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Selection and Properties of Shielding Gases Used for Welding

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Language: en

00:00:00.000
you
00:00:09.170 00:00:09.180 I'm it's an honor to be here today to
00:00:12.370 00:00:12.380 take part in Tom honorary session and
00:00:17.530 00:00:17.540 the topic of the talk that i'm going to
00:00:20.990 00:00:21.000 give today has to do with shielding
00:00:22.519 00:00:22.529 gases and some of the properties that
00:00:24.679 00:00:24.689 that they have and that we can consider
00:00:27.170 00:00:27.180 when we're selecting shielding gas and
00:00:28.940 00:00:28.950 have to give Tom credit for a lot of the
00:00:31.580 00:00:31.590 thoughts in this one of the things
00:00:34.520 00:00:34.530 that's great about Thomas he's able to
00:00:36.250 00:00:36.260 kind of synthesized and simplify
00:00:40.070 00:00:40.080 information and so the thoughts for
00:00:43.520 00:00:43.530 these things came from a lot of
00:00:44.990 00:00:45.000 different resources but you can put them
00:00:46.400 00:00:46.410 together into it something that makes
00:00:48.319 00:00:48.329 sense and I'm grateful to have the
00:00:51.139 00:00:51.149 opportunity to learn that from Tom so
00:00:56.630 00:00:56.640 just as a little bit of background most
00:00:59.330 00:00:59.340 of us are probably familiar with this
00:01:00.950 00:01:00.960 but shielding gases are used extensively
00:01:04.420 00:01:04.430 for a variety of purposes the main
00:01:06.800 00:01:06.810 purpose being prevention of that
00:01:08.749 00:01:08.759 atmospheric contamination when we're
00:01:10.940 00:01:10.950 welding and this is primarily against
00:01:13.910 00:01:13.920 nitrogen so that we don't get porosity
00:01:16.039 00:01:16.049 in our wealth oxygen not quite as
00:01:19.760 00:01:19.770 harmful because sometimes oxygen
00:01:24.260 00:01:24.270 inclusions can even be used to help with
00:01:27.050 00:01:27.060 the formation of things like a sick you
00:01:28.550 00:01:28.560 lurve air I we can also control the weld
00:01:32.750 00:01:32.760 metal composition slightly with
00:01:34.340 00:01:34.350 shielding gases weld shielding history
00:01:39.499 00:01:39.509 started back in eighteen eighty or
00:01:42.469 00:01:42.479 eighteen ninety with acetylene welding
00:01:47.030 00:01:47.040 and bare wire welding acetylene welding
00:01:50.450 00:01:50.460 worked much better because it had its
00:01:52.760 00:01:52.770 own built-in shielding gas bare wire was
00:01:56.450 00:01:56.460 poor quality you can see that the top
00:01:59.450 00:01:59.460 weld up here would look like something
00:02:02.719 00:02:02.729 that was probably made when they were
00:02:04.700 00:02:04.710 trying to do bare wire welding Along
00:02:09.139 00:02:09.149 Came shielded metal arc welding which
00:02:11.150 00:02:11.160 stick welding which has a coating on the
00:02:13.160 00:02:13.170 electrode which decomposes and
00:02:14.920 00:02:14.930 and shields the molten metal from the
00:02:17.530 00:02:17.540 atmosphere and then further along in
00:02:20.080 00:02:20.090 development both gas metal arc welding
00:02:22.449 00:02:22.459 and gas tungsten arc welding were
00:02:24.339 00:02:24.349 developed which have and as a shielding
00:02:27.580 00:02:27.590 gas that's introduced externally and
00:02:30.069 00:02:30.079 some of the shielding gases that are
00:02:31.839 00:02:31.849 typically used or argon helium carbon
00:02:35.289 00:02:35.299 dioxide are the main ones and there are
00:02:37.420 00:02:37.430 a few others and mixtures of these gases
00:02:43.860 00:02:43.870 so I want to spend quite a bit of time
00:02:47.140 00:02:47.150 talking about different properties of
00:02:49.270 00:02:49.280 the arc that are affected by shielding
00:02:51.849 00:02:51.859 gases and a lot of the prior research
00:02:54.039 00:02:54.049 has focused on pure argan arcs over
00:02:58.390 00:02:58.400 water-cooled copper anodes and there's
00:03:01.629 00:03:01.639 some problems with that because they
00:03:03.459 00:03:03.469 don't quite simulate what's going on
00:03:07.319 00:03:07.329 when you're welding over metal and
00:03:09.580 00:03:09.590 you've got a molten metal and you've got
00:03:11.440 00:03:11.450 flow of gas so this talk will mostly
