GAS payload G-169 is discussed. G-169 contains a computer-controlled Gas Tungsten Arc Welder. The equipment design, problem analysis, and problem solutions are presented. Analysis of data gathered from other microgravity arc welding and terrestrial Gas Tungsten Arc Welding (GTAW) experiments are discussed in relation to the predicted results for the GTAW to be performed in microgravity with payload G-169

Kolkailah, Faysal A.

Muir, Arthur H., Jr.

Welcher, Blake A.

English

NASA Technical Reports Server

N87-20306
GAS TUNGSTEN ARC WELDING IN A MICROGRAVITY ENVIRONMENT
WORK DONE ON GAS PAYLOAD G-169
BY
BLAKE A. WELCHER, PAYLOAD MANAGER
FAYSAL A. KOLIC_ILAH, PROJECT ADVISOR
CAL POLY SPACE WELDING PROJECT
CALIFORNIA POLYTECHNIC STATE UNIVERSITY
SAN LUIS OBISPO, CA 93407
ARTHUR H. MUIR JR., PROGRAM MANAGER
SCIENCE CENTER
ROCKWELL INTERNATIONAL CORPORMION
THOUSAND OAKS, CA 91360
ABSTRACT
In this paper the work done on GAS payload G-169 is discussed. G-169 contains
a computer-controlled Gas Tungsten Arc Welder. The equipment design, problem ana-
lysis, and problem solutions are presented. Analysis of data gathered from other
microgravity arc welding and terrestrial Gas Tungsten Arc Welding (GTAW) experi-
ments are discussed in relation to the predicted results for the GTAW to be per-
formed in microgravity with payload G-169.
OBJECTIVE
The objective of GAS payload G-169 is to provide information on the feasibil-
ity of Gas Tungsten Arc Welding (GTAW) in a microgravity environment. A thorough
analysis of the weld produced by the payload in space will be done and compared to
terrestrial welds produced with the same apparatus. To date GTAW has not been
performed in space. The data received from G-169 should have particular signifi-
cance for the assembly of the space station power system tubing.
The material to be welded is stainless steel 316 L (low carbon). The specimen
is a tube 2.00 in. in diameter with a wall thickness of 0.0625 in. A bead on
plate weld will be performed.
EXPERIMENTAL DESCRIPTION
As many of the experimental components as possible are off the shelf, com-
mercially produced products. Using off the shelf products shortens the payload
design time and ensures reliability.
Payload G-169, shown in Fig. 1, consists of: an experimental mounting rack, a
GTAW power supply, a GTAW orbital weld head, a dc to ac power inverter, a pressure
vessel of inert gas, a sealed battery box, a battery pack and a payload controller.
EXPERII_ENTAL MOUNTING RACK
The experimental mounting rack consists of four shelves, 19.75 in. in diam-
eter. Two of the shelves are 0.250 in. thick and two are 0.125 in. thick. Six,
0.375 inch diameter rods are used for support. Tubing, 0.500 in. diameter, is
compression fit between the shelves for spacing and support. All parts of the
rack are made of 6061-T6 Aluminum.
23
https://ntrs.nasa.gov/search.jsp?R=19870010873 2020-03-20T11:20:56+00:00Z
SC37014
f BATTERY BOX
VENTING TURRET
1 J
WELDING POWER
SUPPLY
Fig. 1 Cross section of payload G-169.
GTAW POWER SUPPLY
The GTAW power supply is a modified System Technika International Nika Power
100. The Nika 100 is microprocessor based, allowing the storage and control of
welding schedules. The power supply operates on 115 Vac @ 60 Hz. The power sup-
ply receives ac current and converts it to dc current for welding. The output
characteristics of the power supply are dependent on the weld schedule to be used,
that, in turn, being dependent on the material to be welded.
GTAW ORBITAL TUBE WELD HEAD
The orbital weld head is a modified System Technika International model 6003
Orbital Tube Weld Head. The model 6003 will accommodate up to a two in. outside
diameter tube. The weld head includes a "U" style clamp, that prevents the tube
from rotating. A tungsten electrode is mounted in a gear-driven track that ro-
tates around the tube. The electrode consists of a rod of tungsten with 2% thor-
ium, 0.0625 in. in diameter. A schematic diagram of the weld head is shown in
Fig. 2. The weld head houses a chamber that is purged with an inert gas to pre-
vent oxidation of the weld. A gas line inside the tube gives gas coverage to the
underside of the weld, preventing oxidation of tne specimen.
DC TO AC POWER INVERTER
The Nika 100 requires 115 Vac @ 60 Hz for operation. Since the power source
for the payload is dc batteries, the production of ac current from the dc source
is accomplished by means of a power inverter. The inverter chosen is the SPS
