|Title of Invention||
"AN IMPROVED PROCESS FOR PREPARATION OF CU-TI ALLOYS TOOLS"
|Abstract||An improved process for the preparation of copper and titanium alloy which comprises in adding 1 to 6 wt% of titanium to oxygen free high conductivity copper and heated to a temperature of 1330 to 1370° C, pouring the melt into moulds to form ingots, homogenising the ingots and hot rolling to break the solidification microstructure, subjecting the material to the step of solution treatment as herein described , the solution treated alloys being cold worked and then subjected to an ageing treatment.|
|Full Text||FIELD OF INVENTION
The present invention relates to an improved process for the preparation of a Cu-Ti alloy tools with strength and enhanced electrical conductivity for specific application but without implying any limitation thereto, for preparation of non-sparking tools besides beneficial applications in other areas like springs, valves, connectors and contacts.
Non-sparking tools require very high strength and good electrical conductivity and are used in the areas of mining, gas and petroleum industries and explosive filling in ordnance stores to avoid likelihood of occurrence of unwarranted explosions. Unlike applications for electric conduction such as in wires and strips, the non-sparking tools require making of thicker sections so as to enable manufacture of hammers, chisels, knives, blades etc. typically used in the explosive, petroleum and mining industries. Non-sparking characteristics are obtained in a material, if there is sufficient resistance for formation of ultra-fine abraded particles during impact/sliding of the tool and adequate conductivity to allow withdrawal and dissipation of heat generated during the impact or contact.
Conventionally, Cu-Be alloys are used for making non-sparking tools. However, the manufacture and processing (hot forging and rolling, heat treatment and pickling) of the alloys containing beryllium metal poses health hazard which leads to a disease of lungs known as 'Berylliosis' which is caused due to the toxicity of beryllium metal. Recently, the use of Cu-Ti alloys is being explored extensively as a replacement of Cu-Be alloys, as Cu-Ti alloys are also non-sparking and they do not suffer from the drawback of toxicity associated with Cu-Be alloys. Besides, titanium ores are in abundance compared to those of beryllium and therefore, Cu-Ti alloys cost less than the Cu-Be alloys.
One of the processes known in the art is by Saarivirta (US patent 2,943,960) wherein Oxygen Free copper [OFHC (Oxygen Free High Conductivity) brand] and high purity Ti sponge are used and melting and casting are carried out under inert gas cover, followed by hot working and cold working. Solution treatments, cold working and aging treatments were utilized to obtain strengths of 1448 MPa while exhibiting electrical conductivity of 10-18%.
The main disadvantage of the above process is that the purity of the Cu-Ti alloy obtained by the process is comparatively less, which results in inadequate electrical conductivity.
Another disadvantage of the process is that it uses high purity copper i.e. oxygen free copper (OFHC brand) which is comparatively costly as compared to the commercial grade cathode copper used in the process of the present invention.
One more disadvantage of the process is that it uses protective atmosphere even for heating prior to hot working and solution treatments, as against "air" atmosphere used in the process of the present invention.
Another process known in the art by Ikushima et al (US Patents No. 4,566,915 and 4,599,119) relates to the preparation of Cu-Ti alloys needed for strip conductors having an average crystal grain size of 25 microns and tensile strength of about 1100 MPa.
The main disadvantage of the above process is that it does not specify the exact melting and casting procedures and alloy purity, which affect electrical conductivity.
The limitation of this process is that the Cu-Ti alloy obtained by the process is not suitable for preparation of non-sparking tools such as chisels etc., which require material of higher thickness.
OBJECTS OF PRESENT INVENTION
The primary object of the present invention is to propose an improved process for preparation of Cu-Ti alloys with Ti content preferably from 4 to 6 weight percent, which possess high strength as well as enhanced electrical conductivity specifically suitable for preparation of non-sparking tools, besides other applications.
Another object of the present invention is to propose an improved process for preparation of Cu-Ti alloys, which uses commercial grade copper and titanium.
