JP3899007B2

JP3899007B2 - Method for improving fatigue strength of lap fillet welded joints

Info

Publication number: JP3899007B2
Application number: JP2002294352A
Authority: JP
Inventors: 誠司古迫; 知徳冨永; 順一小林
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2002-10-08
Filing date: 2002-10-08
Publication date: 2007-03-28
Anticipated expiration: 2022-10-08
Also published as: JP2004130313A

Description

【０００１】
【発明の属する技術分野】
本発明は、橋梁、機械品、自動車、自動二輪の部品など、金属板を用いた溶接構造物の溶接部の特性向上方法に関し、具体的には、２枚の金属板を重ね合わせた端部を溶接した重ね隅肉溶接継手の疲労強度向上方法に関する。
【０００２】
【従来の技術】
近年、構造物の軽量化を目的として、使用される鋼材の高強度化と薄肉化が進む中、溶接部の疲労強度を再現性良く、かつ効果的に高める技術の開発が強く望まれている。
一般に、橋梁や自動車車体のような金属板を用いた溶接構造物における溶接継手形状として、２枚の金属板を重ね合わせた端部を溶接した重ね隅肉継手が多く用いられており、溶接方法としては、アーク溶接、レーザ溶接をはじめ、プラズマ溶接、電子ビーム溶接など、多種多様な溶接方法が適用されている。
【０００３】
例えば、図３に、重ね隅肉溶接継手をアーク溶接で作製する場合の例を示す。図３において、アーク溶接のエネルギ密度は低く貫通能力が乏しいため、金属板１，２を重ね合わせ、重ね合わせ部の端部にアークを照射し、溶接ワイヤからの溶融した溶接金属３が金属板１、２の重ね合わせ端部近傍の一部に溶け込むように溶接ビードを形成する、いわゆる重ね隅肉溶接を行う。
図３に示すように、重ね隅肉溶接継手では溶接金属３が金属板２の表面に繋がる部分、つまり溶接止端部４が形状的に急変するために重ね隅肉溶接継ぎ手において特に応力集中部となりやすい。その溶接止端部４の応力集中度は一般に、溶接止端角（θ）や溶接止端部の曲率半径（ρ）に依存すると言われており、これらの値が小さくなる場合に応力集中度が大きくなる。
また、溶接時は、溶接入熱により溶接部の溶融金属とその周囲の鋼板との間に大きな温度勾配が生じ、これよる溶接金属と母材の膨張量の差に起因して溶接止端部で引張残留応力が生じる。特にアーク溶接では入熱が大きいため、溶接止端部にはほぼ母材の降伏強度に相当するほどの引張残留応力が生じやすく、これが溶接継ぎ手の疲労強度を低下させる原因となる。
したがって、従来のアーク溶接による重ね隅肉継手では、時間的に変動する荷重、即ち疲労荷重が継手に加わると、疲労き裂が応力集中が高く、引張残留応力の大きくなりやすい溶接止端部４に発生し、溶接継手の疲労強度は母材のそれと比較し大きく低下するという問題があった。
【０００４】
また、近年、レーザ溶接機の高出力化と高品質化が進み、特に自動車車体へレーザ溶接が適用されるケースが増えつつある。このレーザ溶接を用いて重ね隅肉溶接を行なう場合、アーク溶接に比べて溶接止端部での引張残留応力は多少少なくなるが溶接止端部の形状に起因する応力集中は同じように生じるため、アーク溶接同様、重ね隅肉溶接継手の疲労強度は母材のそれと比べ低下が著しい。
以上のように溶接継手の特性として良好な疲労強度を得ることは極めて重要であり、これまで様々な疲労強度の改善技術が提案されてきた。
例えば、特開２０００−２１８３７０号公報には、アーク溶接を用いた中・厚鋼板のＴ型または十字継ぎ手重ね継手の重ね隅肉溶接において、溶接前に金属板表面にアルミナ(Al₂O₃)などの酸化物を塗布し溶接することで、溶融金属の粘性を低下させて溶接止端角を増大し、応力集中を緩和させる疲労強度改善法が開示されている。この方法では酸化物を鋼板に塗布するための前処理が必要であるから製造コスト及び生産性が低下し、工業的には好ましい方法とは言い難い。
【０００５】
また、特開平１１−１０４８６５号公報には、第１部材の外表面の端部近傍にレーザを照射して第１部材と第２部材の重ね合わせ部分を溶融凝固させ、溶接止端部における溶接止端角を９０度以上に設定することで応力集中を緩和させた、疲労強度に優れたレーザ重ね隅肉溶接継手およびその溶接方法が開示されている。しかし、この方法において溶接止端角が９０度以上の溶接継手形状を実現するための具体的な溶接方法は十分開示されておらず、また、溶接止端曲率半径については全く考慮されてなく、仮にここで言う溶接止端角が実現できたとしても溶接止端曲率半径が小さい重ね溶接継手では、十分な応力集中の緩和は得られずその疲労強度は低いものとなる。
また、特開昭５５−１５３６９２号公報には、重ね継手あるいは当て金溶接継手の隅肉溶接において、補強盛りを行い溶着金属の喉厚と脚長を金属板板厚の√２倍以上とし、疲労強度を向上させる溶接方法が開示されている。しかし、この方法は通常の重ね隅肉溶接継手に比べ喉厚および脚長を増大させるための補強盛をするために溶接効率を低下させ、また、溶接部の外観を低下させるため好ましくない。
【０００６】
また、溶接の後処理による疲労強度の向上手法としては従来、▲１▼グラインディング、▲２▼ＴＩＧドレッシング、▲３▼ショットピーニング、▲４▼ハンマーピーニングが用いられてきた。これらの手法は、応力集中部となる溶接止端部の形状を整形して応力集中を緩和する、もしくは残留応力を変化させて疲労強度を向上するというものである。しかし、これらについても、以下のような問題点があった。
グラインディング、ＴＩＧドレッシングは、止端の形状をよくするものであるが、いずれも著しく作業効率が悪かった。
また、グラインディングは、止端の疲労向上効果には効果があるものの、喉厚が減ることによりルート側の疲労強度を落とす傾向があった。
ショットピーニング、ハンマーピーニングは、疲労強度向上効果はあるが、ショットピーニングは巨大な機械が必要であるうえ、種々のユーティリティが必要となる。
【０００７】
また、ハンマーピーニングは反動が大きく、処理結果が安定せず、時にはかえって疲労強度を低下させてしまうことがある。また、このハンマーピーニングは、あまりに大きな塑性変形を与えるために、薄い板に対しては使いにくいという欠点もあった。
さらに、ハンマーピーニングは、数Ｈｚの低周波の機械加工を継手部に施すため、加工表面の凹凸が激しく、その山部に応力が集中し、継手部に繰り返し荷重がかかると、この応力集中部から亀裂が生じるため継手全体の疲労強度がかえって低下する場合があるという問題点があった。
【０００８】
また、溶接止端部近傍にショットピーニング処理を行うことで圧縮残留応力を付与できる。ここに、ショットピーニング処理は、疲労き裂発生の起点となる部位に、1mm弱の鋼球を多数打ち付け圧縮残留応力を付与する手法である。
さらに、溶接金属の加熱再溶融により溶接止端部形状の改善あるいは引張残留応力の軽減が可能であることも知られている。
しかし、このショットピーニング処理は、大きな機械と鋼球、およびその飛散を防ぐためのチャンバー等の大がかりなユーティリティーを必要とする。
さらに、溶接金属の再加熱を行うと、新たな溶接変形が生じるため高い寸法精度が必要な溶接構造体には採用されないという問題点があった。また、施工効率も著しく低い。
また、溶接部に超音波振動を与えることによって、疲労強度を向上させる方法に関する従来技術としては、例えば、ＵＳＰ６１７１４１５公報に、アーク溶接直後に溶接アークによって熱せられた溶接シーム部に沿って超音波振動を付与する方法が開示されている。
しかし、この従来技術は、溶接直後の高温状態の溶接ビードに超音波振動を与えることを前提としているため、溶接金属及びその周囲の母材部が高温のため降伏強度が低く、打撃応力の付加によっても溶接止端部に圧縮残留応力は導入されにくく、また、一旦圧縮残留応力が導入されたとしても、その後の室温までの冷却過程で熱収縮によりキャンセルされるため、溶接止端部の圧縮残留応力導入による疲労強度向上効果は十分に得られない。また、超音波振動子で打撃する範囲などの溶接止端部の圧縮残留応力導入のための具体的な条件の開示がないため、この方法を用いても安定して所定レベル以上の疲労強度を向上することは難しい。
以上のように、従来の疲労強度の向上技術を、自動車車体をはじめとする重ね隅肉溶接継手に採用することは困難であり、たとえ採用できても疲労強度向上代が低いレベルに留まっていた。
【０００９】
【特許文献１】
