Method of producing a light alloy wheel rim and an improved light alloy wheel rim for a motor vehicle

Abstract

The invention relates to a wheel rim made from a light alloy, such as an aluminium-based alloy, comprising a front face (10) and a tyre tread (20) which are welded together. The inventive method of producing the wheel rim (J) comprises the following steps: moulding of the front face (10) with a light alloy; provision of a weldable alloy strip with structural hardening; curving of said strip to produce a generally cylindrical shape; welding of the ends in relation to the curved strip with fusion of the strip material; forming of the strip by necking and/or spinning in order to form the tyre tread (20); and welding of the front face and the tyre tread to one another.

Description

The present invention relates to a process for producing wheels made of light alloy, especially aluminum alloy, in particular for motor vehicles. More particularly, the invention provides a process for producing a wheel that has improved properties both in terms of lightness and mechanical strength, and also a novel wheel having these improved properties.

PRIOR ART

Most light-alloy wheels are produced at the present time as one-piece castings, usually by employing a low-pressure filling technique in which the mold is filled from the bottom or, more rarely, from the top by gravity.

These two filling techniques have the drawback of requiring the solidification of the alloy forming the wheel to be controlled over the entire length of the latter, and over substantial distances. Thus, during low-pressure filling, the solidification must typically be controlled from the inner wheel flange as far as the center of the wheel, in the middle of the front face (located in the bottom portion of the mold) at which the low-pressure filling with liquid metal takes place and at which the solidification terminates.

Such a control of the solidification is relatively problem-free in the case of small-diameter wheels (typically up to 15 inches in diameter); however, it becomes much more tricky in the case of large-diameter wheels because of the corresponding increase in the mass of metal. This is because such an increase results in a reduction in the molding production rates and a greater difficulty in avoiding molding defects of the shrinkage cavity type.

Moreover, the styles of wheels that are becoming very popular at the present time are what are called “full face” wheels, that is to say those in which the front face does not possess the usual recess relative to the external wheel flange but, on the contrary, approaches, or even becomes tangential with, this wheel flange, thereby creating a large mass under the tire support.

FIG. 1a of the drawings shows, in axial half-section, the appearance of such a full face wheel Ja and the aforementioned substantial mass Ma. In comparison, FIG. 1b shows, in the opposite axial half-section, the appearance of a more conventional wheel Jb.

The existence of this mass Ma, which is difficult to solidify rapidly, is prejudicial not only to the entire production process, since the production rates are reduced, but also to the lightening effect—ever desirable as this is one of the essential strong points of light-alloy wheels—since the mold structure used does not prevent these substantial masses beneath the support for the tire.

It should also be emphasised that molding a one-piece wheel has the drawback of requiring relatively complex molds that are composed, as shown in FIG. 2, of a sole S, an upper core NS and sides C1 and C2, the sole defining an up-filling gate Ch for the low-pressure liquid metal feed.

To conclude, it should be noted that the use of a single material, unavoidable when producing a one-piece wheel, poses problems in terms of impact strength and of behavior in the indentation test, particularly near the wheel flanges, and therefore the thickness of the material at these wheel flanges has to be significantly increased.

It was also proposed many years ago to produce wheels in two parts. The general idea consists in molding the front face (external face) of the wheel by a conventional, especially low-pressure, light-alloy molding process and in producing the rim in a wrought alloy.

However, the techniques used today are not well suited to this type of product. Thus the rim is produced, for example, by butt welding before being rolled, using an approach similar to that for steel wheels. After this part has been produced, the front face, machined beforehand, is then fastened to this rim.

This known technology has many drawbacks. Firstly, the Applicant has demonstrated that the butt welding cannot easily be carried out on structural hardening alloys (such as Al—Si—Mg alloys of the 6000 series, which are subject to precipitation hardening during a solution-quench-temper heat treatment) but can be accomplished in practice only for alloys that do not undergo structural hardening (such as alloys of the 5000 series). Now, these alloys have the drawback of having a low strength potential, which does not exceed that of the casting alloys normally used, such as the alloy known by the name AS7G (aluminum alloy containing 7% by weight of silicon and 0.3% by weight of magnesium).

