JP2017177190A - Extrusion member - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for achieving higher reliability in prevention of stress corrosion crack in a member (for example, an automotive component) made of a 7000 series aluminum alloy extruded material as a raw material.SOLUTION: An extruded member comprises a 7000 series aluminum alloy extruded material having a 0.2% proof stress of 350 MPa or more, and a hollow-cross section, as a raw material. The extruded member comprises: an area to which a residual compression stress is applied, in the whole part or a part of the surface. The residual compression stress in the outermost layer in said area corresponds to 10-30% of durability of the raw material. The residual compression stress may be applied, for example, by a peening process.SELECTED DRAWING: Figure 6

Description

  The present invention relates to an extruded member for automobiles and the like made of a high-strength 7000 series aluminum alloy extruded material, and more particularly to an extruded member having improved stress corrosion cracking resistance.

  In recent years, with the background of the seriousness of energy problems and environmental problems (global warming), movements for improving the fuel efficiency of automobiles are accelerating. Weight reduction of the car body is also effective for improving the fuel efficiency of automobiles, replacing the conventional steel products with aluminum alloy plate materials, extruded materials, forged materials, and cast materials that have a density of about 1/3 in parts. Is progressing.

The aluminum alloy extruded material can be used for automobile frame parts, energy absorbing members, and the like because a long cross-section material having an arbitrary thickness distribution can be obtained without additional processing.
In automobile parts (parts) that require strength, the effect of weight reduction by replacing steel members with aluminum alloy extruded members depends greatly on the mechanical properties of the aluminum alloy. Examples of members that require strength include door reinforcement members, bumper reinforcement members, roof reinforcement members, and body frame members such as side members and pillars. Development of high-strength aluminum alloys for such members is in progress.

In recent years, demand for door reinforcements made of extruded aluminum alloys has been increasing. Since the door reinforcement member is installed together with other mechanism parts in a small portion between the door inner panel and the door outer panel, space restrictions are severe, and a high strength alloy is desired in order to satisfy a predetermined strength. Further, since there is not enough space, the cross-sectional shape of the door reinforcement made of the aluminum alloy extruded material may be a substantially rectangular cross-section composed of two opposed flanges and two webs supporting the flange. Many.
In addition, due to fastening with other members and space constraints, both ends of the door reinforcement made of aluminum alloy extruded material may be subjected to flattening of the cross-sectional shape by crushing processing, oblique cutting processing, etc. It is general (see Patent Documents 1 and 2).

  Commonly used high-strength aluminum alloys are the precipitation hardening type alloys 6000 series (Al-Mg-Si- (Cu) series) and 7000 series (Al-Zn-Mg- (Cu) series). Generally, a 6000 series alloy has a 0.2% yield strength of about 200 to 350 MPa, and a 7000 series alloy has a 0.2% yield strength of about 300 to 500 MPa. The mechanical properties mainly depend on the tempering and the composition of additive elements, and also depend on the casting quality (grain size) and the processing process.

  However, high-strength 6000 series and 7000 series aluminum alloys are known to crack due to tempering, which is called stress corrosion cracking (SCC), where tensile stress is constantly generated in a corrosive environment. . The form of cracks is generally very sensitive and is strongly repelled because of its high stress intensity factor and rapid growth and high risk of unexpected fracture. In general, SCC is more likely to occur as the strength of the material increases. In this sense, the 7000 series alloy has a higher risk of SCC than the 6000 series alloy. With this SCC as a bottleneck, adoption of high-strength 7000 series alloys for automobile parts may be postponed.

SCC occurs at a site where the member is exposed to a corrosive environment and the tensile stress is always above a certain threshold. The tensile direction due to external loads and moments on the member rarely occurs at all times, and SCC may be caused by the residual stress in the tensile direction generated during processing and heat treatment during manufacturing. Many.
Patent Documents 3 to 7 describe that in a 7000 series aluminum alloy extruded material, the residual tensile stress after forming is reduced to prevent the occurrence of SCC.

