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molybdenum wires quality and measures of technology improvements – Part I

Hi, here is TAGUTI news. Molybdenum wire is a common consumable of high speed wire cut electrical discharging machine. Actually a lot of customers can’t tell differences of molybdenum wires among so many brands. Good quality must be first choice, secondly may be the price. So how to distinguish Today I’d like to introduce you some influence factors on molybdenum wires’ quality, and of course measures of technology improvements.

Molybdenum is a rare refractory metal, which has a high melting point(2620℃) and a high density(10.2g/cm³). It has several advantages on physical and chemical properties, such as low coefficient of thermal expansion, high electrical conductivity and excellent thermal conductivity, high strength, high temperature resistance, wear resisting, corrosion resistance and so on. Molybdenum wire has been widely used in field of aerospace, chemical metallurgy, mechano-electronic, automobile making etc as an important metallic meterial. According to concrete purpose, molybdenum wire can be classified into many classes: painting Molybdenum wire, molybdenum wire for fired heater, molybdenum wire for electric light and electron device, and of course molybdenum wire for wire cut edm etc. Optimization of production technology is closely bound up with control of molybdenum wires’ quality.

* Basic technology of molybdenum wires’ production
The raw material of Molybdenum wire is molybdenum powder. Firstly the molybdenum powder has been made to square or round metallic molybdenum blank through technology of powder metallurgy. Then after rolling, rotary swaging, stretching and some other plastic processing methods with various technological means, finished wire products of diferrent specifications(generally d6.5~0.02mm, mainly d3.5~0.05mm) can be obtained.

*Requirments to molybdenum wires’ quality performance
Molybdenum wires’ quality performance reflects on a lot of aspects, such like chemical component, mechanical property, surface smoothness, piece weight, dimensional tolerance and so on. And there are different enphases according to its concrete uses.

Painting molybdenum wire is mainly used for strengthening coating of workpiece surface, such as automobile rocker arm, transmission gear and shift fork, engine pistor ring, simultaneous observation loop, brake pad, air cylinder etc. All these workpieces have specialties like thermostability, corrosion resistance, impact resistance. Comparatively speaking the painting molybdenum wire is thick. Usually there are specifications of d3.17mm, d2.3mm, d1.59mm and some others. At present the unified quality standard has not been formulated yet in China. To ensure the property of coating and avoid pollution to coating basis workpiece from impurities and meet the demands of high automatic and high precision molybdenum coating equipments, so the demands to painting molybdenum wire is strict in the actual production process: Molybdenum wire purity≥99.93%, molybdenum wire density 10.2g/cm³, molybdenum wire hardness HV ≥320; tensile strength is 700-850MPa, tensile elongation 15%; there are no flaws on the surface such as crack, burr, lapping defect, oil contamination; When the finish size of molybdenum wire is d1.59-6.5mm, its tolerance less than or equal to ±0.05mm; there must be a certain length of a single molybdenum wire, eg. d3.17mm of single molybdenum wire should be equal or greater than 20kg, and d1.50mm of single molybdenum wire should be equal or greater than 10kg.

Except painting molybdenum wire, size range of other molybdenum wires is usually d0.02-2.50mm. Among these molybdenum wires, molybdenum wire for electric light is mainly used as filament support and core wire of tungsten twisted spiral. Besides requirements of diameter uniformity, glancing flatness, strength, high temperature property, there should be toughness to avoid brittle fracture in processing and applications of winding and forming. It means this kind of molybdenum wire should possess comprehensive mechanical properties. Its ductility should achieve from 15% to 25%. Molybdenum wire for heating furnace is mainly used as heating element of industrial furnace. It is usually thicker than molybdenum wire for electric light. The most important quality index of molybdenum wire for electric light is high temperature property. Molybdenum wire for vacuum electron device(eg. Core wire of electronic gun in oscillight) and molybdenum wire as electrode used for wire cut edm machine should have high tensile ductility and strength, and allowed dimensional deviation should less than or equal to 1.0%. Because service conditions of high precision and reliability, so there are strict requirments to its quality and specification.