特開２０００−２１８３７０号公報
【特許文献２】
特開平１１−１０４８６５号公報
【特許文献３】
特開昭５５−１５３６９２号公報
【特許文献５】
ＵＳＰ６１７１４１５公報
【００１０】
【発明が解決しようとする課題】
本発明は、前述のような従来技術の問題点を解決し、２枚の金属板を重ね合わせた端部を溶接した重ね隅肉溶接継手の疲労強度向上方法を提供することにより、金属板を用いた溶接構造物の信頼性を一層向上させることを課題とする。
【００１１】
【課題を解決するための手段】
本発明は前述の課題を解決するために鋭意検討の結果なされたものであり、２枚の金属板を重ね合わせた端部を溶接した重ね隅肉溶接継手の近傍の特定範囲を超音波振動端子で打撃する疲労強度向上方法を提供するものであり、その要旨とするところは特許請求の範囲に記載した通りの下記内容である。
【００１２】
（１）２枚の金属板を重ね合わせた端部を溶接した重ね隅溶接肉継手の溶接止端部の近傍を超音波振動端子で打撃する疲労強度向上方法であって、
前記超音波振動端子の直径Ｄが２〜８ｍｍであり、
かつ、前記隅肉溶接継手の溶接止端部を第１の原点とし、前記超音波振動端子の隅肉溶接継手側における側面の、第１の原点に対する相対位置をｘとし、前記金属板表面に平行な隅肉脚長Ｗとするとき、−Ｗ／４≦ｘ≦Ｗ／２を満足する範囲を超音波振動端子で打撃することを特徴とする重ね隅肉溶接継手の疲労強度向上方法。
（２）前記２枚の金属板の板厚をｔ１、ｔ２とし、重ね合わせた金属板のルート端部を第２の原点とし、超音波振動端子の中心軸の、第２の原点に対する相対位置をｙとするとき、
板厚がｔ１の金属板について−ｔ１≦ｙ≦ｔ１を満足する範囲、および／または、板厚がｔ２の金属板について−ｔ２≦ｙ≦ｔ２を満足する範囲を超音波振動端子で打撃することを特徴とする（１）に記載の重ね隅肉溶接継手の疲労強度向上方法。
【００１３】
（３）前記２枚の金属板を溶接する前に、該金属板に機械的な拘束または溶接変形を拘束する曲げ加工を予め施すことを特徴とする（１）または（２）に記載の重ね隅肉溶接継手の疲労強度向上方法。
（４）前記２枚の金属板は、引張強度が４００ＭＰa以上の高強度鋼板であることを特徴とする請求項１乃至請求項３に記載の重ね隅肉溶接継手の疲労強度向上方法。
（５）前記超音波振動端子で打撃するときの隅肉継手の温度が３００℃以下であることを特徴とする請求項１乃至請求項４に記載の重ね隅肉溶接継手の疲労強度向上方法。
【００１４】
【発明の実施の形態】
本発明の実施の形態について、図１乃至図７を用いて詳細に説明する。
＜第１の実施形態＞
図１は、本発明の重ね隅肉溶接継手の疲労強度向上方法における第１の実施形態を示す図である。
図１において、重ね合わされた金属板１および金属板２は、溶接金属３により隅肉溶接されており、４は溶接止端部、５は金属板表面に平行な隅肉脚長Ｗ、６は超音波振動端子、７は超音波振動端子の直径Ｄ、８は超音波振動端子の隅肉継手側における側面、ｘは隅肉継手の溶接止端部（第１の原点）に対する超音波振動端子の隅肉継手側における側面８の相対位置を示す。
このとき、超音波振動端子の直径Ｄを２〜８ｍｍとし、かつ、隅肉継手の溶接止端部（第１の原点）に対する超音波振動端子の隅肉継手側における側面８の相対位置ｘと、金属板表面に平行な隅肉脚長Ｗとの関係で、−Ｗ／４≦ｘ≦Ｗ／２を満足する範囲を超音波振動端子６で打撃することにより、重ね隅肉継手疲労強度を向上させることができる。但し、上記ｘは、溶接止端部を原点とし、原点から重ね合わせ端部側の方向を負（−）とし、その反対方向を正（＋）とする。
【００１５】
まず、超音波振動端子の直径Ｄが２〜８ｍｍとするのは、超音波振動端子６の直径Ｄが、８ｍｍを超えると、超音波振動端子６による打撃面積が大きすぎて打撃圧力が不足し、止端部近傍に圧縮残留応力を効果的に付与できないうえ、超音波振動端子６は通常ある曲率を持った凸形状であり、その最も打撃効果の高い中心部分が溶接止端部４から外れるため、打撃により溶接止端部４に圧縮残留応力を効果的に付与できないからである。また、溶接止端角（図３におけるθ）が大きい場合には、溶接金属３と超音波振動端子６の側面とが接触して、溶接止端部４に超音波振動端子６が届かない場合がある。
他方、端子径Ｄが小さくなり過ぎると打撃圧力が高くなり過ぎ、打撃した部分の凹みが大きく外観不良となるうえ、逆に疲労強度や静的強度の低下をもたらす。さらには超音波振動端子の座屈も生じる。従って、端子径Ｄは２ｍｍ以上とすることが好ましい。
【００１６】
図４は、本発明の第１の実施形態における超音波振動端子による打撃位置と疲労限強度の関係を表す図である。
図４において、横軸は、隅肉溶接止端部に対する超音波振動端子の隅肉継手側における側面の位置ｘ（ｍｍ）を示し、縦軸は、溶接継ぎ手の疲労限強度（ｋN）を示す。なお、ｘは、溶接止端部を原点とし、原点から重ね合わせ端部側の方向を負（−）とし、その反対方向を正（＋）とした。また、使用した超音波振動端子の直径Ｄは３ｍｍで固定し、金属板表面に平行な隅肉脚長Ｗは１．８ｍｍと２．５ｍｍの２水準で行った。
図４から、溶接継ぎ手の疲労限強度を向上するための超音波振動端子の最適な打撃位置は、隅肉止端部に対する超音波振動端子の隅肉継手側における側面の位置ｘと金属板表面に平行な隅肉脚長Ｗとの関係で決まり、ｘが−Ｗ／４＜ｘ≦Ｗ／２の範囲内において、疲労限強度が著しく向上し、この範囲から外れると疲労強度の改善が殆ど、あるいは全く得られない。
本発明の第１実施形態において、図４などの実験結果を技術的根拠として、超音波振動端子６による打撃範囲を−Ｗ／４＜ｘ≦Ｗ／２と規定した。これは、溶接止端部４の近傍のこの範囲を超音波振動端子６で打撃することによって、溶接止端部の形状を良好にして応力集中を緩和させるとともに、溶接時の溶接金属とその周辺の鋼板との熱膨張差によって溶接止端部４の近傍に発生する引張残留応力を低減するか、さらには、圧縮残留応力に変えることができるからである。なお、これらの効果をより顕著に得るためには溶接止端部の打撃範囲に数ｍｍ幅で、かつ打撃前の表面に対して深さ数百μｍ程度の圧痕を形成するように打撃することが好ましい。
なお、本発明においては、金属板の重ね隅肉継手に適用できる溶接方法であれば、溶接方法は問わず、一般に用いられる、アーク溶接、レーザ溶接のほか、プラズマ溶接や電子ビーム溶接などが適用できる。
【００１７】
なお、本発明に使用する超音波振動の発生装置は特に問わないが、発振機により超音波を発振後、トランスデューサーによりその周波数を２０〜６０ｋＨｚに変換し、さらに、ウェーブガイドにてその振幅を増幅させて、直径２ｍｍ〜８ｍｍφのピンからなる超音波振動端子を２０〜４０μｍの振幅で機械的に振動させることによって、打撃部の表面において、平滑性を維持しつつ打撃前の表面に対して深さ数百μｍ程度の圧痕を形成することができる。この超音波振動発生装置は、以下の実施形態にも共通して用いることができる。
【００１８】
＜第２の実施形態＞
図２は、本発明の重ね隅肉溶接継手の疲労強度向上方法における第２の実施形態を示す図である。
図２において、重ね合わされた金属板１および金属板２は、溶接金属３により隅肉溶接されており、９はルート端部、ｔ１は金属板１の板厚、ｔ２は金属板２の板厚、６は超音波振動端子、ｙはルート端部（第２の原点）に対する超音波振動端子の中心軸１０の相対位置を示す。
このとき、上記の本発明の第１の実施形態で規定した条件の他に、さらに、板厚がｔ１の金属板１について、重ね合わせ面におけるルート端部（第２の原点）に対する超音波振動端子の中心軸１０の相対位置ｙが−ｔ１≦ｙ≦ｔ１を満足する範囲、および／または、板厚がｔ２の金属板２について、上記ｙが−ｔ２≦ｙ≦ｔ２を満足する範囲を超音波振動端子６で打撃することにより、重ね隅肉継手疲労強度をより向上させることができる。但し、上記ｙは、重ね合わせ面におけるルート端部を原点とし、原点から溶接止端部側の方向を正（＋）とし、その反対方向を負（−）とする。
【００１９】
超音波振動端子６による打撃範囲を、板厚がｔ１の金属板１について−ｔ１≦ｙ≦ｔ１を満足する範囲とするのは、溶接により引張残留応力が生じやすいルート端部９とのほぼ最短距離の金属板１表面上または溶接金属３表面上の位置を超音波振動端子６により打撃することにより、その部分の引張残留応力を低減するか、さらには、圧縮残留応力に変えることができるからである。なお、上記のように超音波振動端子６による打撃範囲を、重ね合わせ面におけるルート端部（第２の原点）に対する超音波振動端子の中心軸１０の相対位置ｙと金属板１の板厚ｔ１との関係で、−ｔ１≦ｙ≦ｔ１と規定するのは、超音波振動端子６による打撃位置からの荷重や衝撃力の広がりは４５度程度が限界なので、その作用の及ぶ範囲を板厚の２倍の範囲として−ｔ１≦ｘ≦ｔ１とした。
