Forged scroll part and production process thereof

Details Forged scroll part and production process thereof Forged scroll part and production process thereof

2002-03-20

2004-03-09

Wyszomierski, George (Department: 1742)

Metal treatment

Process of modifying or maintaining internal physical...

With casting or solidifying from melt

C148S692000, C148S693000, C148S696000

Reexamination Certificate

active

06702907

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an aluminum alloy-made forged scroll part for a scroll compressor employed mainly in an air conditioner and to a process for producing the part.
BACKGROUND ART
In recent years, scroll compressors have become of great interest as air conditioner compressors, because, for one reason, such a scroll compressor contains a small number of parts and is driven silently. The scroll compressor includes a fixed scroll having a spiral wrap portion
11
provided on a flange portion
12
as shown in
FIG. 2
, and an orbiting scroll having a spiral wrap portion whose shape is similar to that of the portion
11
, the spiral wrap portion of the orbiting scroll being driven for orbital movement such that these spiral wrap portions face each other in a fitted state.
In many cases, a fixed or orbiting scroll (hereinafter referred to simply as a “scroll”) is produced from aluminum alloy in order to reduce the weight of a resultant compressor. The scroll is produced through, for example, casting or forging. In order to provide the scroll with strength and reliability, forging is advantageously carried out for producing the scroll. Since the scroll has a complicated shape, it must be produced through hot forging.
FIG. 3
shows a conventional process for production an aluminum alloy scroll part through forging.
First, an aluminum alloy prepared by mixing alloy components is melted and then cast through continuous casting into a billet (BL) for extrusion having a diameter of 200 mm or more. After the inside of the BL is homogenized through heat treatment, the BL is cut into pieces such that they have an identical length so as to provide round bars each having a predetermined diameter, and each piece is subjected to extrusion to thereby form a round bar (extruded round bar).
Usually, the diameter of the extruded round bar is almost equal to the outer diameter of a forged part. The round bar is cut into pieces, and each piece is employed as a stock material for forging. As will be described below, in order to facilitate production of a scroll part, before forging of the stock material the cut piece may be pre-shaped, if necessary, through forging or machining into a piece having a shape similar to that of the scroll part, so as to employ the pre-shaped piece as a stock material for forging.
The stock material is forged into a scroll part usually through hot forging. In order to provide the forged part with strength, after forging, the part is usually subjected to solution (quenching) and aging heat treatment.
In order to enhance precision in the size of the forged part, a portion of the surface of the part is then subjected to machining, if necessary.
FIG. 4
is a schematic cross-sectional view showing a conventional scroll-forging process. A workpiece
4
placed in a die
2
is pressed downward with a punch
1
to thereby form the wrap portion
11
. Usually, the distance that the punch
1
moves is determined to be consistent in order to make the thickness of a flange portion
12
of the scroll consistent.
JP-A-SHO 54-159712, 59-61542 and 62-89545 disclose a process for forging an aluminum alloy-made scroll, in which, in order to precisely forge a workpiece into a scroll wrap, the workpiece is subjected to forging or machining in advance so as to provide the piece with a preliminary shape, and the workpiece is then forged into the scroll wrap. The reason why such a preliminary process is carried out is that since the wrap portion
11
has a spiral shape and large height and is connected to the flange portion
12
, when a workpiece is forged into a scroll as shown in
FIG. 4
, a wrap portion having a uniform height is difficult to form. Therefore, a workpiece having an intermediate shape is formed in advance. The process can provide a produced scroll with a shape with some degree of precision. However, the process requires designing of an intermediate shape which matches the final shape of the scroll, and preparation of a forging die employed for intermediate processing. Consequently, the process includes complicated steps and involves high costs, presenting difficulty in practice.
JP-A-SHO 60-102243 and JP-A-HEI 06-23474, among other publications, disclose a back-pressure forging process in which a workpiece prepared only by cutting a round bar is employed without being subjected to pre-processing before forging and, during forging of the workpiece, a load is applied to the end portion of a scroll wrap
11
in a direction opposite to the forging direction in order to control material flow so as to realize a uniform flow into a wrap-shaped mold and to reduce variation in the height of the scroll wrap
11
. According to the process, by using a workpiece prepared only by cutting a round bar, a scroll in which variation in the height of a wrap portion
11
is reduced can be produced at low cost with high productivity.
To be specific, the back-pressure forging process for a scroll is schematically shown in the cross-sectional views of
FIGS. 5 and 6
. A workpiece
4
is pressed downward with a punch
1
and forged into a wrap formation space
2
a
of a die
2
while the knockouts are retracted to thereby form a wrap
11
. During the forging, a load lower than a punch pressure is applied as a back pressure through the wrap formation space
2
a
by means of knock pins
7
and knockouts
6
to the end of the wrap in the direction opposite to that of the forging (FIG.
5
). As a result, a scroll part
5
comprising a flange portion
12
with a predetermined thickness L
1
and the wrap
11
with a uniform height L
2
depending vertically from the flange portion can be formed as shown in FIG.
7
.
The back-pressure forging process exerts, to some extent, the effect for making the overall height of a spiral wrap of a forged scroll part uniform.
Although variation in the height of a wrap of a scroll can be regulated to some extent according to the back-pressure forging process, wrap height varies between individual scrolls unless the thickness of individual cut materials, i.e. the weight of individual workpieces, is strictly controlled when cutting the round bar. Therefore, a margin for machining of the end of a wrap must be controlled in every forged part at a post-processing step. Alternatively, in consideration of different wrap heights among scroll products, slightly large-sized scrolls must be forged to provide scrolls with a large margin for machining at a post-processing step. This results in low yield.
In the back-pressure forging process, when a workpiece is forged into a scroll, the thickness L
1
of the flange portion
12
is controlled by a stroke of the punch
1
, and the remaining portion of the workpiece is forged into a wrap portion. Therefore, difference in the volume of the workpieces before forging is reflected in difference in the height L
2
of the wrap portions.
Conventionally, in order to smoothly carry out forging of a workpiece without production loss, the workpiece is prepared by cutting a round bar material having a diameter nearly equal to the outer diameter of a flange portion that will become the maximum outer diameter of a forged scroll. Therefore, variation in the thickness of the cut material is reflected in variation in the volume of the workpiece, i.e. variation in the height of a wrap portion of the scroll.
The horizontal cross-sectional area of a wrap portion is about ⅓ to ⅕ that of a workpiece. Accordingly, the variation in the cut length of the workpiece results in a variation in the height of the wrap portion that is 3 to 5 times the variation in the cut length. Therefore, a margin of the end of the wrap for machining in a post-processing step cannot be reduced because the margin has to include the variation in height. For this reason, a plural number of machining steps are required, resulting in failure to reduce the manhour for machining of scrolls and enhance the material-based yield.
In consideration of conditions under which scrolls are used, an aluminum alloy material containing a large amount of silicon is

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