00:03:15.069 00:03:15.079 discuss gas tungsten arc using Oregon
00:03:18.610 00:03:18.620 argon and helium over molten steel
00:03:20.740 00:03:20.750 granos heat transfer to the weld pool is
00:03:29.909 00:03:29.919 something that's that's very important
00:03:32.229 00:03:32.239 and it's something that can be affected
00:03:34.750 00:03:34.760 by the shielding gases that we use the
00:03:37.539 00:03:37.549 conventional theory that many of us have
00:03:40.449 00:03:40.459 probably learned throughout the years
00:03:42.129 00:03:42.139 and in school is that electrical
00:03:43.479 00:03:43.489 conductivity of the shielding gas and I
00:03:46.689 00:03:46.699 am I anization potential of the
00:03:49.449 00:03:49.459 shielding gas control that heat transfer
00:03:53.110 00:03:53.120 to the weld pool it turns out that both
00:03:54.939 00:03:54.949 of these are incorrect and we'll discuss
00:03:58.179 00:03:58.189 some of the reasons for that some of the
00:04:01.479 00:04:01.489 important arc or plasma properties that
00:04:04.479 00:04:04.489 do contribute to heat transfer the weld
00:04:07.240 00:04:07.250 pool or the gas boundary layer the
00:04:09.339 00:04:09.349 thermal and voltage drop layer that
00:04:12.460 00:04:12.470 exists in the very small layer just
00:04:16.839 00:04:16.849 above the molten pool
00:04:19.249 00:04:19.259 also the thermal conductivity of the
00:04:21.900 00:04:21.910 shielding gas the electrical
00:04:23.249 00:04:23.259 conductivity of the overall welding
00:04:26.310 00:04:26.320 plasma and if and if there is a presence
00:04:29.010 00:04:29.020 of metal vapor in the arc all of these
00:04:31.350 00:04:31.360 things can have a effect on the weld on
00:04:34.590 00:04:34.600 the heat transfer to the weld pool so
00:04:39.210 00:04:39.220 one of the art properties that we often
00:04:42.360 00:04:42.370 think about and might think would have
00:04:45.570 00:04:45.580 an influence but actually doesn't is the
00:04:48.120 00:04:48.130 art column voltage it's represented by
00:04:53.460 00:04:53.470 the the red region here or this region
00:04:55.950 00:04:55.960 here that doesn't have a whole lot to do
00:05:00.810 00:05:00.820 with the transfer of heat to the anode
00:05:04.620 00:05:04.630 itself because you've got a little layer
00:05:06.779 00:05:06.789 down here where you've got both a
00:05:09.420 00:05:09.430 thermal boundary layer and an Anna
00:05:11.430 00:05:11.440 voltage drop and what's happening in
00:05:14.250 00:05:14.260 that very small layer is actually what
00:05:16.529 00:05:16.539 controls the heat transfer to the weld
00:05:19.620 00:05:19.630 pool it turns out that about about
00:05:23.129 00:05:23.139 twenty percent of the heat that gets
00:05:25.080 00:05:25.090 across that layer is from conduction of
00:05:27.930 00:05:27.940 the hot gas across that boundary layer
00:05:30.480 00:05:30.490 81st approximately eighty percent is
00:05:33.719 00:05:33.729 carried by the the current of the
00:05:35.370 00:05:35.380 electrons another property that we often
00:05:42.360 00:05:42.370 think about is our temperature our
00:05:45.570 00:05:45.580 temperature also doesn't control the
00:05:47.520 00:05:47.530 heat transfer to the to the weld pool
00:05:51.920 00:05:51.930 when you think about a welding plasma or
00:05:55.950 00:05:55.960 a welding arc it's actually a partially
00:05:58.050 00:05:58.060 ionized makeup of charged ions electron
00:06:05.250 00:06:05.260 art by ons and electrons mixed with uh
00:06:07.830 00:06:07.840 nyah nyah done ionized atoms and
00:06:11.640 00:06:11.650 molecules it's really only about fifteen
00:06:15.270 00:06:15.280 to thirty percent ionized and so the
00:06:18.960 00:06:18.970 temperature of the plasma is due to the
00:06:21.990 00:06:22.000 most easily ini species to produce that
00:06:25.740 00:06:25.750 current that's required
00:06:28.580 00:06:28.590 and so you might think when you've got
00:06:33.840 00:06:33.850 an argon helium mix of shielding gas
00:06:36.840 00:06:36.850 that they're both going to contribute to
00:06:38.879 00:06:38.889 that it turns out that the Argonaut the
00:06:41.790 00:06:41.800 argon has a it's more easily ionized and
00:06:45.600 00:06:45.610 so you really don't get any helium ions
00:06:48.150 00:06:48.160 when you've got a 50-50 mixture of argon
