METHOD OF HOT FORMING COPPER-BERYLLIUM ALLOY AND HOT FORMED PRODUCT THEREOF

Abstract: A method of hot forming beryllium-copper alloy essentially consisting of from 1.60 to 2.00% by weight of Be, from 0.2 to 0.35% by weight of Co and the balance being essentially Cu, under the conditions of working temperature of 600 - 860°C, work rate of 3.3 x 10⁻⁵ - 10 s⁻¹, and amount of work strain of at least 0.20, to produce by dynamic recrystallisation a hot formed product of an equiaxial grain structure having a uniform stable grain size.

Description: The present invention relates to a method of hot forming beryllium-copper alloy having excellent mechanical strength and reliability and a hot formed product of such an alloy. Various beryllium-copper alloys mainly consisting of beryllium and copper have become widely used as high tensile spring material, electrical conductive material, etc. Such a beryllium-copper alloy is worked mostly by hot forming, but deforming mechanisms of beryllium-copper alloy during hot working have not been clarified and in many cases the working conditions for beryllium-copper alloy have been experimentally determined. Consequently, there are problems that cracks appear during hot working and the grain formed in hot formed articles is coarse and nonuniform and as a result, the strength and the reliability of the articles are not sufficient. US-A-2 266 056 describes treatment of a copper alloy with 1.5 to 2.1% Be and optionally 0.2% or 0.3% Co at 760-816°C to obtain partial re-solution of the beta constituent, followed by hot mechanical deformation while the temperature falls to about 370°C. A reduction in area of 35 to 40% is mentioned. A heat soaking step at 780°C follows. It is an object of the present invention to overcome the aforementioned problems and to provide a method of hot forming beryllium-copper alloy having excellent reliability. The inventors have clarified the behavior of beryllium-copper alloy during hot forming to determine preferred working conditions for preventing cracks and nonuniform grains for occurring during hot working. It is another object of the present invention to provide a hot formed product of beryllium-copper alloy having excellent mechanical strength and reliability. According to a first aspect of the present invention, there is a provision of a method of hot forming beryllium-copper alloy as set out in claim 1. According to a second aspect of the present invention, there is provided a hot formed product of beryllium-copper alloy as set out in claim 3. In the present invention, beryllium-copper alloy having for example a conventional composition is hot formed under a combination of the specified conditions of the working temperature, working rate and amount of work strain to cause dynamic recrystallization to thereby obtain beryllium-copper alloy of a structure of equiaxialed grain having a uniform stable grain size. The hot forming is preferably carried out in such a range that the grain size is not varied and a stable grain size is obtained even if the amount of work strain is increased. The dynamic recrystallization mentioned above means a phenomenon that a new grain structure grows as the deformation progresses during hot working beyond an yield point and such a phenomenon is well known in certain pure metals, but has not been confirmed in alloys consisting of multiple components such as beryllium-copper alloy. The present inventors have made various experiments of hot forming beryllium-copper alloy under a variety of working conditions and have found specified working conditions for ensuring the formation of dynamic recrystallization in beryllium-copper alloy. When beryllium-copper alloy is worked under such specified working conditions, a structure of equiaxialed grain having a uniform stable grain size, which is different from a simple deformation caused by static working of grain can be fabricated to provide a hot formed product having excellent mechanical strength and reliability without occurrence of cracks during hot forming. The reasons why each of the specified conditions according to the present invention is limited will be now described. The reason why the beryllium-copper alloy comprising of from 1.60 to 2.00% by weight of Be, from 0.2 to 0.35% by weight of Co, balance Cu and unavoidable impurities, is selected is that the composition is most industrially utilizable in view of the mechanical strength, electrical conductivity and economics. The reason why the working temperature of 600∼860°C is selected is that if it is lower than 600°C, the dynamic recrystallization does not appear and the grain structure before hot working is only worked, so that the purpose of the present invention can not be attained by the hot working. While, if it is higher than 860°C, the product is molten. The reason why the working rate is limited in a range of 3.3x10⁻⁵∼10 s⁻¹ is that if it is lower than 3.3x10⁻⁵ s⁻¹, the productivity is low and unpractical and the dynamic recrystallized grain becomes coarse, while if it is higher than 10 s⁻¹, there is no time for recrystallizing and the alloy is only worked. It is noted that the working rate means an amount of deformation per one second divided by the original dimension, that is expressed by strain/second. Furthermore, the reason why the amount of work strain is at least 0.20 is that if it is less than 0.20, the dynamic recrystallization does not appear, leaving the grain structure which exists before hot working. The invention will now be described in more detail, by way of example, with reference to the accompanying drawings. Fig. 1 is a diagram illustrating effects of working temperature and working rate on grain structure when a work strain of at least 0.20 is applied; Fig. 2 is a graph showing variation of average grain size with working as well as influence of working rate thereon; and Fig. 3 is a graph showing variation of initial grain size with working. In order to confirm the specified working conditions for forming a homogeneous fine equiaxed grain, various experiments were carried out as mentioned below. Test pieces each having shouldered end portions and a parallel middle portion of 12 mm length and 3 mm width were prepared by longitudinally cutting a beryllium-copper alloy cold strip of 0.5 mm thickness having a chemical composition consisting of Be: 1.80 wt%, Co: 0.25 wt% and the balance being Cu. These test pieces were annealed to form various initial grain sizes in a range of 31∼83 µm. A high temperature tensile test was carried out for each test piece by using a high temperature tensile-quick cooling test machine, in which each test piece was heated and held for twenty minutes at 860°C in vacuum atmosphere and then cooled to an individual predetermined test temperature in the vacuum furnace and held for ten minutes. After deforming, the hot deformed structure which is frozen under hydrogen gas quick cooling was observed by an optical microscope. Thus, the specified working conditions for forming a homogeneous fine equiaxed grain structure were confirmed. Fig. 1 is a diagram illustrating effects of working temperature and working rate on the grain structure when the amount of work strain is not less than 0.20. In condition "A", that is when the working temperature is lower than 600°C or the working rate is higher than 10 s⁻¹, the structure is deformed to change to only an elongated texture. In condition "B", that is when the working rate is less than 3.3×10⁻⁵ s⁻¹, the grain structure becomes homogeneous, but is coarse and such a working rate is too slow to use practically. In condition "C", that is when the working temperature is higher than 860°C, the material melts. While, in the condition "D" according to the present invention, homogeneous fine equiaxialed grain structure can be reasonably obtained. The beryllium-copper alloy having the equiaxialed grain structure obtained under the condition "D" has excellent mechanical strength and reliability. Moreover, cracking does not occur under condition "D". It is noted that Fig. 2, is a graph showing a variation of the average grain size with working and influence of the working rate on the average grain size. It will be seen from the graph that when the amount of working strain is not less than 0.20, stable fine equiaxialed grain having a grain size not more than 50 µm can be obtained corresponding to the working rate. Furthermore, Fig. 3 is a graph showing variation of grain size from initial grain size with working. It will be seen from the graph that the grain size of deformed structure in high strain zone is uniform and stable independent of the initial grain size. Accordingly, in the present invention it is preferable to effect the hot working into the high strain zone in which even if the amount of working strain is increased the grain size does not change as shown by a horizontal line in the graph to provide uniform stable grain size. It will be understood from the above description, according to the present invention deformability and formability of beryllium-copper alloy at high temperature is greatly improved and homogeneous fine equiaxed grain structure can be obtained to improve the mechanical strength and reliability of hot worked products.

