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Content archived on 2024-04-15

FABRICATION AND ASSESSMENT TECHNIQUES FOR AMORPHOUS METAL COMPONENTS

Objective


Amorphous alloys are applied as core materials for magnetic heads because of their favourable magnetic properties and hardness. Tests were performed under real conditions using magnetic heads mounted into recorder systems, and wearing evaluated by measuring material loss. Amorphous alloys are more resistant to abrasive wear than crystalline alloys. Investigation by scanning electron microscopy showed that the magnetic core is worn homogeneously. This is effected by fine oxide particles which are incorporated within the magnetic top layer of the tape and act like a polishing abrasive. The incorporation of hard particles within the core material increases resistance to abrasion and drastically reduces the rate of wear. This was tested by the addition of alumina particles to a crystalline core material. Cobalt and iron based alloys show relatively low wear rates and hence are of interest as core materials.

The bonding of the amorphous metal in the shape of strips to other strips, different metallic substrates, and polymer surfaces has been investigated. Experiments showed that stable bondlines can be achieved by the application of suitable polymer resins. Best results were achieved using modern epoxy resins, some of which give excellent short time shear and long time durability.

Magnetic measurements showed that, for a number of possible applications, the natural oxide layer on the strip's surface in contact with a polymer resin bondline is satisfactory for the high frequency behaviour of magnetic core samples stacked from readily shaped amorphous strips or ring shaped wound strips. For higher requirements in insulation quality, possible solutions were tested using polymer resins with ceramic powder additives for bonding, pretreatment by physical deposition of insulating ceramic layers, or chemical reaction of agents with the amorphous surface. However, all of these options degrade the achievable bondstrengths. The electrodischarge machining technique was shown t o be a reasonable method for the shaping of stacked laminations.

The kinetics of the stress relaxation process were determined for 3 typical amorphous alloys based on iron (Vitrovac 7505), iron-nickel (Vitrovac 4040) and cobalt (Vitrovac 6025). Excellent magnetostrictive properties with enormous potential for technological applications are exhibited by all the amorphous alloys investigated. The best combination of properties is found in the iron based Vitrovac 7505 alloy.

In order to overcome small imposed stresses due to differential thermal expansion when the alloys were employed in magnetometers, the stress sensitivity of the magnetostrictive characteristics of the amorphous alloys was reduced.

The use of surface crystallisation to increase the magnetostriction of Vitrovac 7505 was demonstrated. The increase in coercivity and corresponding decrease in permeability, after correction for sample geometry, to less than 10 000 is undesirable, but is acceptable for practical devices in which the self demagnetisation of the strip limits the permeability to a few thousands.

The production route of amorphous ribbons causes geometric deviations from the ideal rectangular ribbon shape to occur. The assessment of the actual ribbon geometry has been performed by the examination, under the microscope, of the surface, the profile, and the stacking factor. A special device to record the ribbon profile and evaluate characteristic parameters of the ribbon geometry was built and tested. The magnetostriction constant was also characterized by the new small angle magnetization rotation techniques.

2 different cold shaping technologies were investigated with respect to their feasibility and performance. Punching, already in use, can produce poor quality punched lamellae due to early wear damage of the tools. This was confirmed in trials using tools of cemented carbides. Titanium nitride coating of the tool improved performance, but the best results were obtained using tools made of high speed steel. In this case, the cutting edge is rounded off by plastic deformation which results in bulging out of material at the peripheral surface of punch or die, consequently reducing clearance gap width. Thereby, one of the most important requirements for the improvement of punching quality is met, namely narrow clearance gaps to reduce the burr height of punched parts. The advantages of this system are simple tool construction and inexpensive tool material.

Etching was also investigated. This procedure is well established for the photoetching of semifinished products consisting of crystalline alloys. The contour quality of the etched parts of amorphous foils is determined by the chemical composition of the foil, its thickness, and the dimensions of the parts being etched. The feasibility of etching has been confirmed and the advantage of the process for the shaping of relatively filigreed geometries has been demonstrated.

Laser cutting for the bordering of Vitrovac ribbons was investigated. Cutting speeds of up to 500 metres per minute were reached wit h a carbon dioxide laser and ribbon thickness of 25 um. By cutting ribbons of less than 5 mm, heat stresses were introduced and the foils became wavy. Mechanical problems during coiling were observed. Estimations of the cost showed that laser cutting may be more expensive than conventional cutting.
AMORPHOUS METALS EXHIBIT EXTREMELY GOOD SOFT MAGNETIC BEHAVIOUR AND AN EXCELLENT HIGH WEAR RESISTANCE. BUT PROCESSES AND OPERATIONS WITH THE AMORPHOUS STRIPS SUCH AS CUTTING, SHAPING AND BONDING ARE MORE DIFFICULT THAN WITH CONVENTIONAL MATERIALS. THE AIM OF THE PROJECT IS TO MINIMIZE THE DIFFICULTIES EITHER BY MODIFYING EXISTING TECHNIQUES OR BY DEVELOPING NEW ONES.

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VACUUMSCHMELZE GMBH
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Gruener Weg 37
63450 HANAU
Germany

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