00:06:50.700 00:06:50.710 and helium it's all controlled by the
00:06:52.080 00:06:52.090 argon also if you do have a molten metal
00:06:56.370 00:06:56.380 pool you're going to get metal vapours
00:06:57.930 00:06:57.940 and those metal vapours are more easily
00:07:00.360 00:07:00.370 ionized than the shielding gas itself
00:07:02.640 00:07:02.650 and so that will affect it and that will
00:07:04.770 00:07:04.780 drop the temperature by a factor of 2 to
00:07:07.440 00:07:07.450 3 when you've got that molten metal in
00:07:09.600 00:07:09.610 your in your arc or that metal vapor
00:07:14.580 00:07:14.590 would be the right way to say it so this
00:07:18.450 00:07:18.460 is something called astha ha plot and
00:07:20.190 00:07:20.200 what this shows is is particle density
00:07:23.750 00:07:23.760 in in welding arcs we typically say that
00:07:28.620 00:07:28.630 the number of electrons is equal to the
00:07:31.140 00:07:31.150 number of ions and that's because if you
00:07:35.969 00:07:35.979 look at this curve here this is the
00:07:38.070 00:07:38.080 singly ion singly ionized argon curve
00:07:42.480 00:07:42.490 and the temperatures in in argon arcs
00:07:47.580 00:07:47.590 are typically from from here on down and
00:07:50.100 00:07:50.110 so it's all singly ionized atoms that
00:07:54.960 00:07:54.970 that makes up the number of the particle
00:07:59.310 00:07:59.320 density in there also if you've got any
00:08:02.550 00:08:02.560 metal vapor like I mentioned before that
00:08:05.510 00:08:05.520 also that also will lower the metal
00:08:16.320 00:08:16.330 vapor is more easily ionized and will
00:08:18.500 00:08:18.510 lower the temperature
00:08:24.430 00:08:24.440 another property of the ark is the
00:08:27.050 00:08:27.060 current electrons carry 99% of the
00:08:31.490 00:08:31.500 welding current through the arc and the
00:08:36.110 00:08:36.120 current flows from the cathode to the
00:08:39.410 00:08:39.420 anode so in typical gas tungsten arc
00:08:44.720 00:08:44.730 welding that's usually from the tungsten
00:08:47.660 00:08:47.670 to the workpiece more heat is deposited
00:08:51.980 00:08:51.990 in the anode because of that flow and
00:08:54.070 00:08:54.080 the current across the gas boundary
00:08:56.570 00:08:56.580 layer that we talked about earlier
00:08:58.700 00:08:58.710 controls the heat transfer not the
00:09:01.880 00:09:01.890 temperature another art property is
00:09:07.850 00:09:07.860 thermal conductivity the arc is an
00:09:10.850 00:09:10.860 electrically augmented flame and what
00:09:15.410 00:09:15.420 that means is that the electrons flowing
00:09:17.030 00:09:17.040 through the arc coupled with the
00:09:18.740 00:09:18.750 workpiece and that provides tenfold more
00:09:23.030 00:09:23.040 heat intensity than a flame itself or a
00:09:27.140 00:09:27.150 non transfer dark and so in the kinetic
00:09:31.190 00:09:31.200 theory of gases the molecular weight
00:09:34.210 00:09:34.220 controls the thermal conductivity it's
00:09:37.280 00:09:37.290 an inverse relationship to the square
00:09:39.260 00:09:39.270 root of the molecular weight and so
00:09:45.280 00:09:45.290 basically the higher the molecular
00:09:48.500 00:09:48.510 weight to lower the thermal conductivity
00:09:51.740 00:09:51.750 and so a 50-50 argon helium mixture will
00:09:56.750 00:09:56.760 have two times the conductivity of an
00:09:59.510 00:09:59.520 argon arc alone this is another plot of
00:10:06.260 00:10:06.270 thermal conductivity versus temperature
00:10:08.360 00:10:08.370 and you can see that in the eight to ten
00:10:14.600 00:10:14.610 thousand Kelvin temperature range which
00:10:16.790 00:10:16.800 is typical of these arcs that the helium
00:10:21.080 00:10:21.090 thermal conductivity is approximately
00:10:23.840 00:10:23.850 seven times greater
00:10:25.800 00:10:25.810 than the Argonne thermal conductivity
00:10:27.330 00:10:27.340 and this is what results in the greater
00:10:30.420 00:10:30.430 heat transfer to the work piece when
00:10:34.080 00:10:34.090 you're using helium shielding gas rather
00:10:36.120 00:10:36.130 than argon shielding gas and that and
00:10:41.460 00:10:41.470 that is not due to the ionization
00:10:43.950 00:10:43.960 potential differences of those gases as
00:10:46.019 00:10:46.029 has previously been thought I do want to