ルート端部での上記効果は、何れか片方の金属板１のみを打撃しても得られるが、より大きな効果を得るためには、金属板２についても金属板１と同様に、−ｔ２≦ｙ≦ｔ２を満足する範囲を超音波振動端子６で打撃することが好ましい。一般に、重ね隅肉溶接継ぎ手では、重ね合わせ面におけるルート端部に比べ溶接止端部の方が疲労強度が低くなるため、本発明の第２実施形態において、本発明の第１の実施形態で規定した溶接止端部の打撃処理を行わずに、ルート端部に対する打撃処理だけをも、溶接継手の疲労強度は充分に向上することはできない。
但し、本発明の第１の実施形態で規定した溶接止端部の処理条件と第２の実施形態で規定したルート端部の処理条件を同時に満たす条件で実施することができれば、本発明の第１の実施形態のみによって両方の効果を同時に得ることが出来る。
【００２０】
なお、本発明においては、金属板の重ね隅肉溶接継手に適用できる溶接方法であれば、溶接方法は特に問わず、一般に用いられる、アーク溶接、レーザ溶接のほか、プラズマ溶接や電子ビーム溶接などが適用できる。
【００２１】
＜好ましい実施形態＞
通常溶接時には溶接入熱により、金属板は熱変形する。特に鋼材の板厚が薄い場合には、溶接中の金属板の熱変形は大きくなるため、その変形が重ね溶接継手のルート端部近傍にギャップが生じることがある。
一般に重ねすみ肉溶接継ぎ手のルート端部近傍にギャップが生じた場合、継ぎ手使用時に溶接部での変形が大きくなり、ルート端部での発生応力が高くなって、疲労強度が低下する傾向にある。
特に金属板の板厚が9mmより小さい場合、溶接時に金属板が熱変形しやすく、その変形によりルートギャップが増大し疲労強度を低下させる原因となるため、溶接時にルート端部を何らかの方法で拘束することが好ましい。
そこで、このような金属板の熱変形起因の疲労強度低下を抑制するために、金属板を溶接する前に、金属板を機械的な拘束または溶接変形を拘束する曲げ加工を予め施すことが好ましい。
【００２２】
例えば、機械的な拘束の方法としては、施工しやすさという観点から、図５のように金属板１，２同士を、拘束治具１１により金属板１，２の上下から直接荷重を与えて変形を拘束する方法が好ましく、また、曲げ加工の方法としては、図６のように２枚の金属板を重ね合わせて、溶接ビードの進行方向１２に沿って曲加工する方法が好ましい。
本発明の第１実施形態または第２実施形態において、超音波振動端子による打撃によって溶接止端部またはルート端部に対して圧縮残留応力を付与させる効果を充分に発揮させるためには金属鋼板の引張強度が４００ＭＰa以上であることが好ましい。引張強度が４００ＭＰa未満では、超音波振動端子による打撃時に、溶接止端部またはルート端部において発生する残留応力がその外力によって降伏に至り、再分配されてしまう傾向があるが、高強度鋼は降伏しにくいので残留応力が分散しにくいからである。
【００２３】
同様に、本発明の第１実施形態または第２実施形態において、超音波振動端子による打撃によって溶接止端部またはルート端部に圧縮残留応力を付与させる効果を充分に発揮させ、かつ、靱性低下を生じないためには重ね隅肉溶接継手の温度が３００℃以下で溶接止端部またはルート端部の打撃処理を行うことが好ましい。
重ね隅肉溶接継手の温度が３００℃以上では、超音波振動端子による打撃時に、溶接金属および金属板の降伏応力が低くなっているため、残留応力が打撃時の外力によって降伏に至り、再分配されてしまい、さらに打撃処理後も、室温までの温度低下過程での熱収縮により新たに引張残留応力が発生する可能性が高いからである。また、重ね隅肉溶接継手の温度が−１０℃以下では、低温のため継手の靭性が劣化するため、−１０℃以上で打撃処理を行うことがより好ましい。
本発明の重ね隅肉溶接継手の疲労強度向上方法を用いて、作動発生応力の大きな機械品、自動車の足回り品、自動車用ホイールなど、高い疲労強度が要求される金属板加工製品を製造することによって、疲労強度が高く、長寿命の信頼性の高い金属板加工製品を製造することができる。
【００２４】
【実施例】
本発明の重ね隅肉継手の疲労強度向上方法の実施例を以下に示す。
表１に示す板厚、引張強度の鋼板２枚を重ね合わせ、重ねた端部をアーク溶接あるいはレーザ溶接によって隅肉溶接した後、本発明の超音波振動端子による打撃処理を行った。また、比較のために重ね隅肉溶接した後、本発明の超音波振動端子による打撃処理を行わない溶接ままの重ね隅肉溶接継ぎ手も作製した。継手サイズは４０ｍｍ(幅)×２５０ｍｍ(長さ)×板厚(ｍｍ)、重ね代４０ｍｍとした。アーク溶接の場合、通常溶接材料を用いて重ね隅肉溶接を行った。このとき、１．２ｍｍ厚の鋼板に対しては、溶接電流１２０Ａ、溶接速度は９０ｃｍ／ｍｉｎ、また、２．６ｍｍ厚の鋼板については、溶接電流１６０Ａ、溶接速度６０ｃｍ／ｍｉｎとした。
レーザ溶接にはＹＡＧレーザを用い、加工点出力を３．０ｋＷ、溶接速度を５．０ｍ／ｍｉｎ、焦点のビーム直径を０．５ｍｍとした。この際、シールドにはセンターシールドトーチ、ガスとして窒素を用いた。ビームの焦点位置は鋼板表面とし、ビームの狙い位置を重ね端部、あるいは端部より０．２５ｍｍ内側とすることで金属板表面に平行な隅肉脚長Ｗを変化させた。
超音波振動装置は、電源１ｋｗ、周波数２７ｋHzとし、超音波振動端子の振幅は３０〜４０μm、打撃処理速度は５０ｃｍ/minとした。
【００２５】
超音波振動端子による打撃処理を行った後の疲労特性を測定し、その測定値を予め同じ条件で溶接後に打撃処理を施さない場合のそれと比較し、疲労限の強度が１０％以上向上したものを「ＯＫ」(良好)、それ以下のものを「ＮＧ」(不良)としている。なお疲労試験条件は、荷重比(最小荷重／最大荷重)=０．１、繰返し速度=１０Ｈｚの片振り引張とした。
表１のＮｏ．１〜１３が本発明例である。Ｎｏ．１およびＮｏ．２は図１に示す鋼板表面に平行な隅肉脚長５が４．６ｍｍ、２．５ｍｍ、１．８ｍｍ、超音波振動端子の直径７が３．０ｍｍ、５．０ｍｍの場合であるが、超音波振動端子の直径Ｄおよび超音波振動端子の打撃位置ｘが本発明で規定した範囲内であるため、いずれも、良好な疲労特性を得ることができた。
一方、表１のＮｏ．１４〜２７は、超音波振動端子の直径Ｄおよび超音波振動端子の打撃位置ｘの何れかが本発明で規定した範囲から外れるか、全く打撃処理をしない溶接ままの場合の比較例である。
Ｎｏ．１４およびＮｏ．１５は、脚長Ｗが４．６ｍｍ、超音波振動端子の直径Ｄが５．０ｍｍの場合であり、Ｎｏ．１６およびＮｏ．１７は、脚長Ｗが１．８ｍｍ、超音波振動端子の直径Ｄが３．０ｍｍの場合であり、Ｎｏ．１８およびＮｏ．１９は、脚長Ｗが２．５ｍｍ、超音波振動端子の直径Ｄが３．０ｍｍの場合であり、いずれの場合も超音波振動端子の打撃位置ｘが本発明で規定した範囲から外れているために、疲労強度は低く、その評価はどちらも不良（ＮＧ）であった。
Ｎｏ．２０および２２は、超音波振動端子の直径Ｄがいずれも８．５ｍｍと本発明で規定した範囲から外れており、かつ超音波振動端子の打撃位置ｘも本発明で規定した範囲から外れているために、疲労強度は低く、その評価はどちらも不良（ＮＧ）であった。
Ｎｏ．２１および２３は、超音波振動端子の直径Ｄが８．５ｍｍ、１０．０ｍｍと本発明で規定した範囲から外れているために、疲労強度は低く、その評価はどちらも不良（ＮＧ）であった。
また、Ｎｏ．２４〜２７は、溶接止端部およびルート端部、または、溶接止端部に対して超音波振動端子による打撃処理を施さなかった重ね隅肉溶接継ぎ手であり、疲労強度は低く、その評価はどちらも不良（ＮＧ）であった。
【００２６】
次に、表２に示す板厚、引張強度のアルミ板２枚を重ね合わせ、重ねた端部をアーク溶接によって隅肉溶接した後、本発明の超音波振動端子による打撃処理を行った。アルミ板材料は５０８３、アーク溶接は溶加材に５３５６を用いて重ね隅肉溶接を行った。このとき、溶接電流は１５０Ａ、溶接速度は２５ｃｍ／ｍｉｎとした。また、比較のために重ね隅肉溶接した後、本発明の超音波振動端子による打撃処理を行わない溶接ままの重ね隅肉溶接継ぎ手も作製した。材質、継手サイズ、溶接条件および試験方法は鋼板の場合と同様である。
表２のＮｏ．３１〜３６が本発明例であり、図１に示す鋼板表面に平行な隅肉脚長５が４．６ｍｍ、超音波振動端子の直径７が３．０ｍｍ、５．０ｍｍの場合であるが、超音波振動端子の直径Ｄおよび超音波振動端子の打撃位置ｘが本発明で規定した範囲内であるため、いずれも、良好な疲労特性を得ることができた。
一方、表２のＮｏ．３７〜３９は比較例である。
Ｎｏ．３７は、超音波振動端子による打撃処理を行わなかったため、疲労強度は低く、その評価はどちらも不良（ＮＧ）であった。
Ｎｏ．３８およびＮｏ．３９は、超音波振動端子の打撃位置ｘが本発明で規定した範囲から外れているために、疲労強度は低く、その評価はどちらも不良（ＮＧ）であった。
【表１】

