The Orbital Welding of the CBERS (China-Brazil Earth Resources Satellite) Propulsion System

The Orbital Welding of the CBERS (China-Brazil Earth Resources Satellite) Propulsion System

Abstract

The objective of this paper is to obtain a welding procedure capable of attending the manufacturing standards for the Propulsion System of the China - Brazil Earth Resources Satellite - CBERS in terms of geometry, quality and mechanical properties of the welded joints of small diameter pure titanium tubes. The first phase of the study restricted itself to the stages of the infrastructure. Firstly, it involved the setting up of a clean room (class 10,000 Federal Standard 209B) equipped with a distribution system of high purity gases. The second phase consisted of the purchasing of equipment and the training of those involved. Finally, different welding procedures were developed and respectively qualified by means of visual inspections and x-rays, leak detection tests and uniaxial tensile tests of the welded joints. All welds were found to be adequate for use after being subjected to visual and x-ray inspection and undergoing leak detection using helium and tension tests. The best results were presented by the pulsed current welding program which indicate that this is the most appropriate method to be used in the joining of small diameter pure titanium tubes of the aforementioned satellite.

1. Introduction


Figure 1. CBERS satellite.

Down through the years, technological inventions originating from space activities have made a remarkable contribution to the development of various industrial sectors such as the mechanical manufacturing area for example. In this area welding technology has been appointed as being one of most prominent, due to the increasing demand for the use of high quality welding structures for materials with special manufacturing requirements.

This series of demands has permitted the development of a new generation of programmable power supply units. In parallel the development of special orbital weld heads has permitted the use of this kind of technology in the automation of Tungsten Inert Gas - TIG welding of tubes of various diameters and it also has been directly applied in diverse areas such as the aerospace, nuclear power, food, and pharmaceutical industries amongst others. New techniques such as the use of pulsed current synchronized to the filler wire unit and to the movement of the weld head, both of which are pulsed, permit the execution of welds in any position. The qualities of these welds adhere to the most rigorous standards.

The purpose of the present study is to make public the use of the orbital TIG process in the welding joints of small diameter tubes used in the Propulsion System of the CBERS Satellite.
Three distinct welding procedures have been developed, so as to attend the manufacturing standards in terms of quality and mechanical properties. After qualification, all the welding joints were found to be adequate for use; however, the best results which were presented by the pulsed current and the variable rotational speed welding program indicate that this is the most appropriate method to be used in the welding of tubes of the aforementioned satellite.

2. History

National space activities began in the beginning of the nineteen-seventies with the creation of Institutes devoted to studies related to space, the education of personnel and the exchange of scientific information with nations prominent in this field, particularly those dedicated to the use of satellites. The principal objective was to compensate the deficiencies of a country the size of Brazil in terms of integration and knowledge of its territory. From then on the principal objectives were to concentrate on projects involving the receiving and integration of meteorological satellites (MESA Project) remote sensing techniques using satellites and aircraft for the survey of resources (SERE Project) and finally the SACI project responsible for amplifying the country’s educational system.

During the nineteen eighties, Brazilian Institute of Space Research - INPE started to develop priority programs such as the Chinese-Brazilian Earth Resources Satellite-CBERS which is the fruit of cooperation between the Brazilian and Chinese governments and which was successfully launched from the Taiwan base on October 14 1999 by the Long March 4 rocket. Brazil was responsible for 30% of the work and cost of this project and China the rest. The integration and testing of CBERS 1 was carried out in China and that of CBERS 2 at the Laboratory of Integration and Tests (LIT/INPE), Brazil.

The CBERS satellite is composed of two modules. The first, referred to as payload, houses the optical and electronic systems used to observe the earth and collect data. The service module is responsible for the supply of energy, control, telecommunications and other operational functions of the satellite. The principal characteristics of the satellite are as follows.

Weight:    1450 kg
Power:    1100 W
Body dimensions:    1.8 x 2.0 x 2.2 m
Panel dimensions:    6.3 x2.6 m
Orbital Altitude:    778 km
Stabilization:    3 axes
Communication:    UHF and S band
Life span:    2 years

The benefits generated by the Brazilian - Chinese cooperation in manufacturing the CBERS satellite are reflected in the Brazilian participation (represented by INPE) in the construction of the International Space Station -ISS, the greatest undertaking in the world of its kind in this area, which has brought together 16 countries, under the supervision of the USA.
The CBERS 2 was successfully launched on October 21 2003 in China.

3. Orbital Welding

Orbital welding was developed in the nineteen sixties to attend the needs of the aerospace industry, in particular the manufacturing of high integrity components, such as hydraulic systems. The failures found in the welds of these parts during the flight of aircraft close to the speed of sound brought about the development of orbital welding.

From the nineteen-eighties on, orbital welding has been employed in various areas such as semiconductors, pharmaceutical, food and nuclear industries, amongst others, not only in function of the advances in quality and integrity of the welded joints, but also for productivity reasons, smooth surfaces weld bead, increase in the corrosion resistance levels, and practicality when compared to manual welding or other processes.

