Announcement

Collapse
No announcement yet.

High Frequency?

Collapse
X
  • Filter
  • Time
  • Show
Clear All
new posts

  • High Frequency?

    Why is high freq needed to tig weld alluminum? I know thuis may sound stupid but i figured it has been awhile since i asked one.

  • #2
    I guess I'm reading old texbooks that were before inverter welders and such, as they say the high frequency (high voltage) is to stabilize the arc during AC welding (typical of aluminum work), as the arc would extinguish during the crossover (zero volts) of the alternating current between one polarity and the other. The AC being required for cleaning action of oxides (during reverse polarity) and balancing of heat between work and electrode (by switching to straight polarity part time) in aluminum welding.

    Perhaps today's machines with variable frequency squarewave, minimizing the crossover time by basically switching between full positive and negative polarity, the arc is more stable, and don't require a high voltage high frequency to be superimposed on top of the welding current to maintain the arc?

    Comment


    • #3
      Well HF can be used in several ways. For a tig machine which has a sinusoidal ac waveform, you can imagine that the arc toggles between DCEP and DCEN. There is no problem with supporting the arc on DCEN, but there is a problem transitioning to the DCEP portion of the cycle. You see, aluminum and magnesium are poor electron emitters, and during DCEP this is exactly what must happen electrons emitted from the work to the tungsten. A high frequency (high voltage, low amperage) signal is superimposed upon the welding arc and it helps to transition the arc to DCEP so it does not go out. It also helps to ionize the shielding gas and provide for the conductivity needed to maintain the arc during the DCEN to DCEP transition. So for a sine-wave AC tig machine or even a square-wave machine, a continuous HF signal is needed to AC weld aluminum. Now if you have a new inverter with very sharp transitions between the DCEP and DCEN portions of the cycle (Miller calls this advanced squarewave), the transition occurs so quickly there is no time for the arc to go out. So for these machines, HF is only used to start the arc, it is not needed continuously.

      -dseman
      Last edited by dseman; 01-24-2004, 04:35 PM.

      Comment


      • #4
        High frequency is not needed for aluminium, negative polarity is.

        Aluminium is covered with oxide which prevent welding. Negative polarity removes the oxide (by electrolyse). One can weld aluminium with DC, negative polarity.

        The problem with negative polarity is that the electrode is then hotter than the workpiece and tends to melt. So people figured that AC (where the polarity is switched back and forth) was a great way to have negative polarity part of the time (to clean), but not enough to melt the electrode.

        Then with normal AC the arc stops at polarity reversal, because the current drops to zero in the middle of the reverse. This is why high frequency was added, to restart the arc.

        High frequency is actually not needed for inverters, because they reverse polarity much faster and the arc does not have the time to switch off.

        Comment


        • #5
          I have an aold dialarc 250..

          Comment


          • #6
            Originally posted by KIPPER
            I have an aold dialarc 250..
            Then that is a sine-wave ac signal and will require a continuous HF signal for welding aluminum with AC. Yes as the other poster said, you can weld aluminum with DC. In fact, you can weld it with DCEP or DCEN if you understand what is required in terms of changing tungsten sizes and shielding gases. But since you asked about AC welding of aluminum, I limited my previous posts to just that. Used HF boxes are available almost all of the time on ebay. Otherwise expect about $600 for new.

            -dseman
            Last edited by dseman; 01-24-2004, 04:38 PM.

            Comment


            • #7
              Thak you for the responses. What changes would i have to make in equipment to go w/o HF. Thanks again.

              Comment


              • #8
                Kipper,
                In all truthfulness, if you are just beginning to tig weld AL I really wouldn't recommend welding with DC. You would get better and easier results with AC-HF.
                AL can be welded with DCEP. In this process roughly 2/3rds of the heat is in the tungsten and not the work. This is why larger tungstens are used. Basically size up the tungsten by 2 diameters over what you would normally use for tig welding steel with DCEN. With DCEP you will have a lot of cleaning action and this will appear as a pronounced cleaning zone much wider than the actual weld bead. It will be very easy to scratch-start the arc (since you said w/out HF) but you need to practice this so you don't contaminate the tungsten by scratching to deep or hard. With DCEP, you will have a wider and shallower bead. You can use 100% argon for this process. The process is basically limited to 1/8" thickness max, because that woud require a 5/32 dia. tungsten. I can try and dig up a table later if you like.

                Now DCEN of Al is another animal entirely. There is no cleaning action so if you think you needed to clean the material w/acetone and stainless steel brushes for AC welding then get ready to clean even more extensively. In order to help remove the AL oxide during welding it helps if you use Helium instead of Argon as a shielding gas. This will raise the arc voltage by about 66%, which equates to a hotter arc. However, it is more difficult to maintain an arc in Helium because slight variations in arc length correspond to greater changes in arc voltage than the same changes in an Argon shield. Because it is much hotter, it is best for 1/4" or thicker, though with practice this can come down to 1/8" or so. It wil require you to feed the filler into the puddle and not really dip,dip, dip, as usual. Also, you will have to travel faster.

                All of the above is the reason I would recommend AC-HF over DC of any type for someone starting in the tig welding of AL. Unless you really love the dialarc, you may want to sell the dialarc and buy a used synchro tig welder. It will weld considerably better than trying to adapt this for a tig setup--in my opinion. You can also just rent a small tig unit from your welding shop if you just want to try AL welding. Just some food for thought. Good luck.

                -dseman
                Last edited by dseman; 01-24-2004, 09:11 PM.

                Comment


                • #9
                  thanks for the info. I am a little concerned about buying a HF box from ebay, because I am not clear on the connections for power. One person has told me to get the miller HF 252D1 box, It appears to need a 14 pin plug in which i dont have. I might just try renting one and doing that. I do have access to a little century 125 mig welder. What do you think of getting some alluminum wire for that?