【表２】

【００２７】
【発明の効果】
本発明によれば、２枚の金属板を重ね合わせた端部を溶接した重ね隅肉溶接継手の溶接止端部の近傍の特定範囲を超音波振動端子で打撃する疲労強度向上方法を提供することにより、金属板を用いた溶接構造物の信頼性を一層向上させることができ、産業上有用な著しい効果を奏する。
【図面の簡単な説明】
【図１】本発明の第１の実施形態における重ね隅肉溶接継手において超音波振動端子による打撃範囲を示す模式図である。
【図２】本発明の第２の実施形態における重ね隅肉溶接継手において超音波振動端子による打撃範囲を示す模式図である。
【図３】従来の重ね隅肉溶接継手の断面の一例を示す模式図である。
【図４】本発明の第１の実施形態における超音波振動端子による打撃位置と疲労限強度の関係を表す図である。
【図５】金属板を、機械的に（かしめて）拘束する方法の一例を示す図である。
【図６】金属板を曲げ加工して拘束する方法の一例を示す図である。
【符号の説明】
１、２：金属板、
３：溶接金属、
４：溶接止端部、
５：金属板表面に平行な隅肉脚長（Ｗ）、
６：超音波振動端子、
７：超音波振動端子直径（Ｄ）、
８：超音波振動端子の隅肉継手側の側面、
９：ルート端部、
１０：超音波振動端子の中心軸、
１１：拘束治具
１２：溶接ビードの進行方向[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for improving the characteristics of a welded portion of a welded structure using a metal plate such as a bridge, a machine article, an automobile, and a motorcycle part, and more specifically, an end portion obtained by superimposing two metal plates. The present invention relates to a method for improving the fatigue strength of a lap fillet welded joint.
[0002]
[Prior art]
In recent years, with the aim of reducing the weight of structures, the development of technology that enhances the fatigue strength of welds with good reproducibility and effectiveness is strongly desired as the strength and thickness of steel materials used are increasing. .
Generally, as a welded joint shape in a welded structure using a metal plate such as a bridge or an automobile body, a lap fillet joint in which ends of two metal plates are overlapped is welded. As such, various welding methods such as arc welding, laser welding, plasma welding, and electron beam welding are applied.
[0003]
For example, FIG. 3 shows an example in which a lap fillet welded joint is produced by arc welding. In FIG. 3, since the energy density of arc welding is low and the penetration ability is poor, the