Orbital welding is a mechanized version of the Tungsten Inert Gas process, which is used exclusively in tubes and pipes. TIG is one of the fusion welding processes or, that is, those in which the energy applied supplies enough heat to fuse the parts. This energy is provided by an electrical arc, which is defined as being the passage of an electrical current through an ionized atmosphere between two electrodes submitted to a potential difference.


Figure 2. Principles of orbital GTAW.

One of these is the base metal (component) and the other, the so called electrode, non-consumable and made out of pure or alloyed tungsten. In general the joining of small diameter and reduced thickness tubes uses butt welds and single pass welding. In this process the tubes are kept stationary while the electrode passes along the weld joint forming the weld bead. The operation is carried out in an inert gas atmosphere which is responsible for the protection of the extremities of the electrode, the weld pool, and the heated regions of the component from atmospheric contamination.

The orbital system consists of a programmable power source, electric cables and gas hoses, a weld head and optional accessories such as remote control and refrigeration system. For manual operations the insertion of the torch and pedal, the latter being to control the amperage, are indispensable.

The power supply is a piece of programmable equipment, whose welding sequence is controlled by a microprocessor. That is to say the operation is performed automatically without the use of an operator, which guarantees the repeatability and consequently the quality of the beads. Welding skill and welding time are drastically reduced, thereby greatly reducing costs. Due to programming it is possible to adjust different levels of amperage, speed and welding time in function of the diameter and thickness of the material to be welded.

The design of the weld heads is compact and independent. They are cooled or not and are suitable for use in an elevated duty cycle and used to weld a large range of tubes. The weld head components are bipartite and due to the high degree of manufacturing precision, the electrode remains aligned with the weld joint throughout the welding cycle.


Figure 3.  Welding Laboratory


Figure 4.
AMI 207-HP Power Supply
and Model 9AF-750 Weld Head.

4. The Infrastructure Project

The initial phase of the work consists of projecting and implanting a welding laboratory which is capable of attending the manufacturing specifications of the very high purity Propulsion System of the CBERS satellite.

In this type of set up, the use of a clean room with precise humidity, temperature and particulated control is absolutely necessary. The specified values for the above mentioned parameters are achieved by installing filters and equipment which purify the air in the work environment.

The second stage involves the purchasing of imported equipment which provides continuity for the manufacturing stages. Amongst these are, the system for components final cleaning, the particle counter and lastly the orbital TIG power unit from Arc Machines Inc. for joining the Propulsion System components. The welding system is compact, portable and is accurate to plus or minus 1% in relation to the welding parameters, which permits high quality welding once the rest of the items such as the length, the angle of extremity of the electrode, preparation of the weld joints, temperature and metallurgic properties are maintained constant. Figures 2, 3 & 4 show the Welding Laboratory and some of its equipment.

The final phase involves the training of personnel in the previously mentioned manufacturing process, that is to say, the correct method of operating the equipment during the stages of cutting, facing, bending, pickling, integration, tacking and welding, inspection and testing.

5. Special Considerations For Welding Titanium And Its Alloys:


Titanium given its high reactivity to temperatures above 426oC, principally with atmospheric gases, presents difficulty in terms of welding due to the risk of contamination by these gases. Once this happens, it provokes fragility which is characterized by an excessive increase in the ultimate strength and accompanied by a reduction in ductility, toughness and corrosion resistance. Based on this, the minimum condition for any process to applied in welding titanium and its alloys is to make sure of efficient protection in the area when using temperatures equal to or above that already mentioned.

In relation to methods restricting the effect of contamination, it is important to emphasize the adoption of a chemical cleaning procedure to completely remove the oxide layer as well as the residues of organic products which have arisen from the manufacturing process.
The heat input should be kept at reduced levels, so as to diminish the extension of the heat affected zone and consequently the area protected by the inert atmosphere, thereby minimizing the contamination risks. Another characteristic related to the use of elevated heat inputs is the presence of concavity in the weld beads. If the level of concavity is serious, it could lead to the disposal of the welded component.

6. Materials and Consumables

The base metal employed is commercially pure titanium, which is imported in the form of seamless tubes, fittings (union cross, tees and spherical connection) and components (filters, valves, transducers) with a diameter and thickness equal to 6.0 ±0.1mm and 1.0 ±0.05mm respectively.

In relation to the consumables, they are as follows:
- EWCe-2 (2% cerium tungsten); diameter 1.6mm and tip geometry with an 18 degree angle.
- A cylinder of high purity argon (99.999%) for use in protection and purging.

7. Qualification

The initial phase of the experiments involves the surveying of the welding parameters accompanied by qualification. Firstly the tubes are cut into appropriate lengths, which is followed by the facing of both extremities respecting requirements in relation to perpendicularity, section roundness, deburring and bevelling of the edges. Next, argon is injected into the tubes to remove and residues left over from these operations. During the pickling the internal and external diameter of each one of the samples is verified before and after its termination to establish the correct time of pickling. Following this the tubes are placed in plastic bags and protected with high purity argon up to the time of use.

