SURFACE MODIFICATION METHOD FOR LIGHT METAL CASTING

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from Japanese patent application No. 2018-125783, filed on Jul. 2, 2018, the disclosure of which is incorporated herein in its entirety by reference.

BACKGROUND

The present disclosure relates to a surface modification method for a light metal casting.

There is known a surface modification technique using Friction Stir Processing (FSP) for improving variations in properties of material and quality deterioration due to a coarsened solidification structure of a light metal casting, such as an aluminium alloy. Japanese Unexamined Patent Application Publication No. 2004-255440 discloses, as a surface modification technique for a light metal casting using Friction Stir Processing, a technique for adding additives for modifying a metal structure to the metal structure when the surface of a light metal casting is refined with Friction Stir Processing.

SUMMARY

It is desirable that a member that can receive a large impact is broken as designed when the member receives an impact. For example, members used for a vehicle are desirably designed to be broken so that an impact received by a passenger is minimized when the vehicle receives an impact, such as a collision.

In a vehicle, members having complicated shapes are formed of light metal castings in many cases. For example, members in a vehicle, such as a suspension tower, a rear side member, and an engine cylinder block, are formed of light metal castings. Such members formed of light metal castings are also desirably broken as designed when receiving an impact.

When a member formed of a light metal casting has a portion at which stress tends to concentrate, the portion has the possibility of including internal defects and having the lowest strength. For example, when a member formed of a light metal casting has a welded portion at which stress tends to concentrate, the possibility that a fracture occurs at a region in the vicinity of the welded portion when an impact is applied is high. However, it is difficult, in terms of design, to control occurrence of a fracture at a region in the vicinity of the welded portion. Thus, when a member formed of a light metal casting has a welded portion, it is required for a fracture to be caused not at a region in the vicinity of the welded portion but at a portion at which occurrence of a fracture is easily controlled in design when an impact is applied. In other words, the strength at a region in the vicinity of a welded portion needs to be increased to be higher than the strength at a portion at which occurrence of a fracture is easily controlled in design. For this reason, in surface modification of a light metal casting, it is desired to refine the metal structure at a portion at which the strength is especially required as compared to at other portions.

In view of the above circumstance, a purpose of the present disclosure is to provide a surface modification method for a light metal casting that enables, with Friction Stir Processing, to refine the metal structure on the surface at a portion at which the strength is especially required.

The present disclosure is a surface modification method for a light metal casting with Friction Stir Processing in which a rotating shaft and a rotator are rotated and fed while the rotating shaft and the rotator are being pressed against a surface of a casting to modify the surface of the casting, the method including feeding the rotating shaft and the rotator while rotating the rotating shaft and the rotator in a manner such that a side at which a rotating direction of the rotating shaft and the rotator coincides with a feeding direction is positioned at a portion at which increase in the strength is desired with modification of the light metal casting.

The inventor has found that, on a surface subjected to friction stirring, the metal structure on a surface at a side (AS) at which the rotating direction of a rotating shaft and a rotator coincides with the feeding direction is refined as compared to at a side (RS) at which the rotating direction of the rotating shaft and the rotator and the feeding direction are opposite. By positioning a portion at which increase in the strength is desired with modification of a light metal casting at the side AS, it is possible to further refine the metal structure on the surface at the portion at which increase in the strength is desired and to increase the strength after the surface modification.

In addition, the feeding in the same feeding direction may be performed a plurality of times, and each feeding may be performed by shifting, in parallel, a next feeding path from a previous feeding path by a predetermined width equal to or less than a diameter of the rotator. By performing feeding in this manner, a portion which has not been positioned at the side AS in the previous feeding path is to be positioned at the side AS in the next feeding path or subsequent feeding paths. Thus, it is possible to evenly refine the surface of a member formed of a light metal casting.

Furthermore, the rotating shaft and the rotator may be fed according to a path circling along an edge of the light metal casting in a manner such that the side at which the rotating direction of the rotating shaft and the rotator coincides with the feeding direction is positioned at the edge of the light metal casting. By positioning the side AS at the edge of the member formed of the light metal casting and the side RS at the inner side, the metal structure on the surface at the portion in the vicinity of the edge is refined. Thus, it is possible to increase the strength at the outer periphery of the member formed of the light metal casting.

With the present disclosure, it is possible, with Friction Stir Processing, to further refine the surface at a portion at which the strength is especially required.