metal plates

1 and 2 are overlapped, the arc is irradiated to the end of the overlapped portion, and the molten weld metal 3 from the welding wire is the metal plate. So-called overlapped fillet welding is performed in which a weld bead is formed so as to melt into a part of the vicinity of the overlapping end portions of 1 and 2.
As shown in FIG. 3, in the lap fillet welded joint, the portion where the weld metal 3 is connected to the surface of the metal plate 2, that is, the weld toe portion 4 is suddenly changed in shape, so It is easy to become. It is said that the stress concentration degree of the weld toe part 4 generally depends on the weld toe angle (θ) and the curvature radius (ρ) of the weld toe part, and when these values become small, the stress concentration degree Becomes larger.
Also, during welding, a large temperature gradient occurs between the molten metal at the weld and the surrounding steel plate due to the heat input, resulting in a difference in the amount of expansion between the weld metal and the base metal, resulting in a weld toe. Tensile residual stress occurs. In particular, since the heat input is large in arc welding, a tensile residual stress almost corresponding to the yield strength of the base metal is likely to occur at the weld toe, which causes a decrease in the fatigue strength of the weld joint.
Therefore, in a conventional lap fillet joint by arc welding, when a time-varying load, that is, a fatigue load, is applied to the joint, the fatigue crack has a high stress concentration, and the weld toe 4 tends to have a large tensile residual stress. The fatigue strength of welded joints is greatly reduced compared to that of the base metal.
[0004]
In recent years, laser welding machines have been increased in output and quality, and the number of cases in which laser welding is applied particularly to automobile bodies is increasing. When lap fillet welding is performed using this laser welding, the tensile residual stress at the weld toe is slightly less than that of arc welding, but the stress concentration caused by the shape of the weld toe is the same. As with arc welding, the fatigue strength of lap fillet welded joints is significantly lower than that of the base metal.
As described above, it is extremely important to obtain good fatigue strength as a characteristic of the welded joint, and various techniques for improving fatigue strength have been proposed so far.
For example, in Japanese Patent Laid-Open No. 2000-218370, in the lap fillet welding of T-type or cross joint lap joints of medium and thick steel plates using arc welding, alumina (Al ₂ O _Three A fatigue strength improving method is disclosed in which an oxide such as) is applied and welded to reduce the viscosity of the molten metal, increase the weld toe angle, and relieve stress concentration. This method requires pretreatment for applying the oxide to the steel sheet, so that the manufacturing cost and productivity are lowered, and it is difficult to say that this method is industrially preferable.
[0005]
Japanese Patent Application Laid-Open No. 11-104865 discloses that welding is performed at a weld toe portion by irradiating a laser near the end portion of the outer surface of the first member to melt and solidify the overlapping portion of the first member and the second member. A laser lap fillet welded joint excellent in fatigue strength, in which stress concentration is relaxed by setting the toe angle to 90 degrees or more, and a welding method therefor are disclosed. However, a specific welding method for realizing a welded joint shape having a weld toe angle of 90 degrees or more in this method is not sufficiently disclosed, and the weld toe curvature radius is not considered at all. Even if the weld toe angle referred to here can be realized, a lap weld joint having a small weld toe curvature radius will not provide sufficient relaxation of stress concentration and its fatigue strength will be low.
Japanese Patent Laid-Open No. 55-153692 discloses that in fillet welding of a lap joint or a metal-welded joint, a reinforcement is provided so that the throat thickness and leg length of the weld metal are not less than √2 times the thickness of the metal plate. A welding method for improving strength is disclosed. However, this method is not preferable because it lowers the welding efficiency to increase the throat thickness and the leg length as compared with a normal lap fillet welded joint, and lowers the appearance of the weld.
[0006]
Conventionally, (1) grinding, (2) TIG dressing, (3) shot peening, and (4) hammer peening have been used as methods for improving fatigue strength by post-processing of welding. These techniques are to relieve stress concentration by shaping the shape of the weld toe portion that becomes the stress concentration portion, or to improve fatigue strength by changing the residual stress. However, these also have the following problems.
Grinding and TIG dressing improve the shape of the toe, but the working efficiency was remarkably poor.
Grinding was effective in improving the fatigue at the toe, but there was a tendency to reduce the root-side fatigue strength by reducing the throat thickness.
Shot peening and hammer peening have an effect of improving fatigue strength, but shot peening requires a huge machine and various utilities.
[0007]
In addition, hammer peening has a large recoil, the processing result is not stable, and sometimes the fatigue strength is reduced. Also, this hammer peening has a drawback that it is difficult to use for thin plates because it gives too much plastic deformation.
Furthermore, hammer peening is applied to the joints with low frequency machining of several Hz, so that the processed surface is severely uneven, stress concentrates on the peaks, and repeated stress is applied to the joints. Therefore, there is a problem that the fatigue strength of the entire joint may be lowered due to cracks.
[0008]
Further, compressive residual stress can be applied by performing shot peening treatment in the vicinity of the weld toe. Here, the shot peening treatment is a method of applying a compressive residual stress by hitting a large number of steel balls of less than 1 mm at a site where fatigue cracks start.
It is also known that the weld toe shape can be improved or the tensile residual stress can be reduced by remelting the weld metal by heating.
However, this shot peening process requires large utilities such as a large machine and a steel ball, and a chamber for preventing the scattering.
Furthermore, when the weld metal is reheated, a new weld deformation occurs, so that there is a problem that it is not adopted for a welded structure that requires high dimensional accuracy. In addition, the construction efficiency is extremely low.
Further, as a conventional technique related to a method for improving fatigue strength by applying ultrasonic vibration to a welded portion, for example, US Pat. No. 6,171,415 discloses ultrasonic vibration along a weld seam portion heated by a welding arc immediately after arc welding. Is disclosed.
However, since this conventional technology is premised on applying ultrasonic vibration to the weld bead in a high temperature state immediately after welding, the yield strength is low due to the high temperature of the weld metal and the surrounding base metal, and the impact stress is not applied. Also, it is difficult to introduce compressive residual stress to the weld toe, and once the compressive residual stress is introduced, it is canceled by thermal contraction during the subsequent cooling to room temperature. The effect of improving fatigue strength by introducing residual stress cannot be obtained sufficiently. In addition, since there is no disclosure of specific conditions for introducing compressive residual stress at the weld toe such as the range where the ultrasonic vibrator strikes, fatigue strength above a predetermined level can be stably achieved using this method. It is difficult to improve.
As described above, it is difficult to adopt conventional fatigue strength improvement technology for lap fillet welded joints such as automobile bodies, and even if it can be adopted, the fatigue strength improvement margin has remained at a low level. .
[0009]
[Patent Document 1]
JP 2000-218370 A
[Patent Document 2]
JP-A-11-104865
[Patent Document 3]
Japanese Patent Laid-Open No. 55-153692
[Patent Document 5]
US Pat. No. 6,171,415
[0010]
[Problems to be solved by the invention]
The present invention solves the problems of the prior art as described above, and provides a method for improving the fatigue strength of a lap fillet welded joint obtained by welding the end portions where two metal plates are overlapped. It is an object to further improve the reliability of the welded structure used.
[0011]
[Means for Solving the Problems]
The present invention has been made as a result of diligent studies in order to solve the above-mentioned problems, and an ultrasonic vibration terminal is used to specify a specific range in the vicinity of a lap fillet welded joint where two metal plates are overlapped and welded to each other. The present invention provides a method for improving fatigue strength by striking with the following, the gist of which is as follows.
[0012]
(1) A method for improving fatigue strength in which an ultrasonic vibration terminal is used to strike the vicinity of the weld toe portion of the lap corner welded meat joint in which the end portion where two metal plates are overlapped is welded,
The ultrasonic vibration terminal has a diameter D of 2 to 8 mm,
The weld toe of the fillet welded joint is the first origin, the side surface of the ultrasonic vibration terminal on the fillet weld joint side is relative to the first origin, and x is the surface of the metal plate. A fatigue strength improving method for a lap fillet welded joint, wherein when the parallel fillet leg length W is set, a range satisfying −W / 4 ≦ x ≦ W / 2 is hit with an ultrasonic vibration terminal.
(2) The thicknesses of the two metal plates are t1 and t2, the root end of the superimposed metal plates is the second origin, and the relative position of the central axis of the ultrasonic vibration terminal with respect to the second origin Is y
The range of satisfying −t1 ≦ y ≦ t1 for a metal plate with a thickness of t1 and / or the range of satisfying −t2 ≦ y ≦ t2 for a metal plate with a thickness of t2 is hit with an ultrasonic vibration terminal. (1) The fatigue strength improving method of the lap fillet welded joint according to (1).
[0013]
(3) Prior to welding the two metal plates, the metal plates are preliminarily subjected to a bending process for restraining mechanical restraint or welding deformation, as described in (1) or (2) A method for improving the fatigue strength of fillet welded joints.
(4) The method for improving fatigue strength of a lap fillet welded joint according to any one of claims 1 to 3, wherein the two metal plates are high strength steel plates having a tensile strength of 400 MPa or more.
(5) The method for improving the fatigue strength of a lap fillet welded joint according to any one of claims 1 to 4, wherein the temperature of the fillet joint when striking with the ultrasonic vibration terminal is 300 ° C or lower.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described in detail with reference to FIGS.
<First Embodiment>
FIG. 1 is a diagram showing a first embodiment in a method for improving fatigue strength of a lap fillet welded joint according to the present invention.
In FIG. 1, the metal plate 1 and the metal plate 2 that are overlapped are fillet welded by a

