Process for the Forming of Metal Alloy Wheel Rims

The present invention relates to a process for forming wheel rims in metal alloy.

Conventional single-piece forming processes used in the manufacture of wheel rims can be divided broadly into two types.

In processes of the first type, a semi-finished part or rim blank is obtained by a forging process, which consists in hot forming the metal alloy while it is ductile enough to make a rim of the required shape with the minimum of work. Rims made using this process have good mechanical properties but cannot have complex shapes. In forging, the blank is heated and then deformed by repeated application of a compressive force alternated, if necessary, with further heating operations. That means that complicated shapes, such as wheel rims, can be obtained only approximately. Moreover, the process involves several finishing operations which are not only complex but also very time-consuming. A process of this kind applied to the manufacture of wheel rims is described, for example, in U.S. Pat. No. 4,528,734.

In known processes of the second type, the semi-finished part or rim blank is obtained by casting in any of the known modes, for example, low pressure or gravity casting in dies made of refractory sand or metal. Shaping a metal by casting it in its molten state is a short and economical way to obtain products in a wide variety of shapes, even the most intricate. The mechanical quality of such products is, however, lower than that of forgings, mainly on account of defects such as microshrinkage or microporosity in the structure of the metal and, consequently, further processing and heat treatments are required to improve their mechanical properties. A process of this kind applied to the manufacture of wheel rims is described, for example, in U.S. Pat. No. 5,092,040.

The aim of the present invention as characterized in the claims below is to make improvements to casting technology by overcoming the typical drawbacks without affecting the advantages of the casting process constituted by its simplicity and low cost.

This aim is achieved in the present invention by providing a process for the forming of metal alloy wheel rims starting from cast blanks.

Further characteristics and advantages of the invention are apparent from the detailed description which follows, with reference to the accompanying drawings, in which:

FIG. 1 shows a blank and the die used to obtain it, schematically and in cross section;

FIGS. 2 and 3 show, schematically and in cross section half views, the sequence of steps in a single-piece forming cycle of a wheel rim according to the present invention;

FIG. 4 illustrates a wheel rim obtained using the process illustrated in FIGS. 1, 2 and 3.

With reference to the drawings listed above, the process for the forming of metal alloy wheel rims 1 envisages that an unworked piece or blank 2 be obtained by a casting process in which aluminum alloy billets are melted and the molten metal is then poured into the cavity of the die 8 of the blank 2. The blank 2 thus obtained is approximately in the shape of a double Y, in diametral cross section, with nearly constant thickness in all the branches of the Y section. This type of section avoids problems that may arise when thicknesses differ on account of nonuniform cooling of the blank.

When the time necessary to solidify the alloy has elapsed, the blank 2 is extracted from the die 8 and cold machined. The cold machining process (refer to FIG. 2) envisages a first chip removal operation in which the blank is rotated about its axis 20 and material cut away from its central portion 9, to make the hole for the hub, from the inner surface 4 of the blank 2 and from the lateral surface 12 of the blank 2. For clarity, the said first machining process is shown with a dashed line in FIG. 2. This machining process removes material from the blank 2 to generate a reference surface for subsequent machine operations and, at the same time, creates a blank of defined dimensions which hereinafter will be referred to as semi-finished work 3.

The reason for this first machine operation is that the casting process cannot guarantee a constant volume of material in all sections of the blank. The material must, however, be distributed uniformly all round the blank, that is to say, the allowance on the entire circumferential surface of the blank must be the same.

In this regard, it must be stressed that it is very important to guarantee that there are constant volumes of material distributed uniformly along the entire circumference of the blank 2. In fact, this is essential to enable the subsequent compression process known technically as "flow forming", to be carried out on the blank 2, this being a process which requires uniform thicknesses along the entire surface of the blank 2. If the thicknesses are different, the flow forming process applies nonuniform forces to the blank while it is being rotated. This gives rise to vibrations and knocking, preventing the process from proceeding correctly and worsening the quality of the result.

In such an event, the subsequent removal of excess burrs would require further processing, on a lathe, for example. Moreover, since the excess burrs would not be distributed uniformly, even lathing would be hampered by the resulting shocks and vibrations. In all events, therefore, it is very important that the total volume of material of the semi-finished work 3, including the uniformly distributed machining allowance, is within well-defined limits so as to avoid burring due to excess material, which would have to be removed by further machining.

In short, the subsequent flow-forming process is a constant volume process where the work has to have a well-defined, constant volume of material to start with.

The lathe turning process on the lateral surface 12 also creates a groove 6 (see FIG. 2) whose shape matches the outer profile of a flow forming roller 5. The purpose of this groove is explained in more detail below.

Next, the semi-finished work 3, that is, the one shown with the dashed line in FIGS. 2 and 3, is heated in a furnace to a temperature of preferably 380.degree. to 400.degree. C. During the subsequent flow forming process, the semi-finished work 3 heated in this way can be plastically deformed more easily and using less energy. Heating also avoids the problem of cracking which often arises in cold machining processes. Moreover, combined with the subsequent flow forming, it contributes to the elimination of casting defects.

