Metal treatment – Stock – Copper base
Reexamination Certificate
2001-08-06
2004-06-15
Ip, Sikyin (Department: 1742)
Metal treatment
Stock
Copper base
C420S473000, C420S490000, C148S433000
Reexamination Certificate
active
06749699
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a silver containing copper alloy. More particularly, the inclusion of a controlled amount of silver in a copper alloy that further contains chromium, titanium and silicon results in improved resistance to stress relaxation and improved isotropic bend properties without a detrimental effect on either yield strength or electrical conductivity.
2. Description of Related Art
Copper alloys are formed into numerous products that take advantage of the high electrical conductivity and/or high thermal conductivity of the alloys. A partial list of such products includes electrical connectors, leadframes, wires, tubes, foils and powders that may be compacted into products. One type of electrical connector is a box-like structure formed by stamping a predefined shape from a copper alloy strip and then bending the stamped part to form the connector. It is necessary for the connector to have high strength and high electrical conductivity. In addition, the connector should have a minimal reduction in normal force as a function of time and temperature exposure, commonly referred to as resistance to stress relaxation.
Properties important for an electrical connector include yield strength, bend formability, resistance to stress relaxation, modulus of elasticity, ultimate tensile strength and electrical conductivity.
Target values for these properties and relative importance of the properties are dependent on the intended application of products manufactured from the subject copper alloys. The following property descriptions are generic for many intended applications, but the target values are specific for under the hood automotive applications.
The yield strength is the stress at which a material exhibits a specified deviation, typically an offset of 0.2%, from proportionality of stress and strain. This is indicative of the stress at which plastic deformation becomes dominant with respect to elastic deformation. It is desirable for copper alloys utilized as connectors to have a yield strength on the order of 80 ksi, that is approximately 550 MPa.
Stress relaxation becomes apparent when an external stress is applied to a metallic strip in service, such as when the strip is loaded after having been bent into a connector. The metal reacts by developing an equal and opposite internal stress. If the metal is held in a strained position, the internal stress will decrease as a function of both time and temperature. This phenomenon occurs because of the conversion of elastic strain in the metal to plastic, or permanent strain, by microplastic flow.
Copper based electrical connectors must maintain above a threshold contact force on a mating member for prolonged times for good electrical connection. Stress relaxation reduces the contact force to below the threshold leading to an open circuit. It is a target for a copper alloy for connector applications to maintain at least 90% of the initial stress when exposed to a temperature of 150° C. for 1000 hours and to maintain 85% of the initial stress when exposed to a temperature of 200° C. for 1000 hours.
The modulus of elasticity, also known as Young's modulus, is a measure of the rigidity or stiffness of a metal and is the ratio of stress to corresponding strain in the elastic region. Since the modulus of elasticity is a measure of the stiffness of a material, a high modulus, on the order of 150 GPa is desirable.
Bendability determines the minimum bend radius (MBR) which identifies how severe a bend may be formed in a metallic strip without fracture along an outside radius of the bend. The MBR is an important property for connectors where different shapes are to be formed with bends at various angles.
Bend formability may be expressed as, MBR/t, where t is the thickness of the metal strip. MBR/t is a ratio of the minimum radius of curvature of a mandrel about which the metallic strip can be bent without failure. The “mandrel” test is specified in ASTM (American Society for Testing and Materials) designation E290-92, entitled
Standard Test Method for Semi
-
Guided Bend Test for Ductility of Metallic Materials
, and is incorporated by reference in its entirety herein.
It is desirable for the MBR/t to be substantially isotropic, a similar value in the “good way”, bend axis perpendicular to the rolling direction of the metallic strip, as well as the “bad way”, bend axis parallel to the rolling direction of the metallic strip. It is desirable for the MBR/t to be about 0.5 or less for a 90° bend and about 1 or less for a 180° bend.
Alternatively, the bend formability for a 90° bend may be evaluated utilizing a block having a V-shaped recess and a punch with a working surface having a desired radius. In the “V-block” method, a strip of the copper alloy in the temper to be tested is disposed between the block and the punch and when the punch is driven down into the recess, the desired bend is formed in the strip.
Related to the V-block method is the 180° “form punch” method in which a punch with a cylindrical working surface is used to shape a strip of copper alloy into a 180° bend.
Both the V-block method and the form punch method are specified in ASTM designation B820-98, entitled
Standard Test Method for Bend Test for Formability of Copper Alloy Spring Material
, that is incorporated by reference in its entirety herein.
For a given metal sample, both methods give quantifiable bendability results and either method may be utilized to determine relative bendability.
The ultimate tensile strength is a ratio of the maximum load a strip withstands until failure during a tensile test expressed as a ratio of the maximum load to the cross-sectional area of the strip. It is desirable for the ultimate tensile strength to be about 85-90 ksi, that is approximately 585-620 MPa.
Electrical conductivity is expressed in % IACS (International Annealed Copper Standard) in which unalloyed copper is defined as having an electrical conductivity of 100% IACS at 20° C. It is desirable for copper alloys for high performance electrical connectors to have an electrical conductivity of at least 75% IACS. More preferably, the electrical conductivity is 80% IACS or higher.
One copper alloy that approaches the desired properties is designated by the Copper Development Association, New York, N.Y., as C18600. C18600 is an iron containing copper-chromium-zirconium alloy and is disclosed in U.S. Pat. No. 5,370,840 to Caron, et al that is incorporated by reference in its entirety herein. C18600 has a nominal composition by weight of 0.3% chromium, 0.2% zirconium, 0.5% iron, 0.2% titanium and the balance copper and inevitable impurities.
Throughout this patent application, all percentages are expressed as weight percent unless otherwise noted.
Mechanical and electrical properties of copper alloys are highly dependent on processing. If C18600 is subjected to an aging anneal, a 33% cold roll and a relief anneal, the alloy achieves as nominal properties: an electrical conductivity of 73% IACS; a yield strength of 90 ksi; a 90° MBR/t of 1.2 in the good way and 3.5 in the bad way utilizing the mandrel method(“roller bend” method); and a 20% loss in stress when subjected to 200° C. for 1000 hours.
U.S. Pat. No. 4,678,637 to Duerrschnabel et al discloses a copper alloy containing additions of chromium, titanium and silicon and is incorporated by reference in its entirety herein. This alloy, designated by the CDA as C18070, has a nominal composition of 0.28% chromium, 0.06% titanium, 0.04% silicon and the balance copper and unavoidable impurities. When processed by hot rolling, quench and cold rolling interspersed with one or two intermediate bell anneals, the alloy achieves as nominal properties: an electrical conductivity of 86% IACS; a yield strength of 72 ksi (496 MPa), a 90% MBR of 1.6t in the good way and 2.6t in the bad way; and a loss of 32% of the stress when subjected to 200° C. for 1000 hours.
DE 196 00 864 C2 by Wieland-Werke AG, discloses an alloy containing 0.1%-0.5% chromium, 0.01%-0.25% titanium, 0.01%-
Bögel Andreas
Breedis John F.
Caron Ronald N.
Kuhn Hans-Achim
Seeger Jörg
Ip Sikyin
Olin Corporation
Rosenblatt Gregory S.
Wiggin and Dana LLP
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