FIELD OF THE INVENTION
The present invention relates to an improved process and a device for low-pressure die casting of Aluminum alloy Non Drive End ventilation fan for traction motor.
BACKGROUND OF THE INVENTION
In addition to regular spinning & vibratory loads in operation, a traction machine ventilation fan is subjected to shock loads when a train passes over track joints. Hence, failure of a ventilation fan is a common occurrence. Earlier, the fan used to be made by sand casting. In order to improve the quality of the fan and achieve the required microstructure, radiographic quality and mechanical strength, the low pressure die casting method was introduced in prior art for manufacturing of the fan. Accordingly, die set and core box set was developed for casting of the fan.
Among the innovative and conventional foundry processes for Aluminium alloys, low pressure die casting process is characterised by several advantages, including high yield, excellent control of operative parameters, good metallurgical and technological quality. This process is often (and incorrectly) associated only

to the production of automotive wheels, while the process has since improved its potential both towards other automotive components and non-automotive parts.
The increasing number of applications and products is the best proof of the success of Aluminium alloys foundry. This is probably one of the most dynamic fields inside manufacturing and engineering. The well-known advantages associated with the use of Aluminium alloys (light weight, good mechanical behaviour, good corrosion resistance, etc.) constitute the driving force for the introduction, on one hand, of new applications and, on the other hand, development of new processing solutions, to achieve both economically and technologically advantageous production of Aluminium alloys castings.
Among the most interesting processes, the low pressure die casting is certainly worth mentioning, thanks to its uniqueness, allowing, in several cases, an excellent compromise between quality, costs, productivity, geometrical feasibility. Even if such a process is quite old (the first patent, concerning casting of lead alloys, was deposited in England in 1910), its significant industrial application started forty years ago. Nowadays, it is adopted for casting Aluminium-and Magnesium-based alloys. The principle of this process is quite simple:

a permanent die and a filling system are placed over a furnace containing molten alloy (Fig. 1),
The filling of the cavity is obtained by forcing (by means of a pressurized gas, typically ranging from 0,3 to1.5 bars) the molten metal to rise into a ceramic tube (which is called stalk), which connects the die to the furnace. Generally speaking, the pressure used is roughly equivalent to 2 meters of an Aluminium column. Once the die cavity is filled, the overpressure in the furnace is removed, and the residual molten metal in the tube flows again towards the furnace. The various parts of the die are then separated, and the casting is finally extracted.
Specific attention is paid to the design of the die, so as to be controllable by means of proper cooling circuits, the solidification path of the alloy. The massive region of the casting has to be the last one to solidify and must be placed near to the stalk, which acts as a "virtual" feeder and allows to avoid the use of conventional feeders, thus improving the yield of the process, which becomes significantly high. The low injection velocity and the relatively high cycle time lead to a good control of the fluid-dynamics of the process, avoiding the defects originated by turbulence phenomena. Castings up to 70 kg weight can be produced, with tolerances of 0.3-0.6 %.

The die can be designed for the production of a single casting or for multiple castings, according to the size required and to the characteristics of the machine.
However, it should be remembered that the hollow inside part of a casting is formed by a core - this can be remembered by analogy with an apple core. Such shapes in sand are formed in a special sort of pattern called a core box. Specific attention has to be paid to the design of the core box.
Core sand (silica sand mixed with a two-part binder consisting of a phenol formaldehyde resin and a reactive isocyanate) to be blown into a core box and then hardened bypassing an amine catalyst in a carrier gas through it. Thus the core is more-or-less fully hardened by the time it is ejected from the box. This provides high productivity and high accuracy.
A resin-bonded sand core for pressure die casting methods having a first refractory coating, such as of silica, with an inorganic binding agent of colloidal silica and a day such as kaolin, and a second or top coating of a refractory material containing zircon and an organic binding agent, which combination of these two different coatings enables the bonded sand core to have high pressure and temperature resistance, good washout resistance, freedom from surface penetration, and good shake-out properties.

The advantages of low pressure die casting process are several:
- the high yield achievable (typically over 90%)
- the reduction of machining costs, thanks to the absence of feeders,
- the excellent control of process parameters which can be obtained, with a high degree of automation,
- the good metallurgical quality, thanks to a homogeneous filling and a controlled solidification dynamics, resulting in good mechanical and technological properties of the castings.
The applications of low pressure die casting in the automotive field are several, even if this process is often (and reductively) associated only to the production of wheels. Some examples of low pressure die casting products are collected in Fig. 2.
OBJECT OF THE INVENTION
It is therefore an object of the invention is to propose an improved process in and a device for low-pressure die casting of Aluminum alloy Non Drive End ventilation fan for traction motor.