Moreover, the investment needed for this technology is very high, not only as regards the butt welding, but also as regards the subsequent forming operation carried out by rolling over rolls.

Alternative versions of this technology do exist, and relate to various ways of fastening the front face to the rim, but these versions do not solve the main problem mentioned above, and in particular the problem of the mechanical strength of the alloy used for the rim.

Yet another known technology consists in working from a circular blank which is progressively subjected to flowturning on a mandrel in order to produce a one-piece rim. The bottom of the blank, that cannot be used after the rim has been formed, is then cut off and lost. It will be understood that this makes the process extremely expensive and economically ill-suited to the mass-produced automobile market.

SUMMARY OF THE INVENTION

The present invention aims to provide a process for producing a wheel made of light alloy that overcomes or at least alleviates the abovementioned problems and that makes it possible to obtain a light wheel for a minimal cost, while still maintaining suitable mechanical properties.

In this regard, it was surprisingly found that, by using a light alloy that had undergone structural hardening for the rim and by forming this rim by bending, it is possible to use a welding technique involving the fusion of the parts (the facing ends) while still obtaining the final mechanical properties, especially in terms of the strength of the welded region, which are completely satisfactory.

Thus, the present invention proposes, according to a first aspect, a process for producing a wheel made of light alloy, such as an aluminum-based alloy, for a motor vehicle, the wheel comprising a front face and a rim that are welded to each other, characterized in that it comprises the following:

• • the step of casting the front face with a light alloy;
  • the step of providing a strip made of weldable, structural hardening alloy;
  • the step of bending this strip in order to give it a cylindrical general shape;
  • the step of welding the facing ends of the bent strip, with fusion of the material of the strip;
  • the step of forming the strip, by swaging and/or flowturning, in order to form the rim; and
  • the step of welding the front face and the rim to each other.

Preferred, but non-limiting, aspects of this process are the following:

• • the step of forming the strip is preceded by a step of expanding the strip, after its ends have been welded together;
  • the forming step is preceded by an at least partial solution-treatment and quenching operation;
  • the weldable, structural hardening alloy is an aluminum alloy of the 6000 standard series;
  • the step of welding the facing ends of the strip is carried out by a welding technique chosen from the group comprising electron-beam welding, vacuum electron-beam welding, MIG (metal inert gas) welding, tandem MIG welding, laser welding and combined MIG/laser welding;
  • the operation of casting the front face includes the formation of at least one weight-reducing recess, obtained by casting;
  • at least one recess is on an inner side of the front face;
  • at least one recess is near a connection region between the front face and the rim; and
  • at least one recess opens in a demolding direction of the front face.

According to a second aspect, the present invention provides a wheel made of light alloy, such as an aluminum-based alloy, for a motor vehicle, the wheel comprising a front face made of light alloy and a rim also made of light alloy that are welded together, characterized in that the rim is produced by bending and forming a strip made of a weldable, structural hardening alloy, with the facing ends of the bent strip welded together with fusion of the strip material.

Preferred, but non-limiting aspects of this wheel are the following:

• • the weldable, structural hardening alloy is an aluminum alloy of the 6000 standard series;
  • its front face includes at least one weight-reducing recess, obtained during the casting;
  • at least one recess is on an inner side of the front face;
  • at least one recess is near a connection region between the front face and the rim;
  • at least one recess opens in a demolding direction of the front face; and
  • the wheel possesses two wheel flanges, one of which is formed on the front face and the other of which is formed on the rim.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, objects and advantages of the present invention will be more clearly understood on reading the following detailed description of a preferred embodiment of the invention, given by way of example and with reference to the appended drawings.

