Welding in Space and Low-Gravity Environments

by zorro12 (student)

University Degree Engineering

Contents

1. Introduction 3

2. Discussion 3

2.1 General review of welding techniques 3

2.2 Welding in space 4

3. Conclusions 6

References

Introduction

The art of joining metals together goes back to Antiquity with the earliest examples of welding in the Bronze Age and Iron Age. For instance, in the Bronze Age, some decorative pieces and jewelry were fabricated by pressure welding lap joints together. The Middle Ages brought advances in forge welding with the development of blacksmithing techniques.

Welding is by now extensively used in a wide range of industries and relatively mastered when performed in controlled environments. Nevertheless, welding techniques have more recently been developed for applications in more unusual conditions such as underwater or in space. These mediums present additional difficulties. Underwater welding, for instance, requires either to pump away the water around the welding zone (dry underwater welding) or to work with special waterproof equipment (wet welding). In addition, the welder must be a skilled diver and must accommodate with the low-light conditions that exist in deep waters.

This study focuses on welding in vacuums and low-gravity environments such as space. The application of welding techniques in space is relatively recent. In-space welding was first attempted in 1969 by Russian cosmonauts testing numerous different welding techniques and later by the NASA in 1973 aboard Skylab. Until recently, the only joining methods used in space were mechanical fastening and adhesive bonding. However, the improvement of our actual in-space welding capability is crucial for the continuation of major projects such as the International Space Station1 or the Russian Station MIR and the future development of space exploration projects such as FLO (First Lunar Outpost) or MMM (Manned Mission to Mars).

Discussion

2.1 General review of welding techniques

It is difficult to precisely categorize welding techniques as several of them are in between two or more categories. Nonetheless, the most common welding techniques can be classified into five groups:

Electric Arc welding
Oxy-fuel welding
Resistance welding
Energy-beam welding
Solid-state welding

This is a preview of the whole essay

Discussion

2.1 General review of welding techniques

Electric Arc welding
Oxy-fuel welding
Resistance welding
Energy-beam welding
Solid-state welding

Electric arc welding uses a power supply to create an electric arc between an electrode and the main materials to be welded. The electric arc - a discharge of current flowing through the gas between the two electrodes - results in a very high temperature capable of melting the material. A shielding gas is often used in the region surrounding the weld as a protection against atmospheric gases which can reduce the quality of the work.

Oxy-fuel welding is the most common gas welding technique. The heat required to melt the metals is simply provided by combustion of acetylene with oxygen in a torch.

Resistance welding refers to the coalescence of faying surfaces where the heat is generated by the resistance of the material to an electric current. Spot welding is one of the most widely used resistance methods. The weld is made by clamping the overlapping sheets of material between two copper electrodes and passing a pulse of high current between the electrodes. To produce a continuous weld, seam welding uses rotating wheels instead of two static electrodes. These methods use energy efficiently and only generate limited deformation of the work piece.

Energy beam welding methods, namely electron beam and laser welding are relatively recent processes. Electron beam welding (EBW) is a high energy density fusion process accomplished by bombarding the joint to be welded by a beam of high-velocity, strongly-focused electrons. Large amounts of kinetic energy are instantaneously converted into thermal energy during impact and penetration causing the weld-seam surface to melt. Note that a shielding gas is not needed and that the process is more easily performed in a vacuum which prevents dispersion of the electrons. On the other hand, laser beam welding (LBW) achieves the weld through the use of a laser. Like EBW, it uses high energy density but applies the energy through a coherent optical source (typically a carbon dioxide laser). And just as for EBW, the zone affected by the heat is small - the spot size of the laser varies between 0.2 and 0.6 mm* - hence there is little distortion of the work piece. In fact, these two methods are quite similar. They both present the advantages of good penetration (which makes it possible to weld together much thicker pieces than can be achieved with other welding methods) and high welding speed. However, the equipments required are expensive to set up and, as already mentioned, require a vacuum chamber when performed on earth.