                  Comment


                  • #10
                    Kipper,
                    The HF box requires 115vac for power input, leads from your welder's DCEP and DCEN lugs or studs, and the 14pin remote if you have it on your machine. If you don't have that 14pin connection you can still use the HF. The HF will always be on that is all and you won't have remote control of the amperage--you have to make the adjustment at your machine.

                    I see you are also asking about using mig for aluminum, and I searched your userid to find you've also asked about stick welding aluminum. I don't mind helping you, but it kinda sounds that you are un-decided on which way to go. If this is just an experiment for you, I'd suggest you contact the welding dealers in your area and see if they have any machines set up for aluminum work. You can then go into that shop and they can show you the process.

                    To simplify:
                    MIG for al requires new drive rolls,liner,filler metal,argon gas.
                    TIG for al requires at the very least a tig torch,filler metal, and some argon to weld with DCEP.
                    SMAW requires buying a few sticks at your supplier. You can do a search and find the answer I already provided to you on this topic.

                    Good luck with your decision!

                    -dseman

                    Comment


                    • #11
                      the responses i recieved on stick welding alluminum was that it is a bad looking band aid. The mig process i have done with a spoolmatic gun. i already have the argon and the gas kit for the little mig welder, really the argon tank is what i use to tig steel with so it is just a matter of hooking the line up to the mig welder. My concern is would the mig welder be able to push tthe wire fast enough and not get tangled?


                      To throw one more monkey wrench in to the picture I have a hobart cc/cv wire feeder i hopoe to be bringing on line with in a few days any thought on the possibility of alluminum with that?

                      Comment


                      • #12
                        get all the parts you can either for your little mig or the hobart wirefeed. teflon liner, proper drive wheels, etc. Keep the gun's cable as straight as possible and give it a try. I can't tell you that you won't have any problems because after all this is the internet and I can't see you weld.

                        This will tell you what you need to know:
                        http://www.lincolnelectric.com/knowl...compactmig.asp

                        On the topic of the hobart cc/cv feeder. Will it work? Well, yes, since al is a spray process you can use that with a cc machine. Is it optimal? Well, no. You are already using a spoolgun with some other mig, so I'd just stick with that. The spoolgun in combination with a true cv machine will give you the best results of the systems you have described so far.

                        -dseman
                        Last edited by dseman; 01-25-2004, 09:18 AM.

                        Comment


                        • #13
                          Originally posted by dseman
                          Well HF can be used in several ways. For a tig machine which has a sinusoidal ac waveform, you can imagine that the arc toggles between DCEP and DCEN. There is no problem with supporting the arc on DCEN, but there is a problem transitioning to the DCEP portion of the cycle. You see, aluminum and magnesium are poor electron emitters, and during DCEP this is exactly what must happen electrons emitted from the work to the tungsten. A high frequency (high voltage, low amperage) signal is superimposed upon the welding arc and it helps to transition the arc to DCEP so it does not go out. It also helps to ionize the shielding gas and provide for the conductivity needed to maintain the arc during the DCEN to DCEP transition. So for a sine-wave AC tig machine or even a square-wave machine, a continuous HF signal is needed to AC weld aluminum. Now if you have a new inverter with very sharp transitions between the DCEP and DCEN portions of the cycle (Miller calls this advanced squarewave), the transition occurs so quickly there is no time for the arc to go out. So for these machines, HF is only used to start the arc, it is not needed continuously.

                          -dseman
                          Wow! What a great writeup! It just clicked for me. Thank you.

                          Dang I love this board. This response should be in the back of the Miller GTAW book.

                          Also, I notice a lot of people don't realize that electrons flow from the Negative to the Positive. When your electrode is negative, electrons are leaving it. When your work is negative, electrons are travelling up to the electrode.

                          That little bit of info turns a lot of the tech talk into sense.
                          Last edited by rusted; 01-27-2004, 12:33 PM.

                          Comment


                          • #14
                            Rusted,

                            It is in the tig book. I just paraphrased it.

                            Now you know why I keep harping that the newbies should spend $25 and get the student package because it will answer 95% of the questions on this board. They spend hundreds if not thousands of dollars on equipment and yet only a few will spend an extra $25 on some excellent educational resources. Most people would rather let someone else do the thinking for them instead of learning to think for themselves. I guess it's the same with most things in life....spending more money on expensive equipment is always a better substitute than proper education,practice, and a little bit of perspiration.

                            My tirade mode is now disabled...
                            women and children are now free to roam.

                            -dseman
                            Last edited by dseman; 01-27-2004, 01:24 PM.

                            Comment


                            • #15
                              :LOL:

                              Well, I haven't had a chance to really get into the Miller GTAW book, I'm having my mind filled with the GMAW book. Wow, that book really is good. I'm seeing 90% of my questions answered, and I'm sure a re-reading will yeild further information for my 'porous' brain!

                              I agree, that Miller Student Package is the best $25 I've spent in a long time.

                              Originally posted by dseman

                              Rusted,

                              It is in the tig book. I just paraphrased it.

                              Now you know why I keep harping that the newbies should spend $25 and get the student package because it will answer 95% of the questions on this board. They spend hundreds if not thousands of dollars on equipment and yet only a few will spend an extra $25 on some excellent educational resources. Most people would rather let someone else do the thinking for them instead of learning to think for themselves. I guess it's the same with most things in life....spending more money on expensive equipment is always a better substitute than proper education,practice, and a little bit of perspiration.

                              My tirade mode is now disabled...
                              women and children are now free to roam.

                              -dseman

                              Comment

                              Working...
                              X