1607A Static Power Inverter manufactured by K.G.S. Electronics. The inverter
receives 28 Vdc and inverts it to 115 Vac @ 60 Hz at a 1.2 to 1.6 KW output.
SEALED BATTERY BOX
The battery box is a 6061-T6 aluminum core internally coated with a noncon-
ductive, electrolyte resistant material. The bottom plate is recessed out to
24
PURGE
GAS
CHAMBER
SC37042
HEAD COVER
GEAR DRIVEN
ELECTRODE
316L
HOLDER TUBE
WELDING
ELECTRODE
Fig. 2 Schematic diagram of in-place tube weld head.
secure the batteries. A mid-shelf made of plexiglass supports the top of the bat-
teries. The top plate contains a vent that is plumbed up to the venting turret.
Along the box sides are contact points for charging, power output, and thermistor
leads. Before space flight the top and bottom plates will be sealed to the box
via an "0" ring, compression fit to prevent gas leakage.
BATTERY PACK
The battery pack consists of 37 Gates Cyclon "J" cells. The "J" cell is a
2 V, 12.5 Ah battery. The battery pack consists of 3 separate subpacks of bat-
teries all housed in the sealed box. The welding power supply requires 28 cells,
the payload heaters need 6, and the controller needs 3 cells.
INERT GAS PRESSURE VESSEL
Argon is an inert gas commonly used to protect the weld pool from oxidization
during solidification. In order to reduce the environmental parameters affecting
the weld, an inert gas flow over the molten zone is used. During the welding
process, an argon pressure bottle supplies a constant flow of gas purging the
welding area of any other gases. The bottle is a spun aluminum core wrapped with
fiberglass, manufactured by Structural Composite, Inc. A valve and regulator are
used to reduce the bottle pressure to a safe line operating pressure.
PAYLOAD CONTROLLER
The payload controller consists of a Program Logic Sensor (PLS), a timer, and
an analog to digital comparator (see Fig. 3). The PLS has two functions: one is
to activate the welder upon indication of appropriate operating temperature, and
25
SC37015
_-_ START COMMAND
I Ic° P R  °RII
° I ' " ''
DC POWER i TEMP. I 50 TEMP 0
BATTERIES _----_tTHERMISTORS_ I THERM'I'STORS I
VOLTAGELO0,C * [COMPARATORS
I REGULATOR I Jt
AC POWER I'
I INVERTERS I I NAND LOGIC I
_ OK TEMP IOR TIMER
OK TO WELD -'_ WELDER
ii WELD
k___ 32 768 KHz
Fig. 3 Payload G-169 electronics block diagram.
the other is to send a signal to the GCD (gas control decoder) to interrupt power
to the payload in the event of a thermal runaway (battery over-temperature). Upon
payload activation the timer is started. The only function of the timer is to
signal the PLS to attempt a weld start even if proper temperatures are not reached
by a preset time, so at least some sort of data may be obtained. The analog to
digital converter reads the analog signals coming in from the temperature thermis-
tors and converts them to digital signals for use by the PLS.
PROBLEM ANALYSIS
During the development of G-169 the following problem areas were identified
and addressed: (I) battery selection; (2) sealed battery box design and venting;
(3) temperature control; and (4) electromagnetic interference (EMI) production.
BATTERY SELECTION
T_.._ _t_i._.,..._Inn.wrcq_ _-^_,,_ I_,_ VGc _-_ CO :Iz a,,u--_co_ve_ iL tu u_.... puw_r [of wu]d-
ing. The initial battery pack configuration used dc battery power interfaced di-
rectly to the welder dc power input. Unfortunately, the dc to dc interface re-
quires seven different voltages for welder operation, and thus seven different
battery packs. If any one of the seven failed to produce its required power, the
payload could be inoperative. Thus, an alternate power supply approach was
required.
The second option, which was adopted in the final design, is to take the dc
battery power and run it through an inverter to produce the ac current necessary
to operate the standard Nika I00. The Nika 100, a very reliable power supply, is
allowed to distribute the power as needed. This second method is obviously sim-
pler and more reliable.
26
The testing of a battery to show that it is safe for space flight is an ardu-
ous task. Because of this a decision was made to use a battery that had already
met the requirements for space flight. A logical battery choice is one of the
Gates cells which have been used frequently in space payloads. Because of the
payload's large power requirements, the first choice was the Gates Cyclon "BC"
cell. After some research it was discovered that the "BC" cell had not yet met