Claims: A method of hot forming beryllium-copper alloy consisting of from 1.60 to 2.00% by weight of Be, from 0.2 to 0.35% by weight of Co and the balance being Cu and unavoidable impurities, which comprises a step of hot working the beryllium-copper alloy under conditions of a working temperature in a range of 600∼860°C and an amount of work strain of at least 0.20, characterised in that said step of hot working is carried out under conditions effecting dynamic recrystallisation at a working rate in the range of 3.3 x 10⁻⁵ to 10 S⁻¹ so that an equiaxed grain structure having a uniform stable grain size results. A method according to claim 1 wherein the hot working is effected into a work strain zone in which the grain size is independent of the amount of work strain. A hot formed product of beryllium-copper alloy having a composition consisting of from 1.60 to 2.00% by weight of Be, from 0.2 to 0.35% by weight of Co and the balance being Cu and unavoidable impurities, the product having an equiaxed grain structure and a uniform stable grain size which is obtained by dynamic recrystallisation at a working temperature of 600 to 860°C, a working rate of 3.3 x 10⁻⁵ to 10 S⁻¹, and an amount of work strain of at least 0.20. A product according to claim 3 wherein said equiaxed grain structure has a grain size of not more than 50 µm.

Inventors: IWADACHI, TAKAHARU, MURAMATSU, NAOKUNI, SAKAI, TAKU
Applicants: NGK INSULATORS, LTD.
Application year: 1990
Main classification: C22F1
IPC codes: C22F1, C22C9