weld metal

3, 4 is a weld toe, 5 is a fillet leg length W parallel to the metal plate surface, The ultrasonic vibration terminal, 7 is the diameter D of the ultrasonic vibration terminal, 8 is the side surface of the ultrasonic vibration terminal on the fillet joint side, and x is the ultrasonic vibration terminal with respect to the weld toe end (first origin) of the fillet joint. The relative position of the side surface 8 on the fillet joint side is shown.
At this time, the diameter D of the ultrasonic vibration terminal is set to 2 to 8 mm, and the relative position x of the side surface 8 on the fillet joint side of the ultrasonic vibration terminal with respect to the weld toe end (first origin) of the fillet joint is The fatigue strength of the lap fillet joint is improved by striking the range satisfying −W / 4 ≦ x ≦ W / 2 with the ultrasonic vibration terminal 6 in relation to the fillet leg length W parallel to the metal plate surface. Can be made. In the above x, the weld toe is the origin, the direction from the origin to the overlapping end is negative (−), and the opposite direction is positive (+).
[0015]
First, the diameter D of the ultrasonic vibration terminal is set to 2 to 8 mm. If the diameter D of the ultrasonic vibration terminal 6 exceeds 8 mm, the striking area of the ultrasonic vibration terminal 6 is too large and the striking pressure is insufficient. In addition, the compressive residual stress cannot be effectively applied in the vicinity of the toe portion, and the ultrasonic vibration terminal 6 is usually a convex shape having a certain curvature, and the central portion having the highest impact effect is disengaged from the weld toe portion 4. For this reason, the compressive residual stress cannot be effectively applied to the weld toe portion 4 by striking. When the weld toe angle (θ in FIG. 3) is large, the weld metal 3 and the side surface of the ultrasonic vibration terminal 6 are in contact with each other, and the ultrasonic vibration terminal 6 does not reach the weld toe part 4. There is.
On the other hand, if the terminal diameter D is too small, the impact pressure becomes too high, the dent of the impacted portion becomes large and the appearance becomes poor, and conversely, fatigue strength and static strength are reduced. Furthermore, buckling of the ultrasonic vibration terminal also occurs. Accordingly, the terminal diameter D is preferably 2 mm or more.
[0016]
FIG. 4 is a diagram showing the relationship between the hit position by the ultrasonic vibration terminal and the fatigue limit strength in the first embodiment of the present invention.
In FIG. 4, the horizontal axis indicates the position x (mm) of the side surface on the fillet joint side of the ultrasonic vibration terminal with respect to the fillet weld toe, and the vertical axis indicates the fatigue limit strength (kN) of the weld joint. . In addition, x made the welding toe part an origin, the direction from the origin to the overlapping end part was negative (−), and the opposite direction was positive (+). Moreover, the diameter D of the used ultrasonic vibration terminal was fixed at 3 mm, and the fillet leg length W parallel to the metal plate surface was set at two levels of 1.8 mm and 2.5 mm.
From FIG. 4, the optimum striking position of the ultrasonic vibration terminal for improving the fatigue limit strength of the welded joint is the position x of the side surface on the fillet joint side of the ultrasonic vibration terminal with respect to the fillet toe and the surface of the metal plate The fatigue limit strength is remarkably improved when x is in the range of −W / 4 <x ≦ W / 2, and the fatigue strength is almost improved when it is out of this range. Or not at all.
In the first embodiment of the present invention, the impact range by the ultrasonic vibration terminal 6 is defined as −W / 4 <x ≦ W / 2 based on the experimental result of FIG. This is because the ultrasonic vibration terminal 6 strikes this range in the vicinity of the weld toe 4 to improve the shape of the weld toe and relieve stress concentration, and weld metal during welding and its surroundings. This is because the tensile residual stress generated in the vicinity of the weld toe portion 4 due to the difference in thermal expansion with the steel plate can be reduced or further changed to a compressive residual stress. In addition, in order to obtain these effects more remarkably, it is struck so as to form an indentation having a width of several mm in the hitting range of the weld toe and a depth of about several hundred μm with respect to the surface before hitting. Is preferred.
In the present invention, as long as the welding method can be applied to a lap fillet joint of a metal plate, any welding method is generally used, and arc welding, laser welding, plasma welding, electron beam welding, etc. are applied. it can.
[0017]
The ultrasonic vibration generator used in the present invention is not particularly limited. After ultrasonic waves are oscillated by an oscillator, the frequency is converted to 20 to 60 kHz by a transducer, and the amplitude is further changed by a waveguide. By amplifying and mechanically vibrating an ultrasonic vibration terminal composed of a pin having a diameter of 2 mm to 8 mmφ with an amplitude of 20 to 40 μm, the surface of the hitting portion is maintained with respect to the surface before hitting while maintaining smoothness. An indentation having a depth of about several hundred μm can be formed. This ultrasonic vibration generator can be commonly used in the following embodiments.
[0018]
<Second Embodiment>
FIG. 2 is a diagram showing a second embodiment of the method for improving fatigue strength of a lap fillet welded joint according to the present invention.
In FIG. 2, the overlapped metal plate 1 and metal plate 2 are fillet welded by