Analysis of major factors that influence molybdenum wires’ quality
Generally speaking, there are usually some quality problems during production and application of molybdenum wire: chemical component is unqualified, especially content of W, Ni exceeding the standard; processability is unsatisfying, molybdenum wire fracture easily(especially brittle failure appears), low tensile ductivity and strength; bad surface degree of finish, there are always metal burr and microcrack; dimensional precision is not enough, the wire is too short or single weight is too light etc. Through the analysis of these quality problems, the influencing factors could be summed up in several points: compositional characteristics, impurity content, cogging method, processing state and some other factors.

*Molybdenum wire can be divided into three classes according to molybdenum component content: pure molybdenum wire, high temperature molybdenum wire and special molybdenum wire. Purity of pure molybdenum wire should be more than or equal to 99.90%, but the rest impurities should be less than or equal to 0.10%, and each element should be less than or equal to 0.01%. High temperature molybdenum wire(doped molybdenum wire) has obvious high temperature property. Because trace amounts of elements Si, Al, K(or Ca, Mg) are added into molybdenum powder in the preparation of raw material, the forming potassium bubbles could block crystal boundary migration and dislocation motion in the process of large deformation. So that the recrystallization temperature of molybdenum wire is higher, and grain structure with large length-width ratio is relatively easily acquired, the high temperature property of molybdenum wire has been improved visibly. Special molybdenum wire has rare element La(Ti or Zr) added in raw molybdenum powder. It can not only reduce concentration of detrimental impurity in crystal structure, but also the forming La₂O₃ and some other particles present fibrous like after severe plastic deformation processing. And those particles split into small and dispersed second-phase particles with thermodynamic stability via high temperature annealing. They arrange along the direction of wire axis to intensify the wire. Therefore special molybdenum wire has specific characteristics.

However, detrimental impurity elements of molybdenum wire are easily concentrated on the grain boundary to form the internal defect. There will be crack and brittle failure in the situation of deformation and stretch under uneven stress. Like if content of interstitial element (C, N, O, P, S etc.) is relatively high, because of its specification of solid solution and sedimentation, it makes raw material strength increased, but ductility decreased. And ductile brittle transition temperature is raised, finally the processability worsens. Especially when the content of element W, Ni exceeds the standard, mechanical property becomes worse, there will be dimensional deviation, burr and crack on surface of molybdenum wire, and wire easily brokes in the conditions of uncoiling, falling and bending.

*Cogging is an important part of molybdenum wire production. There are two ways of cogging: rolling and rotary swaging. Rolling is like continuous deformation. Because of huge, deep and uniform deformation, crystalline grain could be fully broken up. It is conducive to remove or reduce internal defect, and deformed structure of raw material distributes equably along the transverse fracture surface. The surface quality and the dimensional shape have great consistency. It creates favorable conditions for subsequent process of molybdenum wire. Comparatively speaking, rotary swaging is a traditional way of cogging. The raw material should be forged with high frequency discontinuously and dispersively. Deformation of each forge is minimal and uneven at a certain extent. So that crystal grain of surface differs from the internal ones on size, shape and distributional difference. And the processability and mechanics index of raw material will decline.

There are two ways of cogging to molybdenum powder , rolling and rotary swaging. After cogging and several times of stretching, the raw material could be processed into wire with diameter 0.78mm. Then mechanical property of indoor temperature should be tested after annealing process. The rolling molybdenum wire has higher tensile strength and ductivity. Also it has less deviation. It means the rolling molybdenum wire has better comprehensive mechanical properties and quality stability. Therefore, rolling molybdenum wire has higher production efficiency, and has an obvious advantage in processing raw material of large size. In other words, the rotary swaging molybdenum wire has lower production efficiency and its processing size of raw material is also limited. So in nowadays, most molybdenum wire processing plants would like to choose rolling the molybdenum raw material to get qualified molybdenum wire. No matter choose any ways to produce molybdenum wire, they can be divided into three basic processing way: rolling, rotary swaging and stretching.

This is the first part of Analysis of influence factors on molybdenum wires’ quality and measures of technology improvements. There are something left I’m not telling you. I guess we’ll meet next time. Until then I’ll finish this topic. And you’ll know more and better about the Molybdenum wire processing and quality. Don’t forget come back.

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