Finally, some welding techniques do not involve melting of the base materials. Indeed, solid-state welding consists of a large family of processes which produces coalescence at a temperature essentially below the melting point of the materials being joined. It includes some of the oldest techniques, like forging which is considered as the ancestor of modern welding, and some of the newest. For instance, ultrasonic welding appeared in the mid 1960’s and is being increasingly used. It relies upon high frequency vibrations and high pressure to weld two materials together. This process is used for plastics and, in general, for materials that are too delicate or small for traditional welding techniques. Another common solid-state technique is friction stir welding (FSW) which involves two axially aligned parts. One is a cylindrical-shouldered tool rotating at constant speed while the other, stationary, consists of a hydraulic cylinder which is to apply pressure to the base materials butted together in between the rotating and stationary parts. The heat generated through friction forms a solid-state butt joint. Solid-state welding has several advantages since any problems associated with cooling of the hot liquid phase are avoided. Issues such as porosity, solute redistribution and solidification cracking are of no concern to these methods.

2.2 Welding in Space

A number of experiments of welding in space have already been performed, notably aboard Skylab, Soyuz and Salyut spacecrafts. It has been found that, like on earth, there is no single perfect process which could perform all the welding operations needed. Instead, a variety of processes will be used. Most importantly, the key requirements for a welding technique to be successfully used in space have been determined. The first one is obviously to function in a microgravity environment. Secondly, the process should work both inside pressurized compartments and in the outside vacuum. Thirdly, the welds produced on all aerospace materials for different geometries must be of first quality. Fourthly, the method must equipment must be easy to use, and adapted to manual, semi-automated, teleoperated and robotic operation modes. Finally, it should operate efficiently with low power levels and minimize use of consumable materials. Three welding techniques appropriate to in-space conditions are discussed below

The welding technique which currently appears to be the most promising for in-space conditions is electron beam welding. In the early 1960s, the Paton Welding Institute (PWI) in Kiev, Ukraine, initiated the development of a universal tool (termed Versatile Hand Tool or VHT) capable of heating, welding, brazing and cutting in space. The heart of that tool was an electron beam gun. In fact, environmental vacuum greatly facilitates the use of EBW. On earth, costly vacuum chambers have to be set up especially. They limit the maximum size of the work piece and, moreover, do not achieve the same vacuum level as one finds in space. The life of the cathode in the electron beam gun is proportional to the vacuum level around it and EB guns thus last longer in space than on earth. Several other advantages are to be added to these natural incentives. First, the only equipment needed is an electron emitting filament, deflection plates and a high voltage power supply. Secondly, the beam size and position can be adjusted electronically which totally eliminates the need for moving parts. Finally, the EB gun is an efficient energy source (about 77% of the energy used is transferred to the work piece while only 8% is released into the gun). According to reference I, a power supply of 0.6 KW is sufficient to operate EBW in space (less than a hairdryer). Therefore, EBW seems to be an advantageous process to use in space. Nonetheless, two main problems have to be overcome. Electron beam striking a metal produces x-rays and the welder must hence be protected. But shielding is heavy and weight is a major concern when sending objects into space. Also, the work piece must be effectively connected to ground as the accumulation of negative charges might harm people or damage electronic devices upon discharge. Automatic or robotic solutions exist and would minimize the interaction with the operators.

It is worthwhile mentioning that LBW has been successfully tested in a micro-gravity simulated environment (parabolic flight) by the University of Alabama in Huntsville. However, this method still suffers from its huge power supply requirements and relies on precision-ground mirrors, flash lamp and rod (or gas and heat exchanger), etc. These parts are more numerous, more complex, and demand far greater precision of manufacture than those of an E-beam welder.

Secondly, the Rockwell International Corporation accomplished conclusive ground-based microgravity tests and vacuum simulations with gas tungsten arc welding (GTAW). GTAW is a form of arc welding which uses a non-consumable tungsten electrode to produce the weld. The greatest advantage of this method is that it will weld more types of metals and metal alloys than any other arc welding process. Furthermore, it is especially used for welding aerospace materials. The concentrated nature of the GTAW permits pin-point control of heat input for high quality welds and generates minimum distortion of the work piece. Secondly, there is no flux with this method; therefore, there is no slag to obscure the welder’s visibility.

Thirdly, friction stir welding could potentially lend itself to use in space. It is a versatile process which can be used in arduous environments and is suitable for the types of materials used in space construction (on earth, it is widely used for aerospace applications such as fuel tanks…etc). As mentioned in the previous section, the solid-state nature of friction stir welding results in joints with very few defects. Three other significant advantages are to be mentioned. First, no excessive heat is generated. Secondly, no toxic gases are created and, thirdly, no material incompatibility is involved. Robotic-control of the FSW process for space-based applications provides precise control of involved forces and motions to assure the consistency and quality of the process, minimizes crew time needed for performing the FSW tasks, and eliminates a great amount of risks.

References