JP 2001-301462 A JP 2003-118367 A JP 2002-362157 A Japanese Patent No. 5419401 JP 2010-207832 A Japanese Patent No. 5671422 JP 2014-145119 A

The residual tensile stress of the aluminum alloy extruded material is generated by heat treatment or machining. In heat treatment, when rapid cooling is performed after high-temperature heating, residual tensile stress is generated due to a temperature gradient generated in the member.
Machining can be broadly divided into plastic working and cutting / cutting. Typical examples of plastic working include bending and cross section processing. There are various bending methods such as press bending, pressing bending, tensile bending, and roll bending, but generally, a high residual tensile stress is generated in the longitudinal direction at the bending inner side R (concave side) and a part of the side surface. There are two methods for changing cross-sections: a method of expanding by applying pressure (liquid, solid, electromagnetic force) from the inner surface and a method of reducing (crushing) by a load from the outer surface. High residual stress is generated in the circumferential direction of the cross section of the concave side surface.
In cutting / cutting (including punching), it is known that there are cases in which a high residual tensile stress is generated on the processed surface.

Patent Documents 3 to 7 devise processing conditions and cross-sectional shapes (Patent Documents 3 to 5) as techniques for suppressing residual tensile stress generated in a member such as a door reinforcement made of a 7000 series aluminum alloy extruded material. And a restoration process after plastic working (Patent Documents 6 and 7). However, even when these techniques are applied, the residual tensile stress itself that causes SCC exists on the surface of the member.
Therefore, an object of the present invention is to realize higher reliability in preventing SCC in a member made of a 7000 series aluminum alloy extruded material.

  The present invention relates to extruded members (parts for automobiles, etc.) made of a 7000 series aluminum alloy extruded material having a 0.2% proof stress of 350 MPa or more and having a hollow cross section. Residual compressive stress is present on the entire surface or a part of the surface. It has the provided area | region A, The residual compressive stress of the outermost layer of the said area | region A is equivalent to 10 to 30% of material proof stress, It is characterized by the above-mentioned. In addition to the outer surface and inner surface of the extruded material, and the cut end surface, the surface includes an inner peripheral surface and a cut surface of a hole when cutting / cutting is performed after the extrusion. Further, “A” in the region A is given only to distinguish the region to which the residual compressive stress of 10 to 30% of the material yield strength is given from other regions.

The 7000 series aluminum alloy hollow extruded material has, for example, a substantially rectangular cross-sectional shape including two flanges and two webs connecting them. The extruded member having this cross-sectional shape is, for example, a door reinforcing material, and a deformed cross-sectional portion in which the web is curved may be provided in a part (mainly an end portion) along the extruding direction. It is preferable that the bent inner surface of the web is included in the region A.
Moreover, the said extrusion member may have a hole or a cut surface in a part along an extrusion direction, for example, In this case, the inner surface or cut surface of the said hole, and its vicinity area | region may be included in the said area | region A. preferable.
When the residual compressive stress is applied by the peening process, a surface modified layer into which a large number of subgrain boundaries are introduced is formed on the surface of the region A.

The extruded member according to the present invention is made of a high-strength 7000 series aluminum alloy extruded material having a yield strength of 350 MPa or more, and has a region A to which residual compressive stress is applied on the entire surface or a part thereof. For this reason, compared with the case where a residual tensile stress remains like the conventional example, reliability is high in preventing SCC. The region A may be set at a location where a larger residual tensile stress is generated during heat treatment or machining and in the vicinity thereof. The residual compressive stress in the region A can be applied by a surface treatment such as a peening treatment.
In the extruded member according to the present invention, the residual compressive stress in the region A is set to 10 to 30% of the material yield strength. As a result, the SCC critical stress in the region A is increased by about 0.2 at a 0.2% yield strength ratio compared to a normal case where no residual compressive stress is applied (for example, in the case of a 500 MPa material, the SCC critical stress is increased by about 100 MPa. Can be expected). Moreover, since the maximum value of the residual compressive stress is not so high at the 0.2% yield strength ratio, the risk of disappearance of the residual compressive stress due to external force (described later) can be avoided.

