Metallic glass fluid flow during welding using self-propagating reactive multilayer foils
- PDF / 318,570 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 57 Downloads / 207 Views
MM2.3.1
Metallic glass fluid flow during welding using self-propagating reactive multilayer foils Albert J. Swiston Jr., Timothy P. Weihs, Omar M. Knio*, Todd C. Hufnagel Department of Materials Science and Engineering *Department of Mechanical Engineering The Johns Hopkins University Baltimore MD 21218 ABSTRACT We use a fluid mechanics model to analyze glass fluid flow during the welding of bulk metallic glasses with reactive multilayer foils acting as local heat sources. The resulting welded joints were shear tested, and fracture surfaces were analyzed by optical microscopy. Fracture surfaces of failed metallic glass joints show distinct regions of metal-metal veins that indicate effective metallurgical bonding. We observe a monotonic increase in the failure strength of the joints with the fraction of the joint composed of such veins. For the strongest joint tested (shear strength of 420 MPa), nearly 60% of the fracture surface is comprised of metal-metal veins. We have developed a qualitative fluid mechanics explanation of the welding process, in which shear stresses (due to pressure applied during joining) push the reactive foil from the joint interface and create the metal-metal veins. The welding process is more effective at higher joining pressure and greater foil thickness, leading to increased joint strength. INTRODUCTION Reactive multilayer foils are freestanding nanostructured multilayers comprised of alternating layers of at least two materials that have a large negative heat of mixing [1]. When some form of activation energy is applied to the foil (such as a spark or flame), the layers of the foil intermix, initiating a self-propagating reaction that travels along the foil at velocities ranging from 1-30 m/s and reaches maximum temperatures ranging from 1000-3000°C. Reactive multilayers have been used as precisely controlled local heat sources for joining similar and dissimilar components [2]. By sandwiching a foil between two components, the interfaces are heated rapidly and uniformly, effectively welding or soldering the components. In addition, because the foil is thin, the total heat released is small, making reactive foils well suited for joining temperature-sensitive components such as amorphous metals, with limited risk of crystallization. Previously we have shown the use of reactive multilayer foils to weld Zr-based bulk metallic glass [3]. We observed that the failure stress of the joints increased with increasing foil thickness and joining pressure. Fracture surfaces of failed metallic glass joints showed distinct regions of metal-metal veins, indicating effective metallurgical bonding between the two components. These veins are a result of contraction and cracking of the foil as the reaction front propagates, due to constraints from the surrounding glass components. Once cracking occurs, the viscous supercooled liquid region at each glass interface flows parallel to the bonded interface,
MM2.3.2
dragging the reacted foil out of the joint, and filling existing foil cracks to form continuou
Data Loading...
