Flash Gap Research Articles

Flash Gap Optimization in Precision Blade Forging

Flash Gap Optimization in Precision Blade Forging

Investigation of a moveable flash gap in hot forging

The most common method of bulk forming processes is closed die forging with flash. In these processes a surplus of material is used to ensure a complete filling of the cavity of the forging die. The surplus material is driven out of the die through the flash land, thus the design of the flash land has a major influence on the filling of the die. All dimensions of the flash land are typically fixed during the manufacturing process of the die and can not be changed within the forging process. By use of a moveable flash gap that can be actively changed during the forging process the material flow can be altered. This permits to improve the filling of the cavity. In this paper a moveable flash gap for a hot forging process is described and the influence of such a system on the filling of the die cavity is determined. This is done by a comparison to a conventional forging process with a fixed flash land. Furthermore, the results of experimental trials are compared to results of corresponding FEA simulations. Additionally, the influence of the initial billet temperature is investigated. Experimental trials showed that the moveable flash gap has a distinct influence on the material flow. The higher the flash ratio, the bigger is the influence of the moveable flash gap. The moveable flash gap is designed as a flash brake of a height of 2mm. Its usage lead to differences in height of the parts up to 4.5mm, which correspond to 16.6% of the parts volume, compared to parts forged with a fixed flash land. If the forging temperature is decreased from 1200°C to 1000°C, the influence of the moveable flash gap is reduced. The average differences in height are about 0.5mm (about 3%).

Experimental investigation of a variable flash gap regarding material flow and influence of trigger forces

Closed die forging with flash is the most common method of bulk forming processes. One main property of these processes is the use of a surplus material to ensure a complete filling of the cavity of the forging die. The surplus material is driven out of the die through the flash land. The design of the flash land has a major influence on the filling of the die. Usually, the dimensions of the flash land are fixed during the manufacturing of the die and can’t be changed within the forging process. Additional machining operations are necessary to adapt to e.g. different process parameters or to occurring die wear which deteriorates the filling of the cavity. By use of a variable and moveable flash gap which can be actively changed during the forging process more material can be held in the cavity, thus permitting the improvement of the filling of the cavity. Additionally, such a system can be used to adapt to varying process parameters within the process. In this paper the investigation of a variable flash gap will be described. The influence of such a system on the filling of the die cavity is determined. This is done by comparison with a conventional forging process with a fixed flash land. Additionally, different trigger forces of the variable flash gap were investigated which also have an influence on the material flow during the forming operation. At last, the results of experimental trials are compared to results of FEA simulations. Experimental trials showed that the variable flash gap has a clear influence on the material flow. The higher the flash ratio, the bigger is the influence of the variable flash gap. Differences in height of the parts of 4.6 mm, which correspond to 17.2 %, were reached between a conventionally forged part and a part forged with a variable flash gap. Different trigger forces also have an influence on the height of the parts. The higher the trigger force, the bigger the influence of the variable flash gap. In general, the results match the predictions of FEA simulations. Experimental trials showed that a variable flash gap is able to improve the filling of the cavity. This method can be used to increase the quality of forging parts by a much easier adaption to different process parameters without additional machining operations.

Use of fractional factorial design for flash gap with wall lands in closed die forging

In order to improve the severe service conditions and balance the needs between the better cavity filling and longer die life of traditional closed die forgings, a new flash land named by resistance wall, which integrated the advantages of closed and open die, was introduced into knuckle hot extrusion dies. The fractional factorial design, , was employed to evaluate main effects of the structural parameters on the cavity filling, the maximum forming load and wear depth in bar fillet area, which was simulated by finite element method. Effects of six experimental factors: wall height, clearance, inclination, root fillet, flash thickness and bridge width were studied and optimised. Based on the results, a sort table about main effects was obtained and validated by production trials.

Hot forging of porous preforms in a closed die with a cone-shaped flash gutter: numerical simulation

The hot forging of porous preforms in a closed die with a cone-shaped flash gutter is numerically simulated. The cross-sectional distribution of strains and density over the preform substantially differs at various stages of the process. The maximum axial and radial strains, as well as intermediate density values, are observed near the flash gap.