the requirements for space flight. The next choice was the Gates Cyclon "J" cell.
The "j" cell is a 2 V, 12.5 Ah lead acid battery that has been proven safe for
space flight.
SEALED BATTERY BOX DESIGN AND VENTING
The payload requires 37 Gates Cyclon "J" cells for operation. Because of the
large number of cells, the potential exists for the production of explosive gases.
Thus, as a part of a safety review NASA required that the battery package be
vented out of the canister. A safe venting system requires a sealed metallic,
nonconductive electrolyte resistant battery box, battery supports, and the neces-
sary plumbing to the canister lid.
The battery box has an aluminum core with a nonconductive, electrolyte resis-
tant internal coating. The batteries have top and bottom restraint plates to
ensure stability against vibrational loading. A clearance of 0.50 in. exists
between the top of the batteries and the battery box lid.
The plumbing from the battery box lid to the canister lid consists of VCR
fittings manufactured by Kilsby Roberts Co. The VCR fittings are high pressure,
close tolerance fittings used to connect tubing. All plumbing parts are stainless
steel and are butt welded together.
TEMPERATURE CONTROL
Two temperature control problems have been identified. One problem is the
effect of below 0% temperatures on the microcircuitry. The majority of the elec-
tronic components used on G-169 meet military specification (subzero operation),
but there are a few that do not. Operation of nonmil spec chips in subzero tem-
peratures can cause failure. To prevent failure from occurring, heaters are
located around the temperature critical circuitry. Upon activation of the payload
the heaters are turned on if subzero temperatures are encountered. Once above
zero temperatures are indicated by the thermistors, a signal will be sent from the
PLS to the welder for activation.
The second problem is concerned with battery over-temperature. Because of the
large amount of current required, excessive battery temperatures could occur. To
prevent the batteries from overheating, temperature thermistors are used to sense
temperature extremes. If a thermistor indicates excessive temperature a signal is
sent to the GCD to shut down the system.
ELECTROMAGNETIC INTERFERENCE (EMI)
Electromagnetic Interference (EMI) is a high frequency electromagnetic wave.
When an electrical current is passed through a wire a magnetic field is set up.
The greater the current the greater the magnetic field. A high frequency alter-
nating magnetic field produces an electromagnetic wave.
EMI can travel through air, up electrical cables, or along any other conduc-
tive path. Any type of device capable of receiving high frequency waves can
receive EMI output. Upon receipt of EMI a current is produced. If the current
27
produced is high enough, damageto the system can occur. EMI can activate or
deactivate an idle or running system. EMI can seriously affect computers and
radio communications. The Nika 100 utilizes a high frequency starter to initiate
the arc. This starter is a major producer of EMI.
The standard GAScanister will attenuate up to 70 dB of EMI. It is strongly
suspected that the combination of the power supply and the power inverter, both
EMI producers, will exceed the 70 dB limit. EMI can be measuredby the use of a
spectrum analyzer with a calibrated antenna. EMI is effectively shielded by the
use of copper screen, metal foils and metallized elastomer gaskets. It is desir-
able to entrap EMIas close to the source as possible.
EXPERIMENTAL HYPOTHESIS
To date no data exist on Gas Tungsten Arc Welding in space. The United
States, the USSR and West Germany have done considerable research in other areas
of microgravity welding.
The exact influence that gravity has on weld effectiveness is not known. A
majority of space welding experiments show areas of commonality, but there are
areas of discrepancy. The effects of gravity can be seen in buoyancy driven con-
vection, surface tension, bead geometry, hydrostatic pressure, and removal of
trapped gases. In a microgravity environment some of these conditions have
negligible effects.
In welding, the effects of gravity are most important in determining the prop-
erties of the solidified material. Although gravity has no direct effect on grain
structure or other microstructural properties (these are determined by crystalli-
zation kinetics, which in turn is controlled by short-range intermolecular