The following stage involves the surveying of parameters for the manual tacking and welding operations. Three distinct procedures have been developed for the welding of pure titanium tubes:

1. continuous current
2. pulsed current with variable speed (gradually increase the rpm through a series of levels)
3. pulsed current with decreasing amperage levels
After welding the samples are submitted to visual inspections, (calipers square, boroscope and optical comparator projection) x-rays, leak tests and mechanical uniaxial tensile tests. The non-destructive tests revealed no types of discontinuities or external or internal defects in the weld beads. In accordance with the data from both the tensile and leak tests, the samples, without exception were found to have compatible values to that required by the project, independent of the welding procedure.
Invariably the ruptures during the tensile tests occurred at the base metal. For operational purposes it was decided to apply the pulsed current with variable speed method.


Figure 5. Test coupons.

8. Manufacturing

The CBERS Propulsion System consists of 42 components, various connections and small diameter tubes to interconnect them, as to form the ultra high purity circuit lines of hydrazine. The assembly as a whole is made from pure titanium and has 126 weld joints. The process of orbital TIG welding was decided on for a variety of reasons such as quality, reliability, productivity and versatility of this kind of process in relation to manual welding and others processes.

In order to facilitate the welding operation, the system was divided into groups. Each one was assembled separately following the previously qualified manufacturing sequence and integrating them to the satellite body as the work developed.

As it is an ultra high purity system where one error, no matter how small, can lead to the loss of the whole unit, the precautions begin with the handling and stocking of parts (components, tubes, and connections). These, after detailed inspection to verify their conformity with the established specifications, are segregated in denominated quarantine areas, in which the temperature, humidity and particle levels are maintained in accordance with the adopted standards. As the work progresses, the parts are separated according to their needs and handled with special gloves so as to avoid any contamination.

The first stage consists of the bending of the tubes in accordance with the project design, the pre assembling, the cutting and facing of the ends, the precision of which is indispensable in guaranteeing the quality and consistence of the weld beads.

The next stage concentrates on the integration of the parts with the help of special clamps, so as to guarantee precision in lining up the weld joints (tube to tube, tube to connection, tube to component) in accordance with the permitted standards. The weld joints are individually inspected using a magnifying glass in order to verify the absence of gaps or lack of alignment. On approval the set up is dismantled and the parts are sent for cleaning.

One of the basic conditions for the success when welding the Propulsion System is the inexistence of contaminating material (grease, oil, dust etc.) on the external and internal surfaces of the parts. The pickling phase corresponds to the removal of these impurities as well as the oxide layer.

Next the parts are placed in plastic bags filled with argon, so as to completely eliminate any type of contamination. They are subsequently sent for final cleaning for particulate size control in a machine where isopropyl alcohol circulates.

During the final integration, the parts are positioned and kept aligned using clamps. This is followed by another examination of the weld joints. Following this the parts are tacked manually using a TIG torch so there is no necessity for the use of filler wire. The tack dimensions must be minimum, yet sufficiently resistant to keep the parts in position. Figure 5 demonstrates the tack welding operation.


Figure 6. Manual Tack Welding Operation.


Figure 7. Orbital TIG Welding Operation.

Figure 8.
Orbital TIG Welding Operation.

One of the characteristics of a ultra high purity system is the necessity of purging the inert gas from the interior of the system in tacking operations as well as welding. By means of this technique a positive pressure is created which eliminates the formation of concavity in the weld beads as well as impeding internal oxidation.

When tacking is finished the manual TIG torch is substituted by the orbital weld head along with the corresponding program for each of the operations. These changes induce the tests on samples for both procedures used. Figures 6 and 7 show two distinct welding situations.
After the welding of each group x-ray inspections of the weld beads are carried out. On the completion of these, the assembly of the threaded connections is executed and followed by local and global leak tests of the Propulsion System as a whole.

9. Conclusions

The success obtained after welding with a rejection indices of only 0.7% is attributed to the manufacturing procedures developed for the handling, cutting, facing, pickling, tacking, welding, internal purging stages, the use of standardized electrodes and samples during the welding operation. Simultaneously the use of gases with special characteristics associated with filters, rigorous environmental control, leak tests were decisive in aiding the quality of the final product. Advances in welding technology, materials and manufacturing processes contributed to the success of this work.

10. Acknowledgements

The authors wish to extend their thanks to Dr. Clovis Solano Pereira (Head of the Laboratory of Integration and Tests), Engineer Carlos de Oliveira Lino (INPE/LIT), CAST engineers Meng Song, Chen Xiang Dong, Zhang Yin Yu and Wang He Zhong and the staff of the Contamination and Leak Tests Laboratories (INPE/LIT).
The authors: José Augusto Orlowski de Garcia, Nilton Souza Dias, Gérson Luiz de Lima, Nivio Fernandes Nogueira, Wilson Donizete Bocallão Pereira.



11. Bibliography

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