SUMMARY OF THE INVENTION
Accordingly, there is provided an improved process and a device for low-pressure die casting Aluminum alloy Non Drive End ventilation fan for traction motor, the device comprising a set of die and a two-halved core box wherein the massive region of the casting placed adjacent to the casting stalk to act as a virtual feeder, the process comprising the steps of forming a core for the hollow inside part of the fan in the two halved core box by mixing silica sand with a two part binder consisting of a phenol formaldehyde resin and a reactive isocyanate and blowing the mixture into the core box including hardening the mixture through passing an amine catalyst in a carrier gas; ejecting the substantially hardened core from the box; placing the core in the die-cavity of said two-part die including a filling tube disposed over a furnace containing the molt en aluminum silicon alloy; removing the over-pressure developed in the furnace once the die-cavity is filled; reflowing the residual molten material in the filing tube towards the furnace; extracting the casting by separating the halves of the die; and stress-relieving and heat-treating the casting to obtain the desired microstructure and mechanical strength.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Fig.1 shows a permanent die and the filling system, placed over the furnace containing the molten alloy according to prior art.

Fig.2 shows some examples of low pressure die casting products.
Fig.3 shows a 3d model of Non Drive End ventilation fan
Rg.4 shows a top half of the die according to the present invention.
Fig.5 shows bottom half of the die according to the present invention
Fig. 6a and 6b show the view of the die set assembly of figures 4 & 5
Fig.7 shows the bottom half of a core box according to the invention
Fig.8 shows the top half of the core box according to the invention
Fig.9 shows the core box set assembly of figures 7 & 8.
Fig.lOa and 10b show drawing of Non Drive End ventilation fan(2 views)
DETAIL DESCRIPTION OF THE INVENTION
Die and core box is developed to make high quality casting of Non Drive End ventilation fan using low pressure die casting as per the drawing given in the fig.lOa ad 10b. Specific attention is paid to the design of the die considering shrinkage allowance, material properties and microstructure, to control by means of proper cooling circuits, the solidification path of the alloy. The massive region of the casting has to be the last one to solidify and must be placed near to the stalk, which acts as a "virtual" feeder and allows to avoid the use of conventional feeders, thus improving the yield of the process, which becomes significantly

high. Casting simulation software is used for design of die and gating system. The top and bottom halves of the die are shown in Rg.4 and Rg.5. Specific attention also paid to the design of the core box. Core box set is used for making of core for the hollow inside part of the fan/casting.The top and bottom halves of the core box are shown in fig.7 and fig.8,Core sand (silica sand mixed with a two-part binder consisting of a phenol formaldehyde resin and a reactive isocyanate) is blown into a core box and then hardened bypassing an amine catalyst in a carrier gas through it. Thus the core is more-or-less fully hardened by the time it is ejected from the box. This provides high productivity and high accuracy.
The core is placed in the die. The permanent die and the filling system are placed over the furnace containing the molten alloy(Fig.l). Once the die cavity is filled, the overpressure in the furnace is removed, and the residual molten metal in the tube flows again towards the furnace. The various parts of the die are then separated, and the casting is finally extracted. The casting are suitably stress relieved and heat treated to get the required microstructure and mechanical strength. The number of casting in a lot is decided by considering the thermal expansion of the die, to maintain desired tolerances in the casting.

WE CLAIM :
1. An improved process and a device for low-pressure die casting Aluminum alloy Non Drive End ventilation fan for traction motor, the device comprising a set of die and a two-halved core box wherein the massive region of the casting placed adjacent to the casting stalk to act as a virtual feeder, the process comprising the steps of :-
- forming a core for the hollow inside part of the fan in the two halved core box by mixing silica sand with a two part binder consisting of a phenot phenol formaldehyde resin and a reactive isocyanate and blowing the mixture into the core box including hardening the mixture through passing an amine catalyst in a carrier gas;
- ejecting the substantially hardened core from the box;
- placing the core in the die-cavity of said two-part die including a filling tube disposed over a furnace containing the molt en aluminum silicon alloy;
- removing the over-pressure developed in the furnace once the die-cavity is filled;
- reflowing the residual molten material in the filing tube towards the furnace;
- extracting the casting by separating the halves of the die; and

- stress-relieving and heat-treating the casting to obtain the desired microstructure and mechanical strength.
2. The process as claimed in claim 1, wherein the elongation is greater than 7-9% in the casting.
3. The process as claimed in claim 1, wherein the yield strength > 190 MPa in the casting.
4. The process as claimed in claim 1, wherein the tensile strength > 230 MPa in the casting,
5. The process as claimed in claim 1, wherein the Globular micro-structure is achieved in the casting.