The above and other objects, features and advantages of the present disclosure will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present disclosure.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram schematically showing a configuration of a friction stir apparatus used in a surface modification method for a light metal casting according to the present embodiment;

FIG. 2 is a photomicrograph of the metal structure on the surface of a member formed of an aluminium alloy casting before modification;

FIG. 3 is a schematic diagram for explaining the outline of Friction Stir Processing of a friction stir apparatus;

FIG. 4 is a schematic diagram for explaining the difference in the modification effect by friction stirring;

FIG. 5 is a schematic diagram for a feature point of the surface modification method for the light metal casting according to the present embodiment;

FIG. 6 is a schematic diagram for the feature point of the surface modification method for the light metal casting according to the present embodiment;

FIG. 7 is a schematic diagram for the feature point of the surface modification method for the light metal casting according to the present embodiment;

FIG. 8 is a flowchart showing a procedure of the surface modification method for the light metal casting according to the present embodiment;

FIG. 9 is a schematic diagram for explaining an example of feeding a rotating tool and a probe; and

FIG. 10 is for explaining another example of feeding the rotating tool and the probe.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the present disclosure is described with an embodiment, but the claimed invention is not limited to the following embodiment. In addition, all the configurations described in the embodiment are not essential to means for solving problems. The following description and the drawings are appropriately omitted or simplified to clarify the explanation. In the drawings, the same reference sign is assigned to the same element, and redundant description is omitted as appropriate.

A surface modification method for a light metal casting according to the present embodiment is a method for modifying a surface of a light metal casting with Friction Stir Processing in which a rotating shaft and a rotator are rotated and fed while the rotating shaft and the rotator are being pressed against a surface of a casting to modify the surface of the casting. The light metal casting is a casting formed of light metal, such as an aluminium alloy or a magnesium alloy. The surface modification of the light metal casting is mainly to refine the metal structure on the surface of the light metal casting and to eliminate internal defects, such as cavities. First, a friction stir apparatus used in the surface modification method for the light metal casting according to the present embodiment is described.

FIG. 1 schematically shows a configuration of a friction stir apparatus 1 used in the surface modification method for the light metal casting according to the present embodiment. As shown in FIG. 1, the friction stir apparatus 1 includes a spindle driving part 2, a movement mechanism 3, a rotating tool 4 as a rotating shaft, and a probe 5 as a rotator.

The spindle driving part 2 includes a rotating drive shaft (not shown) to be rotated by an electric motor (not shown). The spindle driving part 2 is supported by a support column 7 fixed perpendicularly on a pedestal part 6 through the movement mechanism 3. On the pedestal part 6, a workpiece table 8 is arranged. On the workpiece table 8, a workpiece W which is a member formed of a light metal casting is placed.

The movement mechanism 3 includes a lifting/pressing mechanism (not shown) and a feeding mechanism (not shown). The lifting/pressing mechanism moves the spindle driving part 2 in the direction perpendicular to the workpiece W (that is, the direction of the rotating drive shaft of the spindle). The feeding mechanism moves the spindle driving part 2 in the feeding direction parallel to the workpiece W.

The rotating tool 4 is a cylindrical member to be rotated by the spindle driving part 2, and the central axis of the cylinder is aligned with the rotating drive shaft of the spindle driving part 2. At the lower end of the rotating tool 4, a pressing surface that is a horizontal face capable of pressing the upper surface of the workpiece W is formed. The rotating tool 4 is formed of metal material, such as stainless steel, having the higher hardness and a higher melting point than those of the base material of the workpiece W.

The probe 5 is a cylindrical member having a smaller diameter than that of the rotating tool 4 and is fixed on the pressing surface of the rotating tool 4 so as to project downward from the center of the pressing surface. The central axis of the cylindrical probe 5 and the central axis of the cylindrical rotating tool 4 are coaxial. The probe 5 is formed of, similarly to the rotating tool 4, metal material having the higher hardness and a higher melting point than those of the base material of the workpiece W. Note that, the side surface of the cylindrical probe 5 may be threaded in the direction opposite to the rotating direction of the probe 5. In this case, when, for example, the rotating direction of the probe 5 is clockwise, the side surface of the cylindrical probe 5 is threaded counterclockwise. This promotes stirring a portion of the workpiece W softened by frictional heat.

Next, the surface modification method for the light metal casting according to the present embodiment is described. Note that, FIG. 1 is referred to, as needed, for the configuration of the friction stir apparatus 1 in the following description.