It is the front view (1A) and II sectional view (1B) of the extrusion member which concern on this invention. It is the front view (2A) and II-II sectional drawing (2B) of another extrusion member which concern on this invention. It is the front view (3A) and III-III sectional drawing (3B) of another extrusion member which concerns on this invention. It is a front view (4A), a top view (4B), and IV-IV sectional drawing (4C) of another extrusion member which concerns on this invention. It is the front view (5A), top view (5B), and VV sectional drawing (5C) of the extrusion member which concerns on this invention. It is a graph which shows the relationship between the residual compressive stress of the outermost layer calculated | required in the Example, and SCC critical stress (all are 0.2% yield strength ratio).

Hereinafter, the extruded member according to the present invention will be described more specifically.
[Aluminum alloy extruded material]
The extruded member according to the present invention is made of a 7000 series (Al-Zn-Mg- (Cu)) aluminum alloy extruded material having a high strength with a 0.2% proof stress of 350 MPa or more. If the 0.2% proof stress is 350 MPa or more, it can be used as an automobile member. For 0.2% proof stress of 350 MPa or more, hot-extrusion material should be subjected to aging treatment after online quenching (T5 tempering), or solution treatment and quenching offline and aging treatment (T6 tempering). Is achieved. However, SCC is generally more likely to occur with higher strength materials.
In general, residual tensile stress or residual compressive stress of about 0 to 20 MPa exists on the surface of the extruded material after tempering. When machining is applied to this, a large residual tensile stress is generated at the above-described locations, and the SCC resistance is lowered.

[Method of applying residual compressive stress by surface treatment]
As a method for imparting residual compressive stress to the surface of the extruded member, a method by surface treatment is the most suitable, and a typical surface treatment is peening. Of the peening treatments, the shot peening treatment, which is usually called a projecting material and usually projects individual fine particles onto a workpiece at a high speed, is the most common, and is widely used in steel springs and steel gears. Japanese Patent Application Laid-Open Nos. 2000-233625 and 2000-343429 disclose a method and apparatus for performing shot peening on the inner surface of a hollow material.
Although it has been conventionally performed to apply residual compressive stress to a member surface by peening treatment to improve fatigue strength, the present inventors know an example that is performed to prevent SCC. Absent.

In addition to the peening treatment, ultrasonic peening that uses the vibration phenomenon caused by ultrasonic waves, laser peening that generates high-pressure plasma by irradiating a laser in water and compressively deforms the workpiece, and cavitation is generated in water. There is cavitation peening that compresses and deforms workpieces. Japanese Patent Application Laid-Open No. 2015-105402 discloses that an aluminum alloy member is subjected to laser peening treatment to form a cured surface modified layer to improve fatigue strength in a high temperature environment.
As a method for applying a residual compressive stress other than the peening treatment, there is a barrel polishing treatment or the like, which has been widely put into practical use in a relatively small member requiring mass production. There is also what is called burnishing.

[Residual compressive stress distribution by peening]
The residual compressive stress generated on the surface of the workpiece by the peening process usually shows a mountain-like distribution in the depth direction. In general, the peak is located at a depth of 50 to 150 μm from the outermost layer (depth of 0 μm), and the residual compressive stress is substantially zero at a depth of about 300 μm. The residual compressive stress of the outermost layer often takes about 30 to 60% of the peak value. Generally, the higher the kinetic energy of the projection material (the projection material is heavier, larger, and higher in speed), the higher the peak value of the residual compressive stress tends to be. Moreover, the higher the kinetic energy of the projection material, the lower the residual compressive stress value of the outermost layer, and the difference between the residual compressive stress value of the outermost layer and the peak value tends to increase.
In the present invention, the residual compressive stress value of the outermost layer of the extruded member made of a 7000 series aluminum alloy extruded material is set to 10 to 30% of the material yield strength based on the results of Examples described later.