In the drawings, in addition to FIGS. 1 and 2 that have already been presented above:

FIG. 3a is a front half-view of a wheel according to one embodiment of the invention;

FIG. 3b is a half-view in axial section of the wheel of FIG. 3a;

FIG. 4 is a perspective view of a strip for forming the rim of the wheel of FIGS. 3a and 3b, after two first steps, namely bending and welding; and

FIG. 5 illustrates, in three cross sections respectively, the appearance of the strip before subsequent forming, and the appearance of the strip after the two subsequent forming operations.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

A process will now be described for producing a light-alloy wheel formed from a front face, preferably having all the lightening features that the one-piece casting of this front face allows, and especially the mass-reducing features beneath the supports for the tire. This front face is welded to a rim with a technique allowing the use of any structural hardening wrought alloy provided that it is weldable.

FIGS. 3a and 3b of the drawings illustrate a wheel J produced according to the invention in two parts, namely a front face 10 and a rim 20. a FIG. 3 of the drawings illustrates the type of lightening that is achieved directly by casting on the rear of the front face of the wheel, avoiding, compared with the one-piece section of FIG. 1a, having to cast metal in regions 11 and 12 that open, in the direction parallel to the axis x-x, toward the inside of the wheel.

FIG. 3b also shows the front wheel flange CJ1, formed on the front face 10 near the recesses 11, 12, and the rear wheel flange CJ2, formed on the rim 20.

It is important to note here that the type of geometry shown in FIG. 3b can be obtained using very simple molds, formed for example from two elements, namely a sole (that is to say the bottom part of the mold, producing the front face of the wheel with its chosen style) and an upper core (producing the rear face of the wheel, and comprising projecting parts that define the lightening regions 11 and 12).

Since the front face 10 and the rim 20 are produced separately, unlike in the one-piece low-pressure casting process or the like mentioned in the introduction, it is possible here to cool the periphery of the front face very rapidly and consequently obtain an increased fineness of microstructure and properties in the core of the wheel that it is not possible to obtain by casting a complete one-piece wheel, the advantage being more appreciable the larger the diameter of the wheel.

As an indication, table I below gives the properties of a low-pressure-cast 15-inch one-piece wheel according to the prior art compared with those of just a front face, having the same style and also produced by low-pressure casting. In both cases, the temperature of the metal in the furnace was 710° C., the alloy used was an AS7G alloy modified with 120 ppm of strontium, refined with titanium diboride TiB₂, and the filling rate was 7 mbar/second.

After unmolding and fettling, a treatment known as a T6 treatment was carried out on the two parts, this comprising the following steps:

• • solution heat treatment for 5 hours at 535° C.;
  • quenching in water at 50° C.;
  • tempering for 3 hours 30 minutes at 160° C.

The properties obtained, measured according to the NF standardized tensile tests on tensile test pieces taken from the arms of the wheel, are given in table I below.

	TABLE I
	Test piece	Rm (MPa)	Rp0.2 (MPa)	A (%)
	(1)	278	220	5.5
	(2)	285	192	13.0
	(1): One-piece cast wheel front face;
	(2): Cast front face alone.

The better properties obtained on the cast front face alone are explained by the fewer shrinkage microcavities, stemming from more effective cooling of the periphery of the front fact. This therefore results in an improved microstructure, which is also favorable to the fatigue strength of the wheel.

Turning now to the rim, the technique used is as follows: starting with a sheet blank or strip made of light alloy, cut to the desired thickness (typically around 3 to 4.5 mm in the case of wheels ranging from 15 to 17 inches, a rim is produced by bending the sheet in order to give it a cylindrical shape and by welding it edge to edge using a welding technique illustrated in FIG. 4. This technique is preferably an electron-beam welding technique, which may or may not be carried out in a vacuum, or else an MIG (metal inert gas) welding technique and its various versions, namely tandem MIG welding, laser welding or combined MIG/laser welding.

