TIG+焊接手册.docx

《TIG+焊接手册.docx》由会员分享，可在线阅读，更多相关《TIG+焊接手册.docx（18页珍藏版）》请在淘文阁 - 分享文档赚钱的网站上搜索。

1、TIG+焊接手册 I. The GTAW (TIG) Process The necessary heat for Gas Tungsten Arc Welding (TIG) is produced by an electric arc maintained between a nonconsumable tungsten electrode and the part to be welded.The heat-affected zone, the molten metal, and the tungsten electrode are all shielded from the atmos

2、phere by a blanket of inert gas fed through the GTAW torch. Inert gas is that which is inactive, or deficient in active chemical properties. The shielding gas serves to blanket the weld and exclude the active properties in the surrounding air. It does not burn, and adds nothing to or takes anything

3、from the metal. Inert gases such as argon and helium do not chemically react or combine with other gases. They possess no odor and are transparent,permitting the welder maximum visibility of the arc. In some instances a small amount of reactive gas such as hydrogen can be added to enhance travel spe

4、eds. The GTAW process can produce temperatures of up to 35,000? F/19,426? C. The torch contributes only heat to the workpiece. If filler metal is required to make the weld, it may be added manually in the same manner as it is added in the oxyacetylene welding process. There are also a number of fill

5、er metal feeding systems available to accomplish the task Advantages of the GTAW Process The greatest advantage of the GTAW process is that it will weld more kinds of metals and metal alloys than any other arc welding process. TIG can be used to weld most steels including stainless steel, nickel all

6、oys such as Monel ?and Inconel ?, titanium, aluminum, magnesium, copper, brass, bronze, and even gold. GTAW can also weld dissimilar metals to one another such as copper to brass and stainless to mild steel. Concentrated Arc The concentrated nature of the GTAW arc permits pin point control of heat i

7、nput to the workpiece resulting in a narrow heat-affected zone. A high concentration of heat is an advantage when welding metals with high heat conductivity such as aluminum and copper. A narrow heat-affected zone is an advantage because this is where the base metal has undergone a change due to the

8、 superheating of the arc and fast cooling rate. The heat-affected zone is where the welded joint is weakest and is the area along the edge of a properly made weld that would be expected to break under a destructive test. Figure 2.2A simple welding circuit showing voltage source and current flow. Fig

9、ure 2.2 shows what a welding circuit using a battery as a power source would look like. The two most basic parameters we deal with in welding are the amount of current in the circuit, and the amount of voltage pushing it. Current and voltage are further defined as follows: CurrentThe number of elect

10、rons flowing past a given point in one second. Measured in amperes (amps). VoltageThe amount of pressure induced in the circuit to produce current flow. Measured in voltage (volts). Resistance in the welding circuit is represented mostly by the welding arc and to a lesser extent by the natural resis

11、tance of the cables, connections, and other internal components. Chapters could be written on the theory of current flow in an electrical circuit, but for the sake of simplicity just remember that current flow is from negative to positive. Early researchers were surprised at the results obtained whe

12、n the battery leads were switched. Well examine these differences in more detail later in the section when we discuss welding with alternating current. Even after alternating current (AC) became available for welding with the use of transformer power sources, welds produced were more difficult to ac

13、complish and of lesser quality than those produced with direct current (DC). Although these AC transformer power sources greatly expanded the use of com-mercial power for SMAW (Stick), they could not be used for GTAW because as the current approached the zero value, the arc would go out. (see Figure

14、 2.4). Motor generators followed quickly. These were machines that consisted of an AC motor, that turned a generator, that produced DC for welding. The output of these machines could be used for both SMAW and GTAW. It was with a motor generator power source that GTAW was first accomplished in 1942 b

15、y V.H. Pavlecka and Russ Meredith while working for the Northrup Aviation Company. Pavlecka and Meredith were searching for a means to join magnesium, aluminum and nickel, which were coming into Figure 2.3The original torch and some of the tips used by Pavlecka and Meredith to produce the first GTAW