After being heated, the semi-finished work 3 is fixed to a spindle 14 of a special flow forming lathe and locked in place by a tailstock 15 which rests against the front 19 of the rim 1, the outer shape of the spindle 14 substantially corresponding to the required end shape of the inner surface 4 of the rim 1. The semi-finished work 3 and the spindle 14 are turned about an axis which corresponds to the axis of rotation 20 of the rim. In this way, the subsequent flow forming process on the lateral surface 12 of the rim guarantees the concentricity of the lateral surface 12 with the axis of rotation 20.

The semi-finished work 3 is machined by flow forming on the areas constituted by the lateral surface 12 to create a channel 21 with an inner edge 17, an outer edge 18 and a middle portion 13, leaving a defined machining allowance depending on the subsequent heat treatments.

The flow forming process consists of a series of axial-symmetric forming operations whereby a rotating workpiece is compressed into shape by a suitable rolling tool 5 which rolls the material down to a predefined thickness.

The roller 5, only a half of which is illustrated in FIG. 3, should preferably be made of hardened steel and be rotated about its axis 5a by a known type of drive motor so that its peripheral speed is the same as that of the surface of the semi-finished work 3 so as to prevent the development of considerable tangential forces due to tangential friction between the two surfaces.

Initially, the roller 5 is inserted into the groove 6, made previously by the cold machining process, without compressing the lateral surface of the rim 1. It is necessary to proceed in this way because the rim, in the area of the disc 7, usually consists of an alternate sequence of spokes 10, that is, parts full of material, and gaps 11, that is, spaces free of material. If the roller 5 is inserted into the lateral surface 12 in an area corresponding to a gap 11, the thickness of the wall 16 is relatively small and contrasts the compressive action of the roller. Therefore, if the groove 6 is not made by cutting away material but by compression instead, the wall 16 of the lateral surface 12 would buckle. Nor would it possible to support the wall 16 of the lateral surface 12 in the area corresponding to the gap 11 since the tailstock 15 rests only against the front 19 of the rim and it would be extremely complicated to construct a tailstock to fit exactly into the gaps 11 of the rim 1 so as to provide adequate support for the wall 16 of the lateral surface 12. The groove 6 is also necessary to start the flow forming process on the lateral surface since the material must be rolled down to the required, smaller thickness in a single pass of the roller 5. According to a general embodiment, when the roller 5 is inserted into the groove 6 it moves sideways first in the direction of the front 19 of the rim 1, thus forming the outer edge 18, and then, after being inserted into the groove 6 again, moves sideways in the direction opposite the front 19, so as to form the inner edge 17 and the middle portion 13.

According to another embodiment, the outer edge 18 of the rim 1 is made beforehand at the casting stage because its axial dimensions are considerably smaller than those of the inner edge 17 and the middle portion 13. In this case, the flow forming process on the outer edge 18 is omitted and this part is only cold processed by removal of material

It should be noted that the profile of the lateral surface 12 of the roller 5 is formed by the combination of the two movements of the roller 5 in the axial and tangential directions with respect to the rim 1; the roller is driven preferably by a computer numerical control (CNC) system.

The compression generated by the flow process on the heated material squeezes out typical casting defects such as microporosity and microshrinkage and, in so doing, greatly improves the mechanical properties of the material. In particular, the wall 16 of the lateral surface 12 of the rim may be made considerably thinner than the corresponding wall of a rim obtained by casting. Moreover, the flow forming process totally eliminates the problem of air leaks through microporosity in the rim material, this being a significant improvement if one considers that practically all tires currently made are tubeless.

The process described above should preferably be followed by a solution heat treatment designed to hold, that is, retain the solid solution of the previously heated alloy. This treatment homogenizes the structure of the material deformed by the earlier flow forming process and relieves internal stress, especially in the area where the lateral surface 12 joins the disc 7. The solution heat treatment may be followed by age hardening in order to further improve the mechanical properties of the alloy.

These heat treatments, however, cause dimensional and geometrical variations in the rim. It is therefore necessary to leave a certain amount of machine allowance so that the rim can be worked down to the required size and shape by a suitable chip removal process.

The rim is then cold processed by cutting material away from the entire lateral surface 12, including the inner edge 17, the outer edge 18 and the middle portion 13, to remove the machining allowance left by the earlier flow forming process and to work the rim down to the required size. This process should preferably be performed using a diamond cutting tool capable of dealing with the considerable hardness of the alloy following the age hardening treatment and of producing a fine surface finish. The latter mechanical process also guarantees the perfect static and dynamic balance of the rim.

Finally, the holes for fixing the rim to the hub and the hole for the valve are made in the rim using known methods.

Although the manufacturing process described above is relatively simple, the rims 1 obtained in this way, illustrated in FIG. 4, have similar mechanical properties to forged rims and are better quality than cast rims.

The invention described can be subject to modifications and variations without thereby departing from the scope of the inventive concept. Moreover, all the details of the invention may be substituted by technically equivalent elements.