[Depth of residual compressive stress]
Automobile members are often exposed to corrosive environments including salt water. In a corrosive environment, even in a member made of an extruded aluminum alloy material having a slow corrosion rate as a raw material, the surface layer loss due to corrosion progresses greatly depending on the alloy type and tempering. From the viewpoint of the reliability of an automobile member made of an aluminum alloy extruded material, it is preferable to have a relatively high residual compressive stress up to a certain depth (about 100 to 150 μm) rather than a high residual compressive stress only in the outermost layer. . The higher the kinetic energy of the projection material, the greater the depth at which the residual compressive stress is introduced.

[Merit of peening process (crystal grain refinement)]
The structure of the surface layer of the aluminum alloy extruded material (depth of about 0 to 100 μm) is likely to be a coarse recrystallized structure because a large strain is given compared to the inside, and the internal fine structure (mainly fibrous) There are many different cases. Coarse crystal grains have a lower SCC critical stress than fine crystal grains. When a large strain is introduced into the surface layer by the peening treatment, many sub-grain boundaries appear in such a coarse recrystallized structure, and an effect similar to crystal grain refinement can be obtained. The grain refinement effect generally tends to increase as the kinetic energy of the projection material increases. The surface layer portion where the effect of crystal grain refinement is obtained in the peening process is called a surface modified layer.

[Disadvantages of applying residual compressive stress]
On the other hand, by applying the residual compressive stress by the surface treatment, the SCC prevention effect can be expected, but there is also a disadvantage. One of the disadvantages is an increase in costs associated with processing. Therefore, it is preferable to limit the processing to a necessary area.
Further, the peening process generally has a demerit that the stress concentration factor increases as the surface roughness increases. Therefore, it is usually assumed that an increase in surface roughness due to the peening treatment leads to a decrease in SCC critical stress. However, as shown in the examples described later, in the 7000 series aluminum alloy extruded material, there is a clear relationship between the surface roughness (arithmetic mean roughness Ra) and the SCC critical stress (0.2% yield strength ratio of the material). I couldn't find it. This means that the increase in surface roughness due to the peening process is not a disadvantage for the present invention.

  In addition, if the residual compressive stress is large (for example, approaching the 0.2% proof stress of the material), plastic deformation occurs due to the compressive stress generated when an external force is applied to the member, and the applied residual compressive stress disappears. There are also risks. Therefore, it is not appropriate to increase the residual compressive stress unnecessarily. In the present invention, the maximum value of the residual compressive stress value of the outermost layer of the extruded member made of a 7000 series aluminum alloy extruded material is relatively small as 30% or less in the 0.2% yield strength ratio of the material, and this risk is avoided. It's easy to do.

[Form of region A in extruded member]
(1) The extruded member shown in FIG. 1 is an example of a door reinforcing material. Residual compressive stress corresponding to 10 to 30% of the material yield strength is applied to the outer surface and inner surface of both ends fixed to the inner panel of the door. Has been granted. In this example, the region A is the outer surface and inner surface (locations indicated by dots) at both ends of the extruded member.
(2) The extruded member shown in FIG. 2 is an example of a door reinforcing material, and a residual compressive stress corresponding to 10 to 30% of the material yield strength is applied to the outer surface and the inner surface over the entire longitudinal direction. In this example, the region A is the entire outer surface and inner surface (location indicated by dots) of the extruded member.

(3) The extruded member shown in FIG. 3 is an example of a door reinforcing material, and both ends fixed to the inner panel of the door are subjected to cross-section processing (crushing processing) in a direction perpendicular to both flanges, Curved outward. In such cross section processing, residual tensile stress is applied to the central part of the concave surface (inner surface) of both curved webs and to the boundary part (both indicated by arrows) of the outer surface of both webs. Occur. In this example, a residual compressive stress corresponding to 10 to 30% of the material proof stress is applied to the entire inner surface and outer surface of the web (both indicated by dots) at both ends including the crushed portion, and this is the region A. It is. In addition, as described in Patent Document 7, in such cross section processing, a high residual tensile stress is generated in the vicinity of the boundary between the portion where the cross section processing is performed and the portion where the cross section processing is not performed (the portion where the extrusion is performed). Tend.