FIG. 2 is a photomicrograph of the metal structure on the surface of a member formed of an aluminium alloy casting (ADT10-F) before modification. The black portion in FIG. 2 is eutectic silicon. As shown in FIG. 2, a member formed of an aluminium alloy casting has portions in which eutectic silicon is not refined and exists as a coarse precipitated grain R1 in the metal structure.

FIG. 3 is a schematic diagram for explaining the outline of Friction Stir Processing of the friction stir apparatus 1. As shown in FIG. 3, the lifting/pressing mechanism of the movement mechanism 3 (see FIG. 1) lowers the rotating tool 4 and the probe 5 in the direction of the arrow A, which is the rotating axis direction of the spindle, and the rotating tool 4 and the probe 5 are rotated at a high speed in the direction of the arrow C (clockwise) while the upper surface of the workpiece W is pressed with the pressing surface at the lower end of the rotating tool 4. Specifically, the rotating tool 4 and the probe 5 are brought into frictional contact with the base material of the workpiece W. The frictional heat generated at this time maintains the temperature range in which the light metal material, which is the base material of the workpiece W, is not melted but plastically deformed, and the base material of the workpiece W is softened and stirred.

Since the coarse precipitated grains (see FIG. 2) in the metal structure on the surface of the workpiece W are thereby crushed, the metal structure is refined, and internal defects, such as cavities, are eliminated.

As described above, the rotating tool 4 and the probe 5 are pressed against the workpiece W and rotated at a high speed, and the rotating tool 4 is moved by the feeding mechanism of the movement mechanism 3 in the direction of the arrow B, which is the feeding direction. As the result, the surface of the workpiece W is modified by the friction stirring along a moving path L of the rotating tool 4. In other words, a surface Wa of the workpiece W subjected to friction stirring is modified.

FIG. 4 is a schematic diagram for explaining the difference in the modification effect of friction stirring. Here, FIG. 4 is a diagram viewed from the direction of the arrow A in FIG. 3. In FIG. 4, the side at which the rotating direction C (C1) of the rotating tool 4 and the probe 5 coincides with the feeding direction B is referred to as an Advancing Side (AS), and the side at which the rotating direction C (C2) of the rotating tool 4 and the probe 5 and the feeding direction B are opposite is referred to as a Retreating Side (RS). The inventor has found that, on the surface Wa subjected to the friction stirring, the metal structure on the surface in the vicinity of the side AS is refined as compared to in the vicinity of the side RS and that the effect of improving the strength and ductility is increased.

FIGS. 5 to 7 are schematic diagrams for explaining a feature point of the surface modification method for the light metal casting according to the present embodiment. Here, FIG. 5 shows a workpiece W 1, which is a member formed of a light metal casting, viewed from the feeding direction of the rotating tool 4 and the probe 5. FIG. 6 shows, in the upper part, a workpiece W1 viewed from the feeding direction of the rotating tool 4 and the probe 5, and FIG. 6 further shows, in the lower part, the workpiece W1 viewed from the axial direction of the rotating tool 4 and the probe 5. FIG. 7 shows the workpiece W1 viewed from the feeding direction of the rotating tool 4 and the probe 5.

As shown in FIG. 5, the workpiece W 1, which is the member formed of the light metal casting, is assumed to have a welded portion S1 at which stress tends to concentrate. It is highly possible that a region S2 in the vicinity of the welded portion S1 includes internal defects and has the lowest strength. Thus, when the workpiece W1 receives an impact, the possibility of occurrence of a fracture at the region S2 in the vicinity of the welded portion S is high. However, it is difficult, in terms of design, to control occurrence of a fracture at the region S2 in the vicinity of the welded portion S1. For this reason, when the workpiece W1 receives an impact, it is undesirable that a fracture occurs at least at the region S2 in the vicinity of the welded portion S1. Thus, the strength at the region S2 in the vicinity of the welded portion S1 needs to be increased to be higher than the strength at regions other than the region S2, such as a region S3 and a region S4.

For this reason, in the surface modification method for the light metal casting according to the present embodiment, as shown in FIG. 6, the region S2 in the vicinity of the welded portion S1 is to be positioned at the side AS at which the rotating direction of the rotating tool 4 and the probe 5 coincides with the feeding direction. In other words, a portion at which increase in the strength is desired with modification of the workpiece W1 is to be positioned at the side AS at which the rotating direction of the rotating tool 4 and the probe 5 coincides with the feeding direction. At this time, the side RS at which the rotating direction of the rotating tool 4 and the probe 5 and the feeding direction are opposite is positioned at the region S3. The rotating tool 4 and the probe 5 are positioned in this manner and fed while the rotating tool 4 and the probe 5 are rotated.