Still another object of the present invention is to propose an improved process for preparation of Cu-Ti alloys wherein insitu refining of commercial grade copper to high purity OFHC (Oxygen Free High Conductivity) grade copper is carried out by carbon monoxide gas bubbles generated insitu during melting in the graphite crucible in a vacuum induction melting furnace.
Still another object of the present invention is to propose an improved process for preparation of Cu-Ti alloys which uses titanium extracted from the abundantly available ores as compared to conventional processes which use costly and toxic Be metal whose resources (ores) are limited.
Further object of the present invention is to propose an improved process for preparation of Cu-Ti alloys, which is trouble-free, non-toxic and does not pose health hazard during the process of preparation as compared to the conventional processes for preparation of Cu-Be alloys used for making non-sparking tools.
Still further object of the present invention is for obtaining Cu-Ti alloys having grain size of 70-80 microns.
Yet another object of the present invention is for obtaining Cu-Ti alloy capable of being worked for obtaining large size implements used in explosive, mining and petroleum industries.
Further object of the present invention is to propose an improved process, for the preparation of Cu-Ti alloys wherein Ti content could be from 1 to 6 wt%, which may be hot/cold worked for obtaining characteristics suitable for use as springs, valves, connectors and contacts.
Yet further object of the present invention is to propose an improved process for the preparation of Cu-Ti alloys, which is energy-efficient requiring solution treatment schedules for very short durations. SUMMERY OF THE PRESENT INVENTION
According to the present invention, an improved process for the preparation of copper and titanium alloy tools wherein commercial grade cathode copper, is melted in a graphite crucible of vacuum induction melting (VIM) furnance, refining the melted copper at 1230 to 1280° C, by bubbling carbon monoxide gas produced insitu during the liquid copper-graphite crucible interaction to produce oxygen free high conductivity (OFHC) copper, adding 1 to 6 wt% of titanium to oxygen free high conductivity copper and heated to a temperature of 1330 to 1370° C, pouring the melt into moulds to form ingots, homogenising the ingots and hot rolling to break the solidification microstructure, subjecting the material to the step of solution treatment as herein described , the solution treated alloys being cold worked and then subjected to an ageing treatment.
The process of the present invention involves solution treatment at temperatures in the range of 750-900° C, preferably in the range of 840 to 900° C for a period of about 2 hours only as compared to the process of the known art which require heating at 500 to 700 C for long durations extending over to about 20 hours. The process follows cold working schedules that lead to formation of deformation twins that enhance strength without decreasing the electrical conductivity compared to that of dislocation cells. During aging carried out at temperatures of 380-480° C preferably at 400-450° C, double precipitation occurs which further enhances electrical conductivity. Cu-Ti alloy obtained by the process of the present invention, has grain size of 75 microns and 1200-1400 MPa strengths, 425 Hv hardness and 25% IACS (International Annealed Copper Standard) electrical conductivity. DESCRIPTION OF PROCESS
According to the present invention, an improved process is provided for the preparation of Cu-Ti alloys with Ti content preferably from 4 to 6 wt%, having high strength and enhanced electrical conductivity specifically suitable for making of non-sparking tools, though the alloy of the present invention can also be used for other applications like springs, valves, connectors, contacts ect. The invention proposes an economical process, which utilises commercial grade cathode copper that is refined insitu by a process of vacuum induction melting and insitu generated carbon monoxide gas bubbling. The reaction between graphite crucible and molten copper generates carbon monoxide (CO) gas bubbles, which are sucked out by application of vacuum of about 10 microns thus, leading to high purity OFHC copper. This technique of preparation of OFHC copper results in removal of impurities to very low levels. Commercially pure titanium is then added to the just refined liquid copper to make high purity Cu-Ti alloys.