(4) The extrusion member shown in FIG. 4 is an example of a door reinforcement, and holes are formed in both flanges by press punching or milling at both ends fixed to the inner panel of the door. In such punching and cutting, residual tensile stress is generated along the processing direction on the inner peripheral surface of the hole. In this example, a residual compressive stress corresponding to 10 to 30% of the material yield strength is applied to the inner peripheral surface of the hole indicated by the dots and the vicinity thereof (outer surface).

(5) The pushing member shown in FIG. 5 is an example of a door reinforcing material, and both ends fixed to the inner panel of the door are cut obliquely. In such a cutting process, a residual tensile stress is generated along the cut surface. In this example, a residual compressive stress corresponding to 10 to 30% of the material yield strength is applied to the cut surface indicated by dots, and this is the region A.

Shot peening was performed on a 7000 series aluminum alloy extruded material made of Al-6.5Zn-1.3Mg-0.15Cu-0.03Cr-0.05Mn-0.15Zr (both numerical values are mass%). Later, it was subjected to the SCC test, the change of the SCC critical stress was investigated, and the effect of the present invention was verified. In addition, even if it is 7000 series aluminum alloy extrusion material other than the said component composition, the effect of this invention is acquired.
The test material is a hollow extruded material having a rectangular cross section subjected to T5 tempering. A JIS No. 5 test piece described in JISZ2201: 2005 (metal material tensile test piece) was sampled by machining from the plate-like portion (wall thickness 1.8 mmt) of the hollow extruded material in the direction parallel to the extrusion, and 0.2 by the following procedure. The% proof stress value was measured.
(Measurement of 0.2% proof stress)
A tensile test was performed in accordance with the provisions of JISZ2241, and a 0.2% yield strength was obtained. The 0.2% yield strength value in the direction parallel to the extrusion was 430 MPa.

Similarly, a plate bending test piece (ISO7539) described in JISH8711: 2000 (stress corrosion cracking test method of aluminum alloy) is processed by machining in the direction parallel to the extrusion from the plate-like portion (wall thickness 1.8 mmt) of the hollow extruded material. Collected. Two specimens were collected for each additional stress in the SCC test described later, and compressive residual stress was applied to these specimens in the following manner (Nos. 1 to 14 in Table 1) or not. (No. 15 in Table 1) and various measurement tests were performed.
(Applying compressive residual stress)
Using an air-type shot peening device (manufactured by Shinto Kogyo Co., Ltd.), shot peening is performed on one side of the test piece under a total of 14 conditions (No. 1 to 14) with different air pressure, projection material, and nozzle type, and compressed. Residual stress was applied. In the column of used nozzles in Table 1, A has a smaller number of projection materials and a lower projection speed than B. In addition, the number of the test pieces to which the compressive residual stress was applied is 12 for each condition (2 for each additional stress in the SCC test).

About each test piece (No. 1-14) after a shot peening process, the residual stress and surface roughness of the surface which performed the shot peening process were measured using one each. The test piece was also subjected to the SCC test. In the SCC test, stress was applied in the direction parallel to the extrusion. For the test piece (No. 15) not subjected to shot peening treatment, the residual stress and the surface roughness were measured in the same manner.
(Measurement of residual stress)
The residual compressive stress in the extrusion parallel direction of the outermost surface was measured using an X-ray stress measurement apparatus MSF-3M (manufactured by Rigaku Corporation). Table 2 shows measurement conditions and analysis conditions.
(Surface roughness measurement)
Using a small surface roughness measuring machine Surf Test SJ-310 (manufactured by Mitutoyo Corporation), the surface roughness in the extrusion parallel direction (corresponding to the arithmetic average roughness Ra of JISB0601: 2001) was measured. The measurement interval was 2.5 mm and the measurement was repeated 5 times.

