrusion process were performed using the commercial software Deform 3D TM with Lagrange formulation.3 Result and discussion Fig. 2 shows the model simulation of chips cold pressing.Fig. 2a shows the shape of chips for high speed machining.Its shape was numerically simulated in 2D (Fig. 2b).Numerical simulation of compaction chips consisted of 5 randomly arranged chips gravity.As can be seen in Fig. 2 at cold pressing of chips there is mutual deformations chips, which are mutually, rearrange and mechanically crimped.Mechanical connection is formed only.This process is dependent mainly on the shape and thickness of the chips.The compaction of the particles in the (semi) a pin are achieved varying bulk density of approximately 80% [4,6], it is according to the size of the pre-product and compacting pressure.In the simulation, the pin is defined as a porous material with a bulk density of 80%.As seen in Fig. 3, the first phase of chip billets pressing leads to complete elimination of porosity cause by the high hydrostatic pressure.This results was also documented by the authors [4].

Fig. 3 Density change of chip billets during forward extrusion
The results of the FEM simulations can be seen in Fig. 4, where the velocity, strain-rate effective and shear stress distributions during the extrusion through the square die and porthole dies are shown on the middle plane of the extrusion.
When square die deformation occurs first in the pre-chamber region.In the main deformation zone (MDZ) is seen intensive strain rate concentrated in a narrow field.Distribution strain after the effective thickness of the billet is shown in Fig. 6.Strain value reached effective deformation, e ≈ 6, in the extrusion ratio 45:1.The extrusion force reached a value F ≈ 1 MN (Fig. 5).
Fig. 4 The stress-strain analysis of forward extrusion chips using three different extrusion dies Implementation of the four ECAP steps leads to the generation of the natural back pressure in the ECAP channel [22].Back pressure is also transmitted to the pre-chamber region, which causes turbulent flow of materials.Effective deformation is almost twice as high (φ ≈ 10) as in the previous case.In place of intersecting canals is the rate of deformation concentrated in the narrow band which promotes the formation of simple shear.Shear deformation contributes to disruption of oxide layers on the particles, and thus helps to a better diffusion connection of chips.Intensive plastic deformation in ECAP channels caused an increase in the extrusion force of nearly 1.7 MN (Fig. 5).The high value of the extrusion force has been well documented in Article C. Plastic deformation in porthole die takes place in several places.First, the material is divided into four feeders and their ends are again combined and weld in welding chamber.To create a high shear stress, the area of the output profile to push feeders, as is schematically shown in Fig. 4. The complex material flow causes high effective strain, where porthole die whit one profile is φ ≈ 20 and for porthole die whit four profile is φ ≈ 30.The extrusion force for both cases is very close to the value of F ≈ 1.2 MN.

Fig. 1
Fig. 1 Schematic illustration of the three different extrusion process for solid state recycling of aluminum chips by hot extrusion, a) square die; b) flate-face die with iECAP; c) porthole die whit one profile and d) porthole die whit four profile.

Fig. 5
Fig. 5 Distribution of effective strain in cross-section of the extrude sample