According to the present invention, the process of preparation of Cu-Ti alloy comprises of the following steps:-
a) Insitu preparation of OFHC copper
For insitu making of high purity OFHC grade copper, commercial purity cathode copper is pickled using dilute hydrochloric acid, washed thoroughly in water, cleaned in acetone and dried in air. This copper is melted and refined insitu in a vacuum induction melting (VIM) furnace at 1200-1250°C under a vacuum of about 10 microns (1 micron=10-3 Torr or mm of Hg) for 20 to 40 minutes. The application of vacuum aids in stirring of the melt by the escaping carbon monoxide gas bubbles which remove all the gaseous and volatile impurities, thus resulting in high purity OFHC grade copper.
b) Preparation of Cu-Ti Alloy
For making Cu-Ti alloys, commercial grade titanium is taken in the range of 1 to 6wt% for general applications and 4 to 6 wt% for non-sparking tools. The titanium is cleaned in acetone and is then added to the copper melt obtained by step (a) and the melting process is continued while the furnace temperature is raised to 1330 to 1370° C, for 20 to 40 minutes. After allowing the temperature to come down to 1200-1240° C, the liquid alloy is poured into graphite mould and allowed to cool in the mould chamber of the VIM furnace.
c) Hot forging and rolling of ingots to wrought forms
The ingots so obtained were homogenized at 800-900° C for 20-30 hours. Hot forgoing/rolling is carried out to break the solidification microstructure, after soaking at 750-900° C for 2 to 3 hours, into sizes customized for making specific non-sparking tools or strips etc,
d) Solution Treatment
Wrought material are then heated for 1.5 for 2.5 hours, preferably 2 hours at high temperature of 750 to 900° C and quenched rapidly into water to obtain uniform grain size of about 75 microns and fine scale precipitation in Cu-4.5 Ti and Cu-5.4 Ti alloys.
e) Cold working/Fabrication of specific tools
Solution treated alloys are cold worked by rolling at room temperature to the desired amounts of deformation so as to fabricate specific tools. Cold working of Cu-4.5 Ti and Cu-5.4 Ti alloys results in deformation twins which leads to enhanced strength without drastically decreasing the electrical conductivity.
0 Aging treatment
Cold worked alloys as obtained above, are aged at 380-480° C preferably at 400 to 450° C, for 1-16 hours to get double precipitation which results in further enhancement of electrical conductivity and strength.
g) Surface Cleaning
The aged tools are then cleaned using 10-20% diluted HC1 acid, washed thoroughly in water and dried with acetone for obtaining bright surface.
The process of the present invention will now be illustrated by specific examples. It is to be understood that the examples given herein are by way of illustration and are not intended to be taken restrictively to imply any limitation on the scope of the present invention.
WORKING EXAMPLES Example 1
The charge is prepared by taking 28.65 kgs of cathode copper and 1.35 kgs of commercial grade titanium metal. The copper cathodes were cut to small sizes to fit into the crucible. The cathode copper pieces were pickled using dilute hydrochloric acid, washed thoroughly in water, cleaned in acetone and dried in air. The titanium metal pieces were also cleaned in acetone. The cleaned cathode copper pieces were charged in a graphite crucible of the vacuum induction melting (VIM) furnace and melted at a vacuum of about 10 microns and refined for 30 minutes at 1250°C wherein insitu carbon monoxide gas bubbling is generated for refining the copper melt. The titanium metal pieces were then added to the liquid copper and the temperature was raised to about 1350°C. The insitu carbon monoxide gas bubbling also ensures uniform dissolution of Ti in the melt. After allowing the temperature to come down to 1200-1240°C, the liquid alloy was poured at 1220°C into a graphite mold and cast in the mould chamber of the VIM furnace (The melting practice described herein can be used to cast ingots of varying sizes weighing 30 kgs on a lab scale). The ingots were homogenized at 850°C for 24 hours and hot forged and rolled after soaking at 850°C for 2 hours, into suitable sizes. Samples from the wrought products are solution treated and quenched rapidly into water to obtain 75 micron grain size as well as fine scale precipitation in Cu-4.5Ti and Cu-5.4Ti alloys. The final properties are imparted when they are cold worked and aged at 400 to 450°C for 1-16 hours.