(SCC test)
The SCC test employs a three-point addition method of a plate bending test (JISH8711: 2001). The test pieces 1 to 14 were made such that the surfaces to which compressive residual stress was applied became convex. No. The applied stress of 1 to 14 is six stages of 220 MPa, 240 MPa, 260 MPa, 270 MPa, 280 MPa and 300 MPa. The applied stress of 15 was set to a lower value. The applied stress was controlled by a strain gauge attached in the longitudinal direction of the convex surface at the center where the tensile stress was maximum. A chromic acid solution (NaCl: 3 g, K ₂ Cr ₂ O ₇ : 30 g, CrO ₃ : 36 g per liter of distilled water) was prepared as a corrosive liquid, and the temperature was maintained at 95 ° C. or higher to promote SCC. The test time was 16 hours.
The test piece after the test was cut at a position where there was a possibility of SCC, embedded with resin, and observed with an optical microscope after polishing to determine the presence or absence of SCC. When the occurrence of SCC is not observed in both of the two specimens having the same applied stress, it is determined that there is no SCC in the applied stress, and the occurrence of SCC is observed in at least one of the two specimens having the same added stress. When it was, it was determined that there was SCC at the applied stress. The maximum stress determined as having no SCC was defined as the SCC critical stress of the test piece.

Each test result is shown in Table 1, and the relationship between the residual compressive stress in the extrusion parallel direction of the outermost layer and the SCC critical stress in the extrusion parallel direction is shown in FIG. In Table 1 and FIG. 6, the residual compressive stress value and the SCC critical stress value were all made dimensionless as the 0.2% yield strength ratio of the material.
From FIG. 6, although the SCC critical stress in the extrusion parallel direction shows some variation, it shows a gentle mountain relation with the residual compressive stress in the same direction, and the 0.2% yield strength ratio of the residual compressive stress is 0. 0.1 to 0.3 showed a substantially constant value, and 0.2 showed a substantially peak. When the 0.2% yield strength ratio of the residual compressive stress is 0.2, the SCC critical stress is No. with no residual compressive stress added. Compared to 15, there is an increase of about 0.2.

As shown in FIG. 6, even when the residual compressive stress of the outermost layer was higher than 0.3 in the 0.2% yield strength ratio, the extrusion parallel direction SCC critical stress was not improved. The reason for such a result is presumed as follows. (1) When SCC proceeds in a corrosive environment, metal elution progresses from the surface layer of the aluminum alloy, and the surface layer gradually disappears. (2) The higher the residual compressive stress of the outermost layer, the smaller the kinetic energy of the projection material as described above, the lower the peak value of the residual compressive stress, and the shallower the depth at which the residual compressive stress is introduced. (3) For this reason, when the residual stress of the outermost layer is high (when the 0.2% proof stress ratio exceeds 0.3), the surface layer portion into which the residual compressive stress is introduced is likely to be lost early in a corrosive environment, As a result, the SCC critical stress was not improved. (4) In addition, the higher the residual compressive stress of the outermost layer, the lower the kinetic energy, so the effect of refining the crystal grains in the surface layer portion was also relatively small.
From the above results, it is considered optimal that the residual compressive stress of the outermost layer of the 7000 series aluminum alloy extruded material is kept within the range of 0.2% proof stress ratio of 0.1 to 0.3.

Claims (6)

Using a 7000 series aluminum alloy extruded material having a 0.2% proof stress of 350 MPa or more and a hollow cross section as a raw material, it has a region A in which residual compressive stress is applied to the whole or a part of the surface, and the outermost layer of the region A An extruded member, wherein the residual compressive stress corresponds to 10 to 30% of the material yield strength. The 7000 series aluminum alloy hollow extruded material has a substantially rectangular cross-sectional shape composed of two flanges and two webs connecting the flanges, and the cross-section portion in which the web is curved in part along the extrusion direction 2. The extruded member according to claim 1, wherein the region A includes a bent inner surface of the web of the variable cross section. The extruded member according to claim 1, wherein the extruded member is a door reinforcement material for an automobile. 2. The extruded member according to claim 1, wherein a hole is formed in a part along the extrusion direction, and an inner peripheral surface of the hole is included in the region A. 3. The extruded member according to claim 1, wherein the extruded member has a cut surface at both ends, and the cut surface is included in the region A. The extruded member according to any one of claims 1 to 5, wherein a surface modified layer in which subgrain boundaries are introduced in the region A is formed.

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