Keywords: high purity nickel, nickel strip, nickel wire, cathode plates, trace elements in nickel, properties of nickel, applications of nickel

Transcription

1 High Purity Nickel (Ni 99.98) in form of Plate, Strip and Wire: Properties, Applications and Fabrication Table of Contents Summary...1 A. Grade and norms...2 B. Properties of high purity nickel...2 I. Lacking trace elements...3 II. Softness...3 III. Low recrystallization temperature of high purity nickel and melting point of other metals..4 IV. Function as diffusion barrier...4 V. Stable properties...4 C. Applications...5 I. Properties of high purity nickel and their applications...5 II. Applications in detail Batteries and capacitors Clad materials Welding and brazing Sensors and controls...7 D. Production of strip and wire from cathode plates...7 I. Starting material...7 II. Fabrication process...7 III. Structure of the weld zone...9 IV. Delivery program...9 Contacts Keywords: high purity nickel, nickel strip, nickel wire, cathode plates, trace elements in nickel, properties of nickel, applications of nickel Summary hpulcas 1 GmbH has developed and implemented a new process to fabricate high purity nickel. The new grade meets and surpasses the requirements for Ni 270. The key to the process is the use of high purity cathode plates instead of nickel powder. The high degree of purity results in special and reproducible properties, which are used in demanding applications ranging from temperature sensors and controls, welding and brazing products, battery and supercapacitor components, diffusion barriers in roll bonded clad metals, contacts and thermobimetals. 1 hpulcas is an acronym for high purity ultra low carbon and sulfur 1

2 A. Grade and norms Hpulcas nickel with a purity of % meets the composition requirements laid down by the norms and technical documentations as UNS N02270, Nickel 270, W.Nr , Ni 99.98, ASTM F3 Grade 4 ( Standard Specification for Nickel Strip for Electron Tubes ) and JIS H 4501:1990 ( Nickel Sheets And Strips For Electronic Tube ). However, the level of trace elements allowed by these norms is drasticcally reduced in hpulcas nickel: Trace element Ni (hpulcas) Nickel 270 ASTM F3 Grade 4 W.Nr JIS H 4501 VNiR/VNiP C wt. % < 0, S wt. % < 0, Cu wt. % < 0, Fe wt. % < 0, Mg wt. % < 0, Mn wt. % < 0, Si wt. % < 0, Ti wt. % < Table 1: Trace elements allowed by high purity nickel grades Whereas the norms assume, that this nickel grade is produced by powder metallurgy, hpulcas uses a proprietary process. B. Properties of high purity nickel Standard grades of Nickel already possess an exceptional range of properties: very good corrosion resistance, good creep resistance 2 and high temperature coefficient of electrical resistance. High purity nickel even shows substantially improved properties. These properties are: Property Unit Degree of purity Ni Ni 99.6 Ni 99.2 Trace elements - C wt. % < 0, Al wt. % < Ti wt. % < < Si wt. % < 0, Mn wt. % < Mg wt. % < Tensile Strength - soft temper Rm [MPa] hard Rm [MPa] Elongation (soft temper) % > Recrystallisation temperature C Electrical resistance µω*cm Temperature coefficient of 10-6 K +6,600 +5,300 to 6,400 +4,700 to +5,800 electrical resistance Curie temperature C 360 +/ Table 2: Properties of various grades of nickel 2 caused by the large number of twins, which hinder dislocation jumps 2

3 I. Lacking trace elements If Nickel is produced by conventional melt metallurgy, C, Al, Si and/or Ti are added to deoxidize the melt, Mn and Mg are added to globalize S. These elements are supposed to be slagged out, but remain partly in the melt. Al, Si and Ti remaining in the melt are mostly present in oxidized form, which makes these particles very hard. As a consequence, the particles don t deform at the same rate as the surrounding metal, resulting in holes, when foil is rolled or in wire breaks, when thin wire is drawn. During the annealing process, the trace elements segregate to the surface, internal layers and grain boundaries. Segregation to layer boundaries is particularly harmful in clad metals, as brittle intermetallic phases impair bond strength and support delamination. High purity nickel is nearly free from trace elements, so little, if any, segregation occurs. II. Softness High purity nickel in the annealed state is extraordinarily soft. The consequences of softness are: - heavy reduction possible between annealing steps - less frequent interim annealing necessary - if nickel is a top and /or bottom layer in clad composites, softness is a critical factor to achieve green bonding and bond strength. This is already the case if the core material is relatively hard in comparison to nickel, as is the case for titanium, stainless steel and steel. It is even more important if the core layer is a soft material as copper or aluminum. hpulcas Strength, MPa Vickers hardness Tensile strength 0,2 yield strength Vickers hardness ,3 43, , ,7 68,6 70,6 72,4 74,1 75,5 Cold work, % Figure 1: Strength and hardness of high purity nickel Even though high purity nickel made by powder metallurgy can be produced having the same degree of purity as hpulcas strip and wire, comparison of the mechanical properties shows that hpulcas nickel products made from cathodes is softer in all tempers. Table 2 represents the comparison of the range of mechanical properties, such as tensile strength (R m ), 0.2% yield strength (R 0.2 ), elongation and Vickers hardness for hpulcas high purity nickel wire, to wire produced by means of powder metallurgy. 3