Effect of extrusion wheel angular velocity on continuous extrusion forming process of copper concave bus bar

The continuous extrusion forming process for producing large section copper concave bus bar under different extrusion wheel angular velocities was studied by three-dimensional finite element technology based on software DEFORM-3D. The rigid-viscoplastic constitutive equation was employed in the model. The numerical simulation results show that the deformation body flow velocity in the die orifice increases gradually with the increase of the extrusion wheel angular velocity. But slippage between the rod and extrusion wheel occurs when the extrusion wheel angular velocity is high. The effective stress near the die orifice enhances gradually with increasing extrusion wheel angular velocity. High stress is concentrated in adjacent regions of the flash gap. The effective strain gradient is greater near the abutment than that near the die orifice. The effective strain of the product increases gradually with increasing extrusion wheel angular velocity. In the deformation process, the deformation body temperature increases remarkably due to friction and deformation. So the cooling is necessary in the region of the die and tools.

Use of SLFET for design of flash gap with V-notched lands in a closed-die forging

The article concerns an influence of the geometrical flash gap parameters on the closed-die forging process. It is focused on the analysis of die forging process where used two kinds of flash gaps with V-notched lands (which have not applied in industries yet). The theoretical calculations were carried out using an authors method of sequential analysis, called slip line fields elemental technique (SLFET). Additionally, in order to determine the best shape of flash gap (depending on forming load and degree of cavity filling by metal) the poly-optimisation was made. The special geometrical model of forging process, which is assumed a forming by extrusion of parts with a central boss (in plane-strain conditions) was assumed in these analyses. Beside, the experimental verification of worked out forging model and obtained selected numerical results was done. That research was realized in laboratory conditions with the use of commercially pure lead workpieces and the special designed tools-set (which assures a forming in conditions similar to plane-strain state). Generally, the obtained experimental results confirm a satisfactory agreement with theoretical results.

Use of UBET for design of flash gap in closed-die forging

An upper bound elemental technique (UBET) has been proposed for the design of the flash gap in closed-die forging operations of axisymmetric shapes. The developed method has been applied to analyze the forging load, die-cavity filling, and effective strain and strain rate distributions in axisymmetric closed-die forging with a rib-web type of cavity. The forging process has been superimposed with two sub-stages called the extrusion and the back-fill stages. The total die load required for extrusion and back fill are calculated separately and a maximum allowable extrusion gap is proposed for a given percentage die-fill. FEM simulations have been carried out with different die geometries and the results are in good agreement with UBET predictions.

Parametric investigation on the curling phenomenon in CONFORM process by three-dimensional finite element analysis

Owing to the circular material flow, a CONFORM continuous extrusion forming process exhibits more complicated process characteristics compared to conventional front- and side-extrusion processes. One remarkable thing is the curling phenomenon in the material flow after it passes through the die, and which is caused solely by the non-uniformity in material velocity profile at the die section. Since this process has been introduced, this phenomenon has not been intensively studied, and furthermore most of analytical and numerical studies for the process have been carried out with the simplified two-dimensional plane-state model. Through the three-dimensional finite element analysis of CONFORM process, we in this paper address the parametric investigation on the curling phenomenon together with the more realistic understanding of the flow characteristics in the die region. For the parametric experiments, we take the wheel velocity, the abutment height and the flash gap size as considering parameters.

Closed die axixymmetric forging of sintered aluminium preforms

The present work deals with an investigation into the closed-die forging of Aluminium powder preforms that have been compacted and sintered from very fine aluminium powder. The sintered preforms, of the same diameter but of different lengths, have been pressed in the cold condition. It is seen that the resistance to flow of the metal into the flash gap can be expressed in terms of the normal stress developed at the junction of the flash and the body of the forging. It appears that there exists a direct relationship between the normal stress and the flash thickness.

Diagnostics of the Profile and Rotational Symmetry of a Detonation Wave

AbstractIt is possible to record a detonation wave profile and to conduct a detailed analysis on its various symmetrical levels and on the rotational symmetry of a detonation wave in only “one” test by using a specific streak mask and applying multi streak or flash gap technique.