Thus, this welding technique avoids having to use the conventional technique called “butt welding” mentioned in the introduction, which it turns out cannot be applied effectively to a structural hardening alloy, and this means that a large variety of weldable, structural hardening alloys can be used.

The rim is then subjected to a series of forming operations, essentially by swaging using the following succession of steps:

• • prior broadening by expansion of the rim; this operation may be carried out on any known type of expander, for example a segment expander or an expander having internal formers pressed against one another; this operation has the advantage, apart from making the subsequent forming of the rim easier, of testing the quality of its butt weld before any subsequent operation; in this regard, a poor weld quality will be revealed at this stage by cracks in line with the welded zone; and
  • creation of the final geometry by a series of swaging steps such as those illustrated for example in FIG. 5, starting from a straight shape running along the direction a-a; these operations are carried out on a steel mandrel of suitable geometry and on conventional swagers (or as an alternative by flowturning).

To take advantage of the optimum deformability specific to structural hardening alloys, the rim is worked in the above forming steps preferably cold in what is called the “T4” state (solution heat treatment followed by a quench) or else in a thermal state corresponding to a partial solution heat treatment, so as to maintain the subsequent structural hardening potential.

Once these operations have been carried out, the rim is welded to the front face. Preferably, for weight reasons a butt welding technique is used, as shown in FIG. 3b, the weld bead being indicated as 30. Other techniques may also be employed. Furthermore, to ensure that the front face and the rim are mutually positioned correctly, a small centering stud may be provided.

Conventional welding techniques, such as electron-beam welding, MIG (metal inert gas) welding and its various versions, namely tandem MIG welding, laser welding or combined MIG/laser welding, may be used to weld the front face to the rim.

Advantageously, before this welding operation, the welding zone of the rim is lightly machined, allowing a clean surface of well-calibrated dimensions to be obtained.

Within the same context, light machining of the other side of the rim ensures that it has a good surface finish and excellent dimensional precision.

A person skilled in the art would have no difficulty in choosing the adjustments to be made in the welding operation so as to minimize the deformations of the wheel, so as to avoid having to machine again the wheel after this operation.

The wheel is then ready for the usual final surface-treatment operations, application of paint and baking, followed by inspection.

In this regard, the final mechanical properties obtained preferably result from the tempering treatment which takes place simply owing to the operation of baking the paint applied to the entire wheel (after the front face has been welded to the rim and after the surface treatment), typically for 20 minutes at a temperature of around 160° to 185° C.

Of course, it is possible, as a variant, for the tempering treatment to be carried out separately from this baking and/or for this tempering to be carried out on only the rim, before it is welded to the front face. In the latter case, the front face itself undergoes, where appropriate, a specific heat treatment such as a solution heat treatment and quench.

It should be noted here that it is preferable to avoid a solution heat treatment and a quench after the step of welding the rim to the front face, so as to avoid any relatively large deformation brought about by the quench, which would require a further annealing of the entire wheel, and therefore of the rim.

As an example, table II below gives the properties of an indented rim belonging to a one-piece cast wheel as described in the introduction of the present application, with a diameter of 15 inches, made of an AS7G casting alloy, compared with those of a rim belonging to a wheel of the same diameter produced according to the invention. The rim according to the invention was made using the following succession of steps:

• • cutting of a starting sheet made of 6081 alloy (the name as per the United States Aluminum Association) with a thickness of 4 mm;
  • bending into a cylinder;
  • welding the cylinder using a tandem MIG process;
  • subsequent forming using the steps illustrated in FIG. 5 to obtain a rim with a thickness of about 3 mm in the wheel recess (near the weld 30) and -a thickness of 4 to 4.5 mm in the region of the flange CJ2; and

tempering at 185° C., carried out on just the rim before it is welded to the front face.