4 Condition Process R 0.2 [MPa] R m [MPa] Elongation [%] Hardness [HV] hpulcas process >45 <65 Soft annealed Powder metallurgy hpulcas process >300 >450 >15 >100 Half hard Powder metallurgy hpulcas process > >5 >150 Full hard Powder metallurgy >203 Table 3: Range of typical mechanical properties of hpulcas Ni 99.98% wire and wire made by powder metallurgy III. Low recrystallization temperature of high purity nickel and melting point of other metals A low recrystallization temperature - is beneficial when nickel is annealed From 500 C upwards, nickel tends to stick, requiring strip annealing. As high purity nickel recrystallizes below 500 C, it can be bell annealed. Bell annealing is more economical than continuous annealing. - is necessary if nickel is clad to a metal with a low melting point Combinations between standard nickel grades and aluminum or magnesium can be clad, but they cannot be subsequently soft annealed due to the low melting points of the Al or Mg. The low recrystallization temperature is even more important, if the metal with a low melting temperature reacts exothermically with nickel upon melting. In addition, magnesium has a tendency to autoignite; the autoignition temperature of magnesium ribbon is approximately 473 C (746 K; 883 F). Metal Melting point Recrystallization C C Less pure Nickel High purity Nickel Magnesium 650 Aluminum 660 Table 4: Recrystallization temperature and melting points of selected metals If aluminum or magnesium are clad to high purity nickel, the composite can be soft annealed. IV. Function as diffusion barrier If clad metals are exposed to elevated temperatures, its layers may diffuse into each other. This is the case for copper and gold, especially, when the gold layer is electrolytically deposited and therefore porous. In addition, constituents of layers may migrate from one layer into another. This happens with C, if high carbon steel and low carbon steel are cladded. Nickel can be used as an interlayer hindering diffusion. V. Stable properties A substantial change in the content of trace elements as carbon, sulfur and silicon has substantial influence on the mechanical as well as electrical properties of pure nickel. 4

5 For high precision electronic applications such as sensing or regulating devices, not only certain properties, but also the stability and reproducibility of the properties are decisive. Standard nickel grades lack stable properties. By ensuring a low content of trace elements stability and reproducibility of the properties are realized. Property Unit Degree of purity Temperature coefficient of electrical resistance (between 0 and 100 C) Ni Ni 99.6 Ni K +6,600 +5,300 to 6,400 +4,700 to +5,800 Curie temperature C 360 +/ Table 5: Stability of properties C. Applications I. Properties of high purity nickel and their applications The special properties of high purity nickel are used in the following applications: Property Degree of purity (Ni-content) Value/characteristics Significance Application Ni Ni 99.2 Decreased amount of impurities Sputtering targets drastically influences the mechanical properties of pure 99.98% 99.2% metal. Segregations of impurities facilitate stress corrosion fracture and inter-granular corrosion. Carbon results in hot-shortness Deep drawn products, and increases hardness and as electrode shells electrical resistance. C-Content <20 ppm 1000 ppm S-content <2 ppm 50 ppm Si-content 0.2 ppm 1000 ppm Tensile strength, MPa Recrystallisation temperature, C Surface segregation of sulfur results into a sulfur induced breakdown of the passive film on nickel facilitating corrosion. Segregation to boundaries results into hot-shortness and reduced mechanical stress. In the presence of silica (as well as Al and Ti oxides) thin products are exposed to breaking (wire) or developing holes (strip). Oxides increase die wear. High purity nickel (HPN) can be deformed by up to 98% without intermediate annealing. Starting from about 500 C, nickel tends to sticking, when bellannealed. HPN annealing 5 Products relying on catalytically active surfaces. Foil, thin wire. Glass molds for optical quality glass. Expanded metal Deep drawn parts (as CCFL-electrodes); brazing spacer taking up compressive stress HPN can be annealed after cladding to metals with low