FIG. 7 further shows photomicrographs of the metal structure on the surface of the workpiece W1 after the surface modification method for the light metal casting described with reference to FIGS. 5 and 6 is performed. Here, the workpiece W1 is an aluminium alloy casting (ADT10-F). In FIG. 7, P1 is a photomicrograph of the metal structure at the region S2 in the vicinity of the welded portion S1 on the surface Wa of the workpiece W1 subjected to the friction stirring. In addition, P2 is a photomicrograph of the metal structure at the region S3 on the surface Wa of the workpiece W1 subjected to the friction stirring, and P3 is a photomicrograph of the metal structure at the region S4. Black portions in P1, P2, and P3 in FIG. 7 are eutectic silicon.

In FIG. 7, eutectic silicon is finely distributed in P1 that is a photomicrograph of the metal structure at the region S2 as compared to in P2 that is a photomicrograph of the metal structure at the region S3 and in P3 that is a photomicrograph of the metal structure at the region S4. The region S3 has been positioned at the side RS when the friction stirring is performed, and has many portions at which the metal structure has been refined as compared to the base material of the workpiece W1 shown in FIG. 2 but has not been refined sufficiently. The metal structure at the region S4 between the region S2 and the region S3 has been refined as compared to at the region S3, but has not been refined as compared to at the region S2.

As described above, by performing the surface modification method for the light metal casting according to the present embodiment, it is possible to further refine the surface at the region S2, which is a portion at which the strength on the surface of the workpiece W1 is especially required, as compared to at regions other than the region S2, such as the region S3 and the region S4, and to increase the strength.

FIG. 8 is a flowchart showing a procedure of the surface modification method for the light metal casting according to the present embodiment. As shown in FIG. 8, first, the rotating tool 4 and the probe 5 are positioned with respect to a workpiece so that the side AS at which the rotating direction of the rotating tool 4 and the probe 5 coincides with the feeding direction is positioned at a portion at which increase in the strength is desired with modification of the light metal casting (step S1). Then, the rotating tool 4 and the probe 5 are pressed against the surface of the light metal casting while being rotated (step S2). Then, the rotating tool 4 and the probe 5 are fed along a predetermined path (step S3).

FIG. 9 is a schematic diagram for explaining another example of feeding the rotating tool 4 and the probe 5 (a different example from FIG. 6). Here, the upper part of FIG. 9 is a diagram viewed from the feeding direction of the rotating tool 4 and the probe 5, and the lower part of FIG. 9 is a diagram viewed from the axial direction of the rotating tool 4 and the probe 5. As shown in FIG. 9, the rotating tool 4 and the probe 5 may be fed in the same feeding direction B a plurality of times, and each feeding may be performed by shifting, in parallel, a next feeding path L2 from a previous feeding path L1 by a predetermined width J equal to or less than the diameter D of the probe 5. By performing feeding in this manner, a portion which has not been positioned at the side AS in the previous feeding path L1 is to be positioned at the side AS in the next feeding path L2 or subsequent feeding paths, and it is possible to evenly refine the surface of the workpiece W.

FIG. 10 is a schematic diagram for explaining another example of feeding the rotating tool 4 and the probe 5 (a different example from FIGS. 6 and 9). Here, FIG. 10 is a diagram viewed from the axial direction of the rotating tool 4 and the probe 5. As shown in FIG. 10, the side AS at which the rotating direction of the rotating tool 4 and the probe 5 coincides with the feeding direction may be positioned at an edge M of a workpiece W2, which is a member formed of a light metal casting, and the rotating tool 4 and the probe 5 may be fed according to a path L3 circling along the edge M. By positioning the side AS at the edge M of the workpiece W2 and positioning the side RS at the inner side, the metal structure on the surface at a portion in the vicinity of the edge M is refined. Thus, it is possible to increase the strength at the outer periphery of the workpiece W2.

The present disclosure has been described above with the above embodiment, but is not limited to the configurations in the embodiment and includes various modifications, amendments, and combinations that can be understood by those skilled in the art without departing the scope of the claimed invention.

From the disclosure thus described, it will be obvious that the embodiments of the disclosure may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the disclosure, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims.

SURFACE MODIFICATION METHOD FOR LIGHT METAL CASTING

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