16、 welds in 1942. Note the torch still holds one of the original tungstens used in those experiments. Although the selenium rectifier had been around for some time, it was the early 1950s when rectifiers capable of handling current levels found in the welding circuit came about. The selenium rectifier

17、 had a profound effect on the welding industry. It allowed AC transformer power sources to produce DC. And it meant that an AC power source could now be used for GTAW welding as well as Stick welding. The realization is that high frequency added to the weld circuit would make AC power usable for TIG

18、 welding. The addition of this voltage to the circuit keeps the arc established as the weld power passes through zero. Thus stabilizing the GTAW arc, it also aids in arc starting without the risk of contamination. The later addition of remote current control, remote contactor control, and gas soleno

19、id control devices evolved into the modern GTAW power source. Further advances such as Squarewave, and Advanced Squarewave power sources have further refined the capabilities of this already versatile process. Alternating Current Alternating current (AC) is an electrical current that has both positi

20、ve and negative half-cycles. These components do not occur simultaneously, but alternately, thus the term alternating current. Current flows in one direction during one half of the cycle and reverses direction for the other half cycle. The half cycles are called the positive half and the negative ha

21、lf of the complete AC cycle. Frequency The rate at which alternating current makes a complete cycle of reversals is termed frequency. Electrical power in the United States is delivered as 60 cycles per second frequency, or to use its proper term 60 hertz (Hz). This means there are 120 reversals of c

22、urrent flow directions per second. The power input to an AC welding machine and other electrical equipment in the United States today is 60 Hz power. Outside of North America and the United States, 50 Hz power is more commonly used. As this frequency goes up, the magnetic effects accelerate and beco

23、me more efficient for use in trans- formers, motors and other electrical devices. This is the advancements in design, the positive and negative amplitude of the waveform can be controlled independently as well as the ability to change the number of cycles per second. Alternating current is made up o

24、f direct current electrode negative (DCEN) and direct current electrode positive (DCEP). To better understand all the implications this has on AC TIG welding, lets take a closer look at DCEN and DCEP. Direct Current Direct current (DC) is an electrical current that flows in one direction only. Direc

25、t current can be compared to water flowing through a pipe in one direction. Most welding power sources are capable of welding with direct current output. They accomplish this with internal circuitry that changes or rectifies the AC into DC. Figure 2.8 shows what one cycle of AC sine wave power would

26、 look like and what it would look like after it has been rectified into DC power. Figure 2.8Single-phase ACsingle-phase direct current (rectified AC). Polarity Earlier in this section it was stated how the earliest welders used batteries for their welding power sources. These early welders found the

27、re were profound differences in the welding arc and the resulting weld beads when they changed the battery connections. This polarity is best described by what electrical charge the electrode is connected for, such as direct current electrode negative (DCEN) or direct current electrode positive (DCE

28、P). The workpiece would obviously be connected to the opposite electrical charge in order to complete the circuit. Review Figure 2.2. When GTAW welding, the welder has three choices of welding current type and polarity. They are: direct current electrode negative, direct current electrode positive a

29、nd alternating current. Alternating current, as we are beginning to under-stand, is actually a combination of both electrode negative and electrode positive polarity. Each of these current types has its applications, its advantages, and its disadvantages. A look at each type and its uses will help t

30、he welder select the best current type for the job. Figures 2.9 and 2.11 illustrate power supply connections for each current type in a typical Direct Current Electrode Negative Figure 2.9Direct current electrode negative. Direct current electrode negative is used for TIG welding of practically all

31、metals. The torch is connected to the negative terminal of the power source and the work lead is connected to the positive terminal. Power sources with polarity switches will have the output terminals marked electrode and work. Internally, when the polarity switch is set for DCEN, this will be the c

32、onnection. When the arc is established, electron flow is from the negative electrode to the positive workpiece. In a DCEN arc, approximately 70% of the heat will be concentrated at the positive side of the arc and the greatest amount of heat is distributed into the workpiece. This accounts for the d

33、eep penetration obtained when using DCEN for GTAW. The elec-trode receives a smaller portion of the heat energy and will operate at a lower temperature than when using alternating current or direct current electrode positive polarity. This accounts for the higher current carrying capacity of a given