The rod samples were drawn to 2 mm diameter wires with intermittent solution treatments. The electrical conductivity of the alloy was determined from resistivity values computed from the experimentally measured resistance of the wires using Kelvin Bridge apparatus. A test for certification of Cu-Ti alloy for non-sparking characteristics was carried out at Central Mining Research Institute (CMRI) Dhanbad in an explosion chamber fabricated for the purpose. The Cu-Ti alloy samples on subjection to abrasive action as per specification laid vide Indian Standard specification IS: 4595-1969, with the environment inside the chamber consisting of 50% each gasoline and oxygen mixture maintained at 28°C, did not result in an explosion. CMRI therefore, issued the certification of "non-sparking" conforming to Indian Standard specification, IS 4595-1969 to the Cu-Ti alloy samples prepared by the process of this invention.
The average grain size of the order of 75 microns is obtainable after solution treatment at 750°C for Cu-1.5Ti, 800°C for Cu-2.7Ti, 860°C for Cu-4.5Ti and 880°C for Cu-5.4Ti alloy. Reduction in grain size either with intermediate annealing or with cold rolling followed by solution treatment gave hardly any incremental improvements. Further treatments carried out to reduce the grain size below 75 microns resulted in recrystallised grains of about 75 microns only even in short duration anneals. Therefore, the process of this invention enables one step solution treatment to obtain above grain size, for thicker sections required for making non-sparking tools like hammer etc.
For subsequent processing 75 microns average crystal grain size was utilized. The afore-mentioned solution heat-treated alloys when quenched in water displayed different microstructures(Fig.l). For example, in Cu-1.5Ti and Cu-2.7Ti alloys, the microstructure is that of a single phase equi-axed alpha solid solution phase revealing dislocations whereas in alloys containing 4.5 and 5.4wt% Ti, the structure consists of modulations and fine scale precipitate (ß1, Cu4Ti) respectively in the quenched state itself. In the present process this difference in microstructure is utilized to obtain distinctly different and enhanced properties during aging.
The strength of Cu-Ti alloys is increased by the presence of Ti in solution and cold working, whereas the electrical conductivity decreases due to the above phenomena. In the process of the present invention, the structure obtained after solution treatment and cold work is significantly altered which enabled improvements in electrical conductivity. The solution treated Cu-1.5Ti and Cu-2.7Ti alloys resulted in microstructure consisting of high density of dislocation cells (Fig.2) on cold working by 90%, which reduced electrical conductivity. In the case of Cu-4.5Ti and Cu-5.4Ti alloys, the presence of fine scale precipitates formed during quenching itself, results in the formation of deformation twins on cold working which while contributing to enhanced precipitation, does not reduce electrical conductivity compared to that by dislocation cells. Age hardening treatments, specifically to obtain double precipitates (Fig.3), namely ß1, Cu4Ti and ß, Cu3Ti were given to simultaneously obtain strengths as high as 1200-1350 MPa as well as electrical conductivity as high as 25% IACS, the highest levels of electrical conductivity of Cu-Ti alloys. These observations are listed in Table-1 displayed hereafter.
The aforesaid processing conditions involving deliberate utilization of prior fine scale precipitation are obtainable in Cu-Ti alloys with Ti contents ranging from 4 to 6 wt%. The properties of Cu-Ti alloys having Ti in the range of 4 to 6wt% and processed by the above said methods compare favorably with those disclosed in the prior art (US patents 2,943,960, 4,566,915 and 4,599,119) as well as Cu-2.OBe-0.6Co alloys, as illustrated in Table 2.
TABLE 1: MECHANICAL PROPERTIES AND ELECTRICAL CONDUCTIVITY OF Cu-Ti ALLOYS.
* : 0.2% offset * : Gauge length: 25mm
ST : Solution Treated PA . Peak aged
CR : Cold worked by rolling, (%).
a : Values reported for deformed alloys aged at 450°C/24h. b : Tensile strength of deformed alloys aged at 450°C/24h.
