Electric heating – Metal heating – For bonding with pressure
Reexamination Certificate
2000-05-11
2002-04-09
Elve, M. Alexandra (Department: 1725)
Electric heating
Metal heating
For bonding with pressure
C219S117100
Reexamination Certificate
active
06369347
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an aluminum alloy sheet that is suitably useful as a body sheet for automotive vehicles and is highly weldable by means of continuous resistance spot welding.
BACKGROUND ART
In automotive vehicles used as transport means, weight saving in respect of gravity has heretofore been taken as a target for development. This weight saving contributes greatly to increased transport quantity and hence reduced energy cost, and also to improved environmental protection.
Aluminum alloy materials are light in weight, good in formability and resistant to corrosion subject to surface treatment, and they offer high strength depending upon the chemical compositions and fabricating conditions. Thus, it has been proposed to substitute such an aluminum alloy for a steel sheet commonly accepted as an automobile body sheet, thereby making the finished automobile light in weight.
Products for general use such as automobiles have today been mass-produced as the advent of a certain production method with good efficiency. Namely, the body shape of an automotive vehicle is divided into a plurality of predetermined segments, followed by stamping of a body sheet matched with each such segment in order to prepare a stamped segment. The segment is then lapped in side-by-side relation to the mating adjoining segment, after which resistance spot welding is carried out. Repeated lapping and welding lead eventually to a complete automobile body. The resistance spot welding noted here is of an electric resistance type explained below in more detail. Electrode chips are used which are formed of a Cu alloy (also referred to hereunder as “electrodes”), and the lapped side portions of two stamped segments are firmly clamped between and pressed against the electrodes in tandem, followed by supply of a large capacity of current to the electrodes, so that a large amount of heat is generated out of the electrode in a short period of time. Thus, a hot melt pool or a nugget commonly called so is provided on a given surface of the lapped sides of the stamped segments, and ultimate solidification of the nugget brings about the two stamped segments welded on their respective sides. A production system designed to adopt this resistance spot welding is advantageous in that a production line is made feasible with automation.
Such production system is currently in common use for a conventional steel sheet since the latter is highly adaptable to resistance spot welding in continuous manner, say with a continuous run of 10,000 to 20,000 weld spots prior to the need for dressing of the electrode chip.
An aluminum alloy sheet of an Al—Mg series has been reputed as a substitute for the steel sheet recently. This alloy sheet is excellent in respect of corrosion resistance, strength and formability.
However, such aluminum alloy sheet is smaller in specific resistance and larger in thermal conductivity than a steel sheet, and besides is liable to suffer from seizing in operation with the corresponding electrode chip. This alloy sheet fails to warrant resistance spot welding with a continuous run of weld spots meeting with mass production.
In order to solve this problem or otherwise improve weldability by means of continuous resistance spot welding, there have been proposed several techniques. For instance, one such technique is directed to an aluminum alloy sheet containing Mg in an amount of from 0.5 to 6% by weight and having a specific resistance at 20° C. of 5.5 &mgr;&OHgr;·cm or more (Japanese Unexamined Patent Publication No. 5-279781). Another technique is directed to a method in which an aluminum alloy sheet of an Al—Mg series or an Al—Mg—Si series is chemically etched on both of its surfaces to remove oxide films therefrom, followed by heating of the resulting alloy sheet in the atmosphere so as to form uniform oxide films on both sides (Japanese Unexamined Patent Publication No. 6-55280).
However, an electrode chip reacts on its surface with a portion of an aluminum alloy sheet to be exposed to resistance spot welding, eventually forming an adverse alloy layer thereon and moreover involving deformed tip of the electrode chip with the result that a current is difficult to be applied at a stable value, and hence, resistance spot welding is difficult to be continuously conducted. These difficulties cannot be satisfactorily obviated even with reliance upon the techniques cited above.
DISCLOSURE OF INVENTION
Continued research leading to the present invention, the present inventors have found that an electrode chip on its surface can be made difficult to form an alloy layer which would arise from reaction of Cu of the electrode with an aluminum alloy sheet to be subjected to resistance spot welding, particularly in the case where the density of a particulate intermetallic compound is preset at a specific value. The compound is defined as existing in one surface layer of such alloy sheet to be pressed against the electrode.
One principal object of the invention, therefore, lies in the provision of an aluminum alloy sheet for automotive use that exhibits stable weldability by means of continuous resistance spot welding with rather a small likelihood of the tip of an electrode chip becoming deformed.
More specifically, the invention provides an aluminum alloy sheet for use in an automobile body sheet having excellent weldability by means of continuous resistance spot welding, which comprises a starting aluminum alloy sheet which has an alloy composition containing from 2 to 6% by weight of Mg, 0.15 to 1.0% by weight of Fe and from 0.03 to 2.0% by weight of Mn, and the rest of which balanced up with an aluminum and unavoidable contaminants; and a surface layer disposed over one surface of the starting aluminum alloy sheet to be pressed against electrodes for use in spot welding, the surface layer containing a particulate intermetallic compound which has a particle diameter of 0.5 &mgr;m or more and a density of 4,000 pieces of particles per one mm
2
or more.
BEST MODE FOR CARRYING OUT THE INVENTION
The aluminum alloy sheet having the above specified composition shows a rise in electric resistance and, even with a small supply of current when in initiating resistance spot welding, can generate a large heat at the lapped side portions of two such alloy sheets to be subjected to spot welding with consequential formation of a desirable nugget between the alloy sheets. Additionally, a lot of particulate intermetallic compounds are formed due to the presence of Fe and Mn in the aluminum alloy sheet. A particulate intermetallic compound of 0.5 &mgr;m or more in particle diameter is allowed to exist in a high density in a surface layer of the aluminum alloy sheet. This leads to least formation of an alloy layer between the electrode and the alloy sheet, and hence, results in reducing the alloy layer on the electrode surface, and also to least deformation of the electrode tip, ultimately permitting a stable continuous operation of resistance spot welding. If the particulate intermetallic compound were present in too small a content in the surface layer on the aluminum alloy sheet, then an alloy layer would take place in a greater extent on the electrode surface, resulting in deformed tip of the electrode chip with eventual failure to effect resistance spot welding in a stably continuous fashion.
The aluminum alloy sheet is pressed, against the electrode, on its surface layer in which the particulate intermetallic compound is contained as stated above. It is desired that such surface layer be made to have a depth or thickness of 20 &mgr;m or more as determined from its outer surface. A depth or thickness of 20 &mgr;m or more could render the resultant particulate intermetallic compound containing surface layer sufficiently thick even upon pressing against the electrode, thus avoiding an alloy layer from getting formed between the electrode and the aluminum alloy sheet. Smaller thickness than 20 &mgr;m would be liable to frequently produce an objectionable alloy layer.
Moreover, the oxide film shou
Hayashi Noboru
Lloyd David James
Moriyama Takeshi
Wycliffe Paul
Yasunaga Kunihiro
Elve M. Alexandra
Nippon Light Metal Company Ltd.
Parkhurst & Wendel L.L.P.
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