34、 size tungsten electrode with DCEN than with DCEP or AC. At the same time the electrons are striking the work, the positively charged gas ions are attracted toward the negative electrode. Figure 2.10GTAW with DCEN produces deep penetration because it concentrates the heat in the joint area. No clean

35、ing action occurs with this polarity. The heat generated by the arc using this polarity occurs in the workpiece, thus a smaller electrode can be used as well as a smaller gas cup and reduced gas flow. The more concentrated arc allows for faster travel speeds. Welding with Alternating Current When us

36、ing alternating current sine waves for welding, the terms electrode positive (reverse polarity) and electrode negative (straight polarity) which were applied to the work-piece and electrode lose their significance. There is no control over the half cycles and you have to use what the power source pr

37、ovides. The current is now alternating or changing its direction of flow at a predetermined set frequency and with no control over time or independent amplitude. During a complete cycle of alternating current, there is theoretically one half cycle of electrode negative and one half cycle of electrod

38、e positive. Therefore, during a cycle there is a time when the work is positive and the electrode is negative. And theres a time when the work is negative and the electrode is positive. In theory, the half cycles of alternating current sine wave arc are of equal time and magnitude as seen in Figure

39、2.14. Figure 2.14One complete cycle of AC sine wave showing reversal of current flow that occurs between the positive and negative half cycles. The degree symbol represents the electrical degrees. The arc goes out at 0?, 180? and 360? and maximum amplitude is at 90? and 270?. Arc Rectification When

40、GTAW welding with alternating current, we find that the equal half cycle theory is not exactly true. An oscilloscope Figure 2.15 will show that the electrode positive half cycle is of much less magnitude than the electrode negative half cycle. There are two theories accounting for this. One is the o

41、xide coating on nonferrous metals such as aluminum. The surface oxide acts as a rectifier, making it much more difficult for the electrons to flow from the work to the electrode, than from the electrode to the work. The other theory is that molten, hot, clean aluminum does not emit electrons as easi

42、ly as hot tungsten. This results in more current being allowed to flow from the hot tungsten to the clean molten weld pool, with less current being allowed to flow from the clean molten weld pool to the electrode. This is referred to as “arc rectifi-cation” and must be understood and limited by the

43、welder as indicated in Figure 2.16. Figure 2.15A reproduction of an actual unbalanced AC sine wave. Note the positive half cycle is clipped off. The missing portion was lost due to rectification of the arc. What can also be seen is a high current spike which can lead to tungsten breakdown and tungst

44、en spitting. Arc Rectification *Power source of proper Advanced Squarewave design will eliminate this phenomenon. Figure 2.16Arc rectification. Balanced and Unbalanced Waveforms Squarewave AC power sources have front panel controls which allow the welder to alter the length of time the machine spend

45、s in either the electrode positive (cleaning) portion of the half cycle or electrode negative (penetration) portion of the half cycle. Machines of this type are very common for TIG welding in industry today. Very few industrial GTAW AC sine wave power sources are being produced today. Waveform Balan

46、ce Control *This time controls the penetration and is most advantageous. Set to as high a percentage as possible without losing the cleaning. Very rare to set below 50%. *Note the expanded electrode negative time available on the Advanced Squarewave machine. % Time Electrode Negative*% Time Electrod

47、e Positive AC s ine w a ve power s o u rce S q ua rew a ve Adv a nced S q ua rew a ve Not a pplic ab le, control not a v a il a b le 4568 1090* Not a pplic ab le, control not a v a il a b le 3255 1090 Figure 2.22 shows an illustration of the frequency effects on a welding arc and the resultant weld

48、profile. This can be beneficial in automated welding by reducing the amount of deflection and wandering that occurs in the direction of travel when fillet welding. Figure 2.22 Normal 60 Hz arc compared to a 180 Hz arc. The current is changing direction 3 times faster than normal with a narrower arc cone and a stiffer more directional arc. The arc does not deflect but goes