forces), it can, however, indirectly affect solidification through its effects on
fluid motion [1]. The motion of the liquid metal is a major factor in the sta-
bility of the pool and also, through convection, affects heat transfer within the
pool thereby controlling the penetration and cross-sectional profile of the weld
fusion zone [2]. The elimination of the buoyancy driven convection may have the
effect of substantially reducing or eliminating macro- and micro-segregation from
the solidified weld [3].
Previous experiments show a correlation between a microgravity environment and
a more uniform grain size throughout the heat effected zone. This could possibly
be accounted for by the lack of buoyancy driven convection, thus creating a slower
solidification rate. The solidification rate is shown to also be a function of
the thermal conductivity of the specimen.
The lack of buoyancy driven convection could prevent the removal of entrapped
n_c k,,_k]_ "_ _^ " _ '
......... _...... field _atal. The er,trdppud yd_e_ cuu_u cause poten_iai VOldS
and an increase in the porosity in the space-welded specimen. The entrapped gases
could chemically alter the corrosion-resistant properties of the stainless steel.
The shape of the fluid weld pool in a microgravity environment is a subject of
controversy. One theory suggests that the absence of gravitational and hydrosta-
tic forces will cause the liquid pool to take on a shape that minimizes surface
energy. A minimum surface energy configuration would be spherical in shape. This
analysis suggests that the bead shape will be determined solely by surface
tension.
An alternate theory suggests that in a microgravity environment a liquid me-
dium has a greater affinity for its parent material then it would in a one "g°'
environment. This affinity could cause the fluid metal pool to be drawn back to
28
the parent material, and thus result in a thinning of the weld pool. A horizontal
cross-sectional view of the weld pool would show an hourglass shape. This theory
states that surface tension is the driving force in weld pool shape but that the
bead geometry is affected by the affinity for the parent material.
The specimen to be welded is stainless steel 316 L, a material with low ther-
mal conductivity. From the data analyzed and the fact that the specimen has a low
thermal conductivity, one is lead to believe that the space-welded specimenwill
exhibit a heat-effected zone almost entirely comprised of small grains. At the
interface of the heat-affected zone with the parent material, macro- and micro-
segregation should exist producing a grain size gradient.
This analysis suggests that the beadgeometry of the space-welded specimen
should be about the sameas its earth-based counterpart, a shape somewherebetween
what the two above stated theories suggest. G-169 utilizes an argon purge to
protect the weld from oxidation. This purge, being a gas flow, could possibly
create convective cooling, thus allowing convective heat transfer. The temper-
ature gradient produced by the convective cooling could cause a migration of any
entrapped gases to the weld surface. Overall, the weld produced on G-169 should
be a better weld in terms of strength and penetration. The physical appearance
should show a bead that is similar to its earth-based counterpart but with a
smaller, more uniform granular structure.
ACKNOWLEDGMENT
Without the significant support of the Rockwell International Science Center,
through its Independent Research and Development Program, this work would not have
been possible. Additional assistance from Rockwell's Rocketdyne and Space Sta-
tions Systems Divisions as well as from ST International is also gratefully
acknowl edged.
REFERENCES
[i] Bourgeois, S.V., "Physical Forces Influencing Skylab Experiments M551, M552,
and M553." Under contract NAS8-27015 to Lockheed Missile and Space Co.,
Huntsville, AL.
[2] Wang, Y.H. and Kou, S., "Driving Forces for Convection in Weld Pools,"
presented at 1986 International Conference on Trends in Welding Research,
Gatlinburg, TN, May 18-22, 1986.
[3] Garland, J.G., McKeown, D., "Metallographic Assessment of Skylab Experiment
M551." Proceeding of Processing and Manufacturing in Space Symposium,
Frascati, March 25-27, 1974, p. 45-64.
29


Gas tungsten arc welding in a microgravity environment: Work done on GAS payload G-169

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server