weld metal

3, 9 is the root end, t 1 is the plate thickness of the metal plate 1, and t 2 is the plate thickness of the metal plate 2. , 6 indicates the ultrasonic vibration terminal, and y indicates the relative position of the central axis 10 of the ultrasonic vibration terminal with respect to the root end (second origin).
At this time, in addition to the conditions defined in the first embodiment of the present invention, the ultrasonic vibration with respect to the root end (second origin) on the overlapping surface of the metal plate 1 having a thickness t1 The range in which the relative position y of the central axis 10 of the terminal satisfies −t1 ≦ y ≦ t1 and / or the range in which the above y satisfies −t2 ≦ y ≦ t2 for the metal plate 2 having a thickness t2. By hitting with the sonic vibration terminal 6, the fatigue strength of the lap fillet joint can be further improved. However, the above y is the origin at the root end on the overlapping surface, the direction from the origin to the weld toe side is positive (+), and the opposite direction is negative (-).
[0019]
The strike range by the ultrasonic vibration terminal 6 is set to a range satisfying −t1 ≦ y ≦ t1 for the metal plate 1 having a thickness t1, which is almost the shortest distance from the root end portion 9 where tensile residual stress is likely to occur due to welding. By hitting the position of the distance on the surface of the metal plate 1 or the surface of the weld metal 3 with the ultrasonic vibration terminal 6, the tensile residual stress of the portion can be reduced or further changed to the compressive residual stress. It is. Note that, as described above, the striking range of the ultrasonic vibration terminal 6 is set such that the relative position y of the central axis 10 of the ultrasonic vibration terminal with respect to the root end (second origin) on the overlapping surface and the thickness t1 of the metal plate 1 are. Therefore, -t1 ≦ y ≦ t1 is specified because the spread of the load and impact force from the striking position by the ultrasonic vibration terminal 6 is limited to about 45 degrees. As a double range, −t1 ≦ x ≦ t1.
Although the above-mentioned effect at the root end can be obtained by hitting only one of the metal plates 1, in order to obtain a greater effect, the metal plate 2 is also −t2 ≦ It is preferable to hit the ultrasonic vibration terminal 6 in a range satisfying y ≦ t2. Generally, in a lap fillet welded joint, the weld toe portion has a lower fatigue strength than the root end portion on the lap surface. Therefore, in the second embodiment of the present invention, in the first embodiment of the present invention, The fatigue strength of the welded joint cannot be sufficiently improved even by performing only the striking process on the root end without performing the prescribed welding toe striking process.
However, if the processing can be performed under the conditions that simultaneously satisfy the processing conditions for the weld toe defined in the first embodiment of the present invention and the processing conditions for the root end defined in the second embodiment, Both effects can be obtained simultaneously by only one embodiment.
[0020]
In the present invention, the welding method is not particularly limited as long as the welding method can be applied to the lap fillet welded joint of the metal plate, and generally used arc welding, laser welding, plasma welding, electron beam welding, etc. Is applicable.
[0021]
<Preferred embodiment>
During normal welding, the metal plate is thermally deformed by welding heat input. In particular, when the steel plate is thin, thermal deformation of the metal plate during welding increases, and this deformation may cause a gap near the root end of the lap weld joint.
Generally, when a gap occurs near the root end of a lap fillet welded joint, deformation at the weld becomes large when the joint is used, and stress generated at the root end tends to increase, resulting in a decrease in fatigue strength. .
In particular, if the thickness of the metal plate is less than 9 mm, the metal plate is likely to be thermally deformed during welding, and this deformation increases the root gap and decreases the fatigue strength. It is preferable to do.
Therefore, in order to suppress such a decrease in fatigue strength due to thermal deformation of the metal plate, it is preferable to perform in advance a bending process for mechanically restraining the metal plate or restraining the welding deformation before the metal plate is welded. .
[0022]
For example, as a method of mechanical restraint, from the viewpoint of ease of construction, the