Computer-aided forging-die design for two-dimensional cross-sectional components

In the closed-die forging process which allows flash into the flash gap of the finisher die, forging design starting with the machined section geometry is the first procedure and is influenced by the nature of the metal to be processed, and the capabilities and limitations of the forging process. This paper describes some developments of an automatic forging-die design system for two-dimensional components and discusses several applications of the system. According to the design sequences, empirical guidelines which are concerned with the design of the forging and the finisher die, obtained from an extensive literature review, were first compiled in a structured and systematic form as design rules and database, some of them being introduced in the paper. Two-dimensional cross-sections (axisymmetric and plane-strain sections) can be dealt with by this system, being automatically decomposed, recognized and modified for the design of the forging and the finisher die. Accordingly, forging design which considers the geometric features of the section of the machined part can be carried out. The results obtained from the system developed at authors' laboratory have been compared with those from other sources, the results corresponding well, apart from a few minor differences. The design results of the system were investigated according to forging type, forging material and flat-web location.

Study of metal flow in extrusion forging of rectangular billets

The deformation of a rectangular billet between a groovd extrusion-forging mode in which the billet material is forced simultaneously into the die orifice and the flash gap. This mode of deformation is similar to the initial stages of the closed-die forging process, the understanding of such deformation process being important for both product quality control and forming tool design. In this paper, the metal flow behaviour during the extrusion forging of a rectangular billet between a grooved die and a flat die has been studied using the finite-element method with a view to extending the application to more complicated practical cases. The emphasis has been on the deformed profiles rather than on the forging load. Experiments using two sets of dies with different parameters and lead billets were performed, good agreement being found between the experimental and numerical results in terms of the deformed profiles.

Review of flash design rules for closed-die forgings

In an extensive literature review the authors have found 16 different rules or procedures for calculating the flash gap dimensions T (thickness of flash), W (width of flash land) and ratio W/T. These rules were implemented in an application program in an interactive CAD environment. For 10 existing axi-symmetrical industrial forgings, the flash dimensions were calculated with these rules and compared with the flash dimensions actually used to produce these forgings in industry.

Detection of the helium flash gap in the integrated (U - B) versus (B - V) diagram for 624 Large Magellanic Cloud clusters

The number of Large Magellanic Cloud clusters was measured with integrated UBV photometry by more than a factor of 4, now totaling 624 objects. A gap in the cluster distribution through the color-color diagram appears in the region of the equivalent SWB type IV. The amplitude of the gap is ∼0.1 mag in both colors. The turnoff ages of a few clusters near the gap edges perfectly fit theoretical predictions of a red giant branch phase transition. This color jump is due to the first appearance of stars suffering the helium flash, which form a bright and populous red giant branch that persists through the subsequent cluster evolution. As an additional result, evidence is found that Hodge 7 (SL 735) might be a classical globular cluster

Flow of metal into the flash gap in the last stages of a plane-strain closed-die forging operation

In closed-die forging the flash geometry should be such as to ensure that the cavity is completely filled just as the two dies come into contact at the parting plane. If metal is caused to extrude through the flash gap as the dies approach the point of contact — a practice generally resorted to as a means of ensuring complete filling — dies are unnecessarily stressed in a high-stress regime (as the flash is quite thin and possibly cooled by then), which reduces the die life and unnecessarily increases the energy requirement of the operation. It is therefore necessary to carefully determine the dimensions of the flash land and flash thickness — the two parameters, apart from friction at the land, which control the lateral flow. The dimensions should be such that the flow into the longitudinal cavity is controlled throughout the operation, ensuring complete filling just as the dies touch at the parting plane.The design of the flash must be related to the shape and size of the forging cavity as the control of flow has to be exercised throughout the operation: it is possible to do this if the mechanics of how the lateral extrusion into the flash takes place is understood for specific cavity shapes and sizes.The work reported here is part of an ongoing programme investigating flow in closed-die forging. A simple closed shape (no longitudinal flow) which may correspond to the last stages of a real forging operation is analysed using the stress equilibrium approach. Metal from the cavity (flange) flows into the flash by shearing in the cavity in one of the three modes considered here: for a given cavity the mode with the least energy requirement is assumed to be the most realistic. On this basis a map has been developed which, given the depth and width of the cavity as well as the flash thickness, will tell the designer of the most likely mode (of the three modes considered) in which metal in the cavity will shear and then flow into the flash gap. The results of limited set of experiments, reported herein, validate this method of selecting the optimum model of flow into the flash gap.