6 Property Curie-point, C Electrical resistance, µω*cm Value/characteristics Significance Application Ni Ni 99.2 temperatures are below the temperature, where sticking melting points as Al and Mg. starts. 360± Consistent Curie temperature HPN can be used for HPN nickel component can be reduced in size. Less Joule heating during charging and discharging of batteries. temperature sensing. current collectors in batteries, battery tabs; Litz wire for use in aggressive climate and high temperature Temperature coefficient of electrical resistance Table 6: Properties of high purity nickel, significance and use Sensors: Resistance thermometers, e.g. in E-Cigs Controls: Regulator coil in glow plugs for Diesel engines II. Applications in detail 1. Batteries and capacitors Nickel is used as - electrode substrate in hearing aid batteries and supercapacitors - anode current collector in rechargeable batteries - battery tabs 2. Clad materials Metal combination Application Ni/Al Terminal for prismatic batteries with aluminum case Examples: Thermobimetals Ni/FeNi36 In a two layer composite, the layer with the high coefficient of expansion may consist of nickel, e.g. ASTM TM22. MnCu18Ni10/Ni/FeNi36 In case of a three layer structure, an intermediate layer consisting of copper or nickel is used either to reduce the electrical resistance or to increase thermal conductivity. For interlayers of nickel see: ASTM B TM9 to TM17; DIN 1715, Tl. 1 TB 1425, 1435, 1555). Ni/Ti/Ni Brazing material for ceramics and metals CuNi25/Ni/CuZn20Ni5 Magnetic signature as a security feature in coinage Table 7: Clad metal combinations using Ni as one of the layers 3. Welding and brazing Pure nickel is used for welding in form of electrodes, wire and sheath for cored wire. 6

7 In brazing, nickel is used in form of foil, expanded metal/metal mesh and layered or clad composite. Nickel functions as solder and spacer to take up stress. 4. Sensors and controls The temperature coefficient of electrical resistance is used for resistance thermometers and the progressive limitation of current flowing through a wire subject to increasing temperature. The magnetic properties (Curie point and Villari effect) are used for temperature sensing resp. as tensile stress sensor. D. Production of strip and wire from cathode plates I. Starting material hpulcas has developed a proprietary process to manufacture high purity nickel plate, strip, wire and flatwire directly from full plate cathodes, avoiding melt metallurgy. Cathode plates are produced by an electro-winning process resulting in fully dense nickel plates with a minimum of trace elements. This method achieves an initial purity of 99.98% nickel. Cathode plates are produced by inserting a thin starter sheet in the electrolytic bath, both sides of which are receiving nickel ions in the process of the electrolytic deposition. Therefore, the cathode plate has a three layer structure, the outer layers being composed of columnar grains, which resist deformation against their axis. The surface of the as-deposited material is covered with an orange peel, dotted with occasional warts. Cathode plates are offered in various sizes, e.g. 720 mm wide, 1280 mm long, 12 to 15 mm thick. Figure 2: Full plate nickel cathode II. Fabrication process The surface defects are mechanically removed by milling. Due to the stress resulting from the electrolytic deposition process, grain recrystallization can be achieved by annealing without prior reduction. Through a proper choice of annealing parameters, the grains, during recrystallization, grow across the layer boundaries, dissolving the previous layered 7

8 structure. This is important, as otherwise the plates could delaminate or even split at the layer boundaries. The milled cathode plates are heat treated under ambient air at 1100 C in order to achieve recrystallization. They are then hot rolled at about 1000 C from 12 to 15 mm down to about 6 mm in one pass. At this fabrication step, the three-layer structure is still noticeable but the previously sharp separation of layers has disappeared. Figure 3: Hot rolling of full plate nickel cathode After cooling, the plates are leveled and the oxidized surface is removed by brushing. The plates are then cut to a rectangle of constant width by a table shear. For the production of strip, the plates are beveled, frontally welded by TIG-welding with a high purity nickel feed wire and coiled. Figure 4: Coiler at the end of the welding line Figure 5: Cold rolled and slit coil 8

9 The hot rolled coil is then cold rolled and slit by subcontractors. For producing wire, the 6 mm thick plates are cut into 6 mm wide sticks. Special emphasis has to be given in further processing to avoid edges being folded into the body, which would result in deep reaching delamination. The sticks are frontally hot or cold pressure welded. Afterwards the raw wire can be treated in the same way as hot rolled wire produced by melt metallurgy. Figure 6: Spool with 0.2 mm diameter high purity nickel wire III. Structure of the weld zone The welding zones of both strip and wire have the same chemical composition as the starting material. The difference between the structures of the welding zone, the heat affected zone and the initial material can be removed by a multi-step reduction with in-between annealing. The resulting microstructures are shown in Fig. 3. (a) (b) Figure 7: Microstructure of as-welded (a) and worked and annealed material (b) IV. Delivery program Product Plate Width Thickness Strip/Foil Width Thickness Temper 9 mm inch up to up to soft annealed - quarter hard

10 Product mm inch - full hard - mill finish Surface - mirror finish - brush finish Rod Diameter up to 4 up to Length up to 2,500 up to 98.4 Wire Diameter Flat wire Width Thickness Contacts hpulcas GmbH Frauensteiner Strasse Freiberg Germany Tel.: Fax: Angaben gemäß 37a HGB, 35a GmbHG: Geschäftsführer: Ralf Lummer; Amtsgericht Chemnitz, Reg.-Nr. HRB USt-ID-Nr. DE