Behavior of interfacial oxide during diffusion bonding of aluminum alloys and its influence on joint strength

Abstract

In order to investigate the interfacial phase formed during the diffusion bonding of the aluminum alloy and to discuss its influence on the bond strength, TEM (Transmission Electron Microscope) observations have been carried out for joint interfaces of commercial aluminum alloys: Al-Mg-Si alloy 6063, Al-Mg alloy 5005, and Al-Zn-Mg alloy 7N01. At the joint interfaces of these alloys, amorphous oxide films were observed, and they were altered gradually to crystalline oxide particles, as the bonding temperature was increased. The bonding temperature at which the amorphous oxide film disappeared lowered with increasing the Mg content. As the area occupied by the amorphous oxide film was decreased, the tensile strength of the joints of all the alloys increased significantly, suggesting that the amorphous oxide was a major factor that decreased the bond strength. At all bonding temperatures employed, the crystalline oxides at the joint interfaces of 6063 and 7N01 alloys were identified as Al2MgO4 and MgO, respectively. In contrast, at the joint interface of 5005 alloy, both the crystalline oxides were observed depending on bonding temperature T(sub w); i.e., Al2MgO4 and MgO at T(sub w) below 853 K, and Al2MgO4 at T(sub w) above 853 K. In addition, particulate intermetallic compounds much larger than the crystalline oxides were precipitated preferentially at the joint interface even compared with the grain boundary. These precipitates observed in the joints of 6063 and 7N01 alloys were identified as Mg2Si and MgZn2, respectively. Although the fracture morphology of the joint suggested that these precipitates acted as initiation sites of dimples in tensile tests, it can be considered that they have much less influence on the bond strength than the amorphous oxide film, since solution treatments for dissolving these precipitates resulted in only slight improvement in the tensile strength.

アルミニウム合金の拡散接合中に形成される界面相を調べ、その接合強度に及ぼす影響を考察するために、市販のアルミニウム合金、Al-Mg-Si合金6063、Al-Mg合金5005、およびAl-Zn-Mg合金7N01の接合面についてTEM(透過電子顕微鏡)観察を行った。これらの合金の接合面には非晶質の酸化物膜が観察された。そして、この酸化物膜は、接合温度の上昇に連れて徐々に結晶性酸化物粒に変化した。非晶質酸化物膜が消失する接合温度は、Mg含有量が増すにつれて低下した。非晶質酸化物が占める面積が減るにつれて、いずれの合金においても、引張強度は著しく向上した。これは、非晶質酸化物膜が接合強度を低下させる主因であることを示唆する。試験した温度全域において、接合面での結晶性酸化物は、6063および7N01合金で、それぞれAl2MgO4およびMgOと同定された。これに対して、5005合金の接合面では、接合温度T(sub w)に応じて、2種類の酸化物が認められた。すなわち853K以下のT(sub w)ではAl2MgO4とMgOが、853K以上のT(sub w)では、Al2MgO4が認められた。更に、結晶性酸化物よりも遥かに大きい金属間化合物が、結晶粒界よりもむしろ接合面に優先的に析出した。6063および7N01合金の接合部に認められる析出物は、それぞれMg2SiおよびMgZn2と同定された。接合部の破壊形態から、これらの析出物は、引張試験でのディンプルの発生サイトとして働くことが示唆されたが、これらの析出物は、溶液処理でこれら析出物を溶解しても、引張強度の向上は極めて僅かでしかないので、非晶質酸化物膜に比べれば、接合強度への影響は遥かに低いと考えられる。