The influence of flash design on material flow and tool pressure in closed-die forging: a practical example

The closed-die hot-forging of components for the automobile industry involves heavy forces, high energies and severe wear conditions in the die. After a relatively short period of time in service — corresponding to the production of about 5000 components — the dies must be re-engraved. A conventional die is usually re-engraved twice and consequently the tool can only be used for the production of about 15 000 components. The cost of the tools amounts to about 10% of the total expenses for closed-die forged products. The material yield in closed die forging is low: the material losses are about 30%, the main part of which constitutes the flash. Considering the total expenses for a product, the cost of the material makes up about 50%. From the above, it is clear that the material yield and the tool life are two factors of utmost importance in considering the economy of a closed-die forging process. In the present work, plane-strain numerically-optimized upper-bound solutions for various flash designs are presented for the closed-die forging of a spindle. Metal flow just before and just after filling of the die cavity is analysed. The theoretical results are compared with experimental results obtained by the use of plasticine. It is concluded that from the material-yield point of view it is favourable to use a flash land with a V-notch. When the material flows through this kind of flash gap the notches are filled with material and intense shear takes place within the material itself. This kind of braking or restraining device is found to improve the material yield by restricting the material flow out into the flash gap. After die filling, however, the mean die-pressures are much heavier than those for dies with conventionally lubricated flat flash-lands. V-notched flash-lands are recommended provided that it is possible to stop the deformation process just before die filling. This statement pre-supposes, however, that the initial shape and weight of the workpiece can be accurately controlled and reproduced.

Shock-compression parameters for uranium dioxide from 5 to 330 GPa

Calculations related to liquid metal, fast-breeder-reactor, excursion analyses based on extrapolations of published Hugoniot data for UO2 are in serious error because the experimental techniques used to obtain the data did not resolve the strong multiple-wave shock structures present. We have used the inclined prism, flash gap, and two-stage gas-gun techniques to determine shock-compression parameters for UO2 up to 330 GPa. The Hugoniot elastic limit for UO2 was found to be 5.7 GPa. The low-pressure shock velocity, Us, can be described in terms of particle velocity, Up, by Us = 3.88 + 1.11 (UP−0.3) for 0.3 < UP < 1.05 km s−1. At high pressure, a plot of shock versus particle velocity displays a discontinuity between 1.05 < Up < 1.8 km s−1, which appears to be a manifestation of a solid-solid phase transition. For 1.8 < UP < 4.0 km s−1, the plot is given by Us = 5.8 + 1.28 (UP−1.8). This curve differs substantially from those previously reported.

Pressure-induced phase transition in GaAs under shock compression

Shock compression experiments for GaAs are performed up to about 400 kbar. Hugoniot data are obtained both by the argon flash gap technique and by the inclined mirror technique. Inclined mirror experiments reveal the three-wave structure, yielding the Hugoniot elastic limit (HEL) and apparent phase transition pressure to be 84±8 kbar and 203±11 kbar, respectively. A correction to remove the effect of HEL on the phase transition pressure reduce the latter value to 162±11 kbar, which corresponds to the static phase transition pressure. Preliminary argon flash gap experiments for GaP show the phase transition at about 260 kbar.

The Filling of the Flash Gap in Closed-Die Forging

The pressures required to drive overflow material into a flash gap of annular shape and ever-decreasing thickness are calculated in the presence of thermal conduction into the die. The behavior of the plastic boundary layer near the die surface is also examined in relation to the transient flow of heat into the die. The energy expended in material acceleration is considered in the analysis and is shown to have a dominant influence on power requirement in some instances.