	TABLE II
	Test piece	Rm (MPa)	Rp0.2 (MPa)	A (%)
	(1)	269	203	4.5
	(2)	260	210	16.0
	(1): Guttered rim on a one-piece wheel;
	(2) Guttered rim on a two-piece wheel according to the invention.

It follows from tables I and II above that a wheel produced in two parts according to the present invention is capable of meeting standard specifications substantially as well as a one-piece wheel.

If we now compare, in terms of weight, these two 15-inch wheels, it may be seen that there is a significant weight saving; specifically, the one-piece wheel of the prior art weighs about 7.9 kg, whereas the two-piece wheel according to the invention weighs about 6.5 kg, i.e. a weight saving of almost 20%, which is considerable.

Of course, the present invention is in no way limited to the embodiment described and illustrated, and a person skilled in the art will know how to make many alternative versions or modifications thereof.

Claims

1. A process for producing a rim (J) made of light alloy, such as an aluminum-based alloy, for a motor vehicle, the rim comprising a front face (10) and an outer rim piece (20) that are welded to each other, characterized in that it comprises the following:

the step of casting the front face (10) with a light alloy;

the step of providing an outer rim piece made of weldable, structural hardening alloy;

the step of bending this outer rim piece in order to give it a cylindrical general shape;

the step of welding the facing ends of the bent outer rim piece, with fusion of the material of the outer rim piece;

the step of forming the outer rim piece, by swaging and/or flowturning, in order to form the outer rim piece (20); and

the step of welding the front face and the outer rim piece to each other.

2. The process as claimed in claim 1, characterized in that the step of forming the outer rim piece is preceded by a step of expanding the outer rim piece, after its ends have been welded together.

3. The process as claimed in claim 1 or claim 2, characterized in that the forming step is preceded by an at least partial solution heat treatment and quenching operation.

4. The process as claimed in claim 1, characterized in that the weldable, structural hardening alloy is an aluminum alloy of the 6000 standard series.

5. The process as claimed in claim 1, characterized in that the step of welding the facing ends of the outer rim piece is carried out by a welding technique chosen from the group comprising electron-beam welding, vacuum electron-beam welding, MIG (metal inert gas) welding, tandem MIG welding, laser welding and combined MIG/laser welding.

6. The process as claimed in claim 1, characterized in that the operation of casting the front face (10) includes the formation of at least one weight-reducing hollow (11, 12), obtained by casting.

7. The process as claimed in claim 6, characterized in that at least one hollow (11, 12) is on an inner side of the front face (10).

8. The process as claimed in claim 6, characterized in that at least one hollow (11, 12) is near a connection region between the front face (10) and the outer rim piece (20).

9. The process as claimed in claim 6, characterized in that at least one hollow (11, 12) opens in a demolding direction of the front face.

10. A rim (J) made of light alloy, such as an aluminum-based alloy, for a motor vehicle, the rim comprising a front face (10) made of light alloy and an outer rim piece (20) also made of light alloy that are welded together, characterized in that the outer rim piece (20) is produced by bending and forming a part made of a weldable, structural hardening alloy, with the facing ends of the bent outer rim piece welded together with fusion of the material of the outer rim piece.

11. The rim as claimed in claim 10, characterized in that the weldable, structural hardening alloy is an aluminium alloy of the 6000 standard series.

12. The rim as claimed in claim 11, characterized in that the front face (10) includes at least one weight-reducing hollow (11, 12), obtained during the casting.

13. The rim as claimed in claim 12, characterized in that at least one hollow (11, 12) is on an inner side of the front face.

14. The rim as claimed in claim 12, characterized in that at least one hollow (11, 12) is near a connection region between the front face and the outer rim piece.

15. The rim as claimed in claim 12, characterized in that at least one hollow (11, 12) opens in a demolding direction of the front face.

16. The rim as claimed in claim 10, characterized in that it possesses two rim flanges (CJ1, CJ2), one (CJ1) of which is formed on the front face (10) and the other (CJ2) of which is formed on the outer rim piece (20).