metal plates

1 and 2 are applied directly to each other as shown in FIG. A method of constraining deformation is preferable, and a method of bending is preferably a method of bending two metal plates along the traveling direction 12 of the weld bead as shown in FIG.
In 1st Embodiment or 2nd Embodiment of this invention, in order to fully exhibit the effect of giving a compressive residual stress with respect to a weld toe part or a root end part by the impact | damage by an ultrasonic vibration terminal, a metal steel plate is used. The tensile strength is preferably 400 MPa or more. When the tensile strength is less than 400 MPa, the residual stress generated at the weld toe or root end tends to yield and redistribute due to the external force when hit by the ultrasonic vibration terminal. This is because the residual stress is difficult to disperse because it is difficult to yield.
[0023]
Similarly, in the first embodiment or the second embodiment of the present invention, the effect of imparting compressive residual stress to the weld toe or root end by striking with the ultrasonic vibration terminal is sufficiently exerted, and the toughness is reduced. In order to prevent the occurrence of the failure, it is preferable to perform the hitting process at the weld toe or root end when the temperature of the lap fillet weld joint is 300 ° C. or lower.
When the temperature of the lap fillet welded joint is 300 ° C or higher, the yield stress of the weld metal and the metal plate is low at the time of impact by the ultrasonic vibration terminal. This is because, even after the impact treatment, there is a high possibility that a new tensile residual stress is generated due to thermal contraction in the process of lowering the temperature to room temperature. In addition, when the temperature of the lap fillet welded joint is −10 ° C. or lower, the toughness of the joint deteriorates due to the low temperature.
Using the method for improving the fatigue strength of a lap fillet welded joint according to the present invention, a metal plate processed product that requires high fatigue strength, such as a mechanical product having a large operating stress, an automobile undercarriage product, and an automobile wheel, is manufactured. As a result, it is possible to manufacture a metal plate processed product having high fatigue strength and long life and high reliability.
[0024]
【Example】
Examples of the method for improving the fatigue strength of the lap fillet joint of the present invention are shown below.
Two steel plates having the thickness and tensile strength shown in Table 1 were overlapped, and the overlapped ends were fillet welded by arc welding or laser welding, and then subjected to a striking process using the ultrasonic vibration terminal of the present invention. In addition, for the purpose of comparison, after lap fillet welding, a welded lap fillet welded joint that was not subjected to the impact treatment using the ultrasonic vibration terminal of the present invention was also produced. The joint size was 40 mm (width) × 250 mm (length) × plate thickness (mm), and the overlap allowance was 40 mm. In the case of arc welding, lap fillet welding was usually performed using a welding material. At this time, the welding current was 120 A and the welding speed was 90 cm / min for a 1.2 mm-thick steel plate, and the welding current was 160 A and the welding speed was 60 cm / min for a 2.6 mm-thick steel plate.
A YAG laser was used for laser welding, the processing point output was 3.0 kW, the welding speed was 5.0 m / min, and the focal beam diameter was 0.5 mm. At this time, a center shield torch was used for the shield, and nitrogen was used as the gas. The focal position of the beam was the surface of the steel plate, and the fillet leg length W parallel to the surface of the metal plate was changed by setting the aimed position of the beam to the overlap end or 0.25 mm inside the end.
The ultrasonic vibration device had a power supply of 1 kW and a frequency of 27 kHz, the amplitude of the ultrasonic vibration terminal was 30 to 40 μm, and the impact treatment speed was 50 cm / min.
[0025]
Fatigue characteristics after the impact treatment with ultrasonic vibration terminals was measured, and the measured value was compared with that in the case where no impact treatment was performed after welding under the same conditions in advance, and the fatigue limit strength was improved by 10% or more. “OK” (good), and “NG” (bad) below. The fatigue test conditions were one-way tension with a load ratio (minimum load / maximum load) = 0.1 and a repetition rate = 10 Hz.
No. in Table 1 Reference numerals 1 to 13 are examples of the present invention. No. 1 and no. 2 is the case where the fillet leg length 5 parallel to the steel plate surface shown in FIG. 1 is 4.6 mm, 2.5 mm, and 1.8 mm, and the diameter 7 of the ultrasonic vibration terminal is 3.0 mm and 5.0 mm. Since the diameter D of the ultrasonic vibration terminal and the striking position x of the ultrasonic vibration terminal are within the range defined in the present invention, good fatigue characteristics can be obtained in both cases.
On the other hand, no. 14 to 27 are comparative examples in the case where either the diameter D of the ultrasonic vibration terminal and the striking position x of the ultrasonic vibration terminal are out of the range defined in the present invention, or the welding is not performed at all.
No. 14 and no. No. 15 is a case where the leg length W is 4.6 mm and the diameter D of the ultrasonic vibration terminal is 5.0 mm. 16 and no. No. 17 is a case where the leg length W is 1.8 mm and the diameter D of the ultrasonic vibration terminal is 3.0 mm. 18 and no. 19 is a case where the leg length W is 2.5 mm and the diameter D of the ultrasonic vibration terminal is 3.0 mm. In any case, the striking position x of the ultrasonic vibration terminal is out of the range defined in the present invention. Furthermore, the fatigue strength was low, and both of the evaluations were poor (NG).
No. In Nos. 20 and 22, the diameter D of the ultrasonic vibration terminal is 8.5 mm, which is outside the range defined by the present invention, and the striking position x of the ultrasonic vibration terminal is also outside the range defined by the present invention. Therefore, the fatigue strength was low, and both of the evaluations were poor (NG).
No. In Nos. 21 and 23, the ultrasonic vibration terminal diameter D is 8.5 mm and 10.0 mm, which are out of the range defined in the present invention. Therefore, the fatigue strength is low, and both evaluations are poor (NG). It was.
No. Nos. 24-27 are lap fillet welded joints that were not subjected to the hammering treatment by the ultrasonic vibration terminal on the weld toe and root ends, or the weld toe, and the fatigue strength was low. Both were bad (NG).
[0026]
Next, two aluminum plates having the thickness and tensile strength shown in Table 2 were overlapped, and the overlapped end portions were fillet welded by arc welding, and then subjected to a striking process using the ultrasonic vibration terminal of the present invention. Overlap fillet welding was carried out using 5083 for the aluminum plate material and 5356 for the arc welding for the filler metal. At this time, the welding current was 150 A, and the welding speed was 25 cm / min. In addition, for the purpose of comparison, after lap fillet welding, a welded lap fillet welded joint that was not subjected to the impact treatment using the ultrasonic vibration terminal of the present invention was also produced. The material, joint size, welding conditions and test method are the same as in the case of the steel plate.
No. in Table 2 31 to 36 are examples of the present invention, where the fillet leg length 5 parallel to the steel plate surface shown in FIG. 1 is 4.6 mm and the diameter 7 of the ultrasonic vibration terminal is 3.0 mm and 5.0 mm. Since the diameter D of the ultrasonic vibration terminal and the striking position x of the ultrasonic vibration terminal are within the range defined in the present invention, good fatigue characteristics can be obtained in both cases.
On the other hand, no. Reference numerals 37 to 39 are comparative examples.
No. In No. 37, the fatigue treatment was not performed by the ultrasonic vibration terminal, so the fatigue strength was low, and both of the evaluations were poor (NG).
No. 38 and no. In No. 39, since the striking position x of the ultrasonic vibration terminal was out of the range defined in the present invention, the fatigue strength was low, and both of the evaluations were poor (NG).
[Table 1]

[Table 2]

[0027]
【The invention's effect】
According to the present invention, there is provided a fatigue strength improving method in which a specific range in the vicinity of a weld toe of a lap fillet welded joint in which ends of two metal plates are welded is hit with an ultrasonic vibration terminal. As a result, the reliability of the welded structure using the metal plate can be further improved, and a remarkable industrially useful effect can be obtained.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing a striking range by an ultrasonic vibration terminal in a lap fillet welded joint according to a first embodiment of the present invention.
FIG. 2 is a schematic view showing a striking range by an ultrasonic vibration terminal in a lap fillet welded joint according to a second embodiment of the present invention.
FIG. 3 is a schematic view showing an example of a cross section of a conventional lap fillet welded joint.
FIG. 4 is a diagram illustrating a relationship between an impact position by an ultrasonic vibration terminal and a fatigue limit strength in the first embodiment of the present invention.
FIG. 5 is a view showing an example of a method for mechanically (caulking) restraining a metal plate.
FIG. 6 is a diagram illustrating an example of a method for restraining a metal plate by bending.
[Explanation of symbols]
1, 2: Metal plate,
3: Weld metal
4: Weld toe,
5: Fillet leg length (W) parallel to the metal plate surface,
6: Ultrasonic vibration terminal,
7: Ultrasonic vibration terminal diameter (D),
8: Side surface of fillet joint side of ultrasonic vibration terminal,
9: root end,
10: central axis of ultrasonic vibration terminal,
11: Restraint jig
12: Travel direction of weld bead

Claims

It is a fatigue strength improving method of hitting the vicinity of the weld toe of the lap fillet welded joint where the ends of the two metal plates overlapped with each other with an ultrasonic vibration terminal,
The ultrasonic vibration terminal has a diameter D of 2 to 8 mm,
The weld toe of the fillet welded joint is the first origin, the side surface of the ultrasonic vibration terminal on the fillet weld joint side is relative to the first origin, and x is the surface of the metal plate. A method for improving the fatigue strength of a lap fillet welded joint, characterized in that when the parallel fillet leg length is W, a range satisfying −W / 4 ≦ x ≦ W / 2 is hit with an ultrasonic vibration terminal.

The thicknesses of the two metal plates are t1 and t2, the root end portion of the overlapped metal plates is the second origin, and the relative position of the central axis of the ultrasonic vibration terminal with respect to the second origin is y. and when,
The range of satisfying −t1 ≦ y ≦ t1 for a metal plate with a thickness of t1 and / or the range of satisfying −t2 ≦ y ≦ t2 for a metal plate with a thickness of t2 is hit with an ultrasonic vibration terminal. The method for improving fatigue strength of a lap fillet welded joint according to claim 1.

3. The lap fillet welding according to claim 1, wherein before the two metal plates are welded, the metal plate is preliminarily subjected to bending processing for restraining mechanical restraint or welding deformation. Method for improving the fatigue strength of joints.

The method for improving fatigue strength of a lap fillet welded joint according to any one of claims 1 to 3, wherein the two metal plates are high-strength steel plates having a tensile strength of 400 MPa or more.

The method for improving fatigue strength of a lap fillet welded joint according to any one of claims 1 to 4, wherein the temperature of the fillet welded joint when striking with the ultrasonic vibration terminal is 300 ° C or lower.