00:10:51.480 00:10:51.490 talk a little bit about ionization
00:10:52.980 00:10:52.990 potential the ionization potential by
00:10:57.810 00:10:57.820 definition is the energy necessary to
00:10:59.790 00:10:59.800 remove an electron from a neutral atom
00:11:02.390 00:11:02.400 so you're left with in an ion and
00:11:05.190 00:11:05.200 electron conventional theory that
00:11:09.600 00:11:09.610 ionization potential of the shielding
00:11:12.480 00:11:12.490 gas control the heat in or the heat
00:11:16.980 00:11:16.990 transfer for a molten weld pool is
00:11:19.530 00:11:19.540 actually incorrect because the effect of
00:11:22.590 00:11:22.600 the metal vapor that comes from the
00:11:25.620 00:11:25.630 molten weld weld pool overcomes the
00:11:27.630 00:11:27.640 ionization potential of the gas that
00:11:30.390 00:11:30.400 conventional theory is okay for pure
00:11:32.850 00:11:32.860 argan over a water-cooled copper anode
00:11:35.579 00:11:35.589 which is where much of the research has
00:11:37.620 00:11:37.630 been done and also electrical electrical
00:11:45.750 00:11:45.760 conductivity is also a function of the
00:11:48.630 00:11:48.640 shielding gas composition and the
00:11:50.760 00:11:50.770 ionization potentials that are present
00:11:56.090 00:11:56.100 metal vapours have around a 5 e.v.e
00:11:59.400 00:11:59.410 ionization potential whereas argon is 15
00:12:03.840 00:12:03.850 to 16 helium is 24 25 and the higher the
00:12:09.930 00:12:09.940 ionization potential the lower the
00:12:13.640 00:12:13.650 electrical conductivity basically and so
00:12:17.960 00:12:17.970 the lower ionization potential of an
00:12:21.329 00:12:21.339 argon art for example means that it has
00:12:24.300 00:12:24.310 a higher degree of ionization at a lower
00:12:27.300 00:12:27.310 temperature than a helium arc and so
00:12:32.280 00:12:32.290 argon arc will have a greater electrical
00:12:34.800 00:12:34.810 conductivity than
00:12:36.240 00:12:36.250 julia mark another thing that affects
00:12:41.160 00:12:41.170 the electrical conductivity as it has
00:12:42.840 00:12:42.850 many of these features is the metal
00:12:45.900 00:12:45.910 vapor in the arc if there's metal vapor
00:12:48.080 00:12:48.090 president in the arca twin crease the
00:12:50.190 00:12:50.200 electrical conductivity as well and so
00:12:52.590 00:12:52.600 you can see in this plot of electrical
00:12:55.740 00:12:55.750 conductivity versus temperature when
00:12:58.680 00:12:58.690 you've got a little bit of aluminum
00:13:00.620 00:13:00.630 vapor in your helium that the electrical
00:13:04.890 00:13:04.900 conductivity for any given temperature
00:13:06.360 00:13:06.370 is much higher the higher the amount of
00:13:09.540 00:13:09.550 metal vapor you've got prison so in
00:13:16.830 00:13:16.840 conclusion shielding gases do affect the
00:13:20.670 00:13:20.680 property of welding arcs some of the
00:13:25.610 00:13:25.620 some of the features of the arc or
00:13:29.190 00:13:29.200 properties of the arc that don't control
00:13:32.700 00:13:32.710 or that aren't controlling in the heat
00:13:34.560 00:13:34.570 transfer our temperature and voltage of
00:13:39.210 00:13:39.220 the arc some of the important factors
00:13:41.700 00:13:41.710 that do control are our current thermal
00:13:44.550 00:13:44.560 conductivity and electrical conductivity
00:13:46.380 00:13:46.390 of the plasma and so differences in
00:13:50.579 00:13:50.589 argon and helium and mixtures of those
00:13:53.190 00:13:53.200 gases can be explained the arcs that we
00:13:57.300 00:13:57.310 get from mixtures of those gases can be
00:13:59.070 00:13:59.080 explained in terms of the conductivity
00:14:01.440 00:14:01.450 differences both electrical and thermal
00:14:03.450 00:14:03.460 and finally arcs containing a metal
00:14:07.950 00:14:07.960 vapor do too actually welding over a
00:14:10.829 00:14:10.839 molten weld pool will will cause the
00:14:15.600 00:14:15.610 shielding gas to behave differently than
00:14:17.730 00:14:17.740 if it were a pure gas arc over a molten
00:14:21.240 00:14:21.250 copper anode and that's the end of my
00:14:24.390 00:14:24.400 client presentation
00:14:27.140 00:14:27.150 [Applause]
00:14:31.290 00:14:31.300

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