Table - 2 Comparison of the properties of Cu-Ti alloy with those of conventional Cu-2.OBe-0.5Co alloy as well as Cu-Ti alloys of prior art.
1 : Alloy of Present Invention
2 : Alloy of US patent 2,943,960
3 .Alloy of US patent 4,566,915 and 4,599,119. NR : Not reported
SCA : ST + Cold work (90%) + prak aged (400°C/I h) The time, temperature and % cold work given here correspond to Cu-Ti alloy of present invention only
* : Conductivity and tensile strength achieved when the deformed alloy was aged at 450°C/24h
** : Conductivity and tensile strength achieved when the deformed alloy was aged at 400°C/17h
1. An improved process for the preparation of copper and titanium alloy tools wherein commercial grade cathode copper, is melted in a graphite crucible of vacuum induction melting (VIM) furnance, refining the melted copper at 1230 to 1280° C, by bubbling carbon monoxide gas produced insitu during the liquid copper-graphite crucible interaction to produce oxygen free high conductivity (OFHC) copper, adding 1 to 6 wt% of titanium to oxygen free high conductivity copper and heated to a temperature of 1330 to 1370° C, pouring the melt into moulds to form ingots, homogenising the ingots and hot rolling to break the solidification microstructure, subjecting the material to the step of solution treatment as herein described , the solution treated alloys being cold worked and then subjected to an ageing treatment.
2. A process as claimed in claim 1 wherein commercial grade
titanium sponge in the range of preferably 4-6 wt% is added to
the insitu generated oxygen free high conductivity (OFHC)
copper liquid metal.
3. A process as claimed in claim 1 wherein the step of solution
treatment is carried out for 15 to 25 minutes, preferably 20
minutes to complete the alloying.
4. A process as claimed in claim 1 where the liquid alloy is poured
at 1200-1240° C, preferably at 1230° C and then allowed to cool
in the mould chamber of the VIM furnace to form alloy ingots.
5. A process as claimed in claims 1 and 2 where the alloy ingots
are homogenized at temperature of 800-900° C, preferably at
850° C for 20-30 hours, preferably 24 hours.
6. A process as claimed in claims 1 and 3 where the ingots are hot
forged and rolled at a temperature of 750- 900° C, preferably at
850° C to wrought products of desired sizes and shapes.
7. A process as claimed in claims 1 and 4 where the wrought
products are solution treated at a temperature of 750-900° C,
preferably at 860° C for 1.5 to 2.5 hours, preferably 2.0 hours
and quenched rapidly into water to obtain a grain size of 75
microns and fine scale precipitation.
8. A process as claimed in claims 1 and 5 wherein the solution
treated samples are cold worked and desired types of tools are
9. A process as claimed in claims 1 and 6 wherein the fabricated
tools/shape are aged at a temperature of 380-480° C, preferably
at 400-450° C for 1 to 16 hours, to obtain non-sparking tools
having strength of 1300 MPa and electrical conductivity of 25%
IACS via double precipitation.
10. A process as claimed in claims 1 and 7 wherein the aged
tools are cleaned by pickling in a 10-20%, preferably 15% dilute
HCI acid, washed thoroughly in water and dried with acetone,
to obtain bright surface.
11. An improved process for preparation of copper-titanium alloy
tools substantially as herein described and illustrated.
|Indian Patent Application Number||1228/DEL/1999|
|PG Journal Number||11/2008|
|Date of Filing||15-Sep-1999|
|Name of Patentee||THE CHIEF CONTROLLER, RESEARCH AND DEVELOPMENT|
|Applicant Address||MINISTRY OF DEFENCE, GOVT. OF INDIA B-341, SENA BHAWAN, DHQ-P.O. NEW DELHI-110011, INDIA.|
|PCT International Classification Number||C22C 9/00|
|PCT International Application Number||N/A|
|PCT International Filing date|