Abstract:
This invention is to plasma arc weld using a keyhole mode to build a partially-penetrated keyhole and then a melt-in mode to finally reach the full penetration before switching to the base period. The full penetration is thus established during the peak period in two stages: keyhole stage and then melt-in stage. While the keyhole stage helps reduce the heat inputs and weld puddles, the melt-in stage finishes the full penetration at reduced impacts from the plasma jets producing the desired weld bead geometry and regularity. The duration of the melt-in stage is automatically determined using arc signals to assure the full penetration. In comparison with keyhole PAW, bead geometry and regularity are significantly improved with slightly increased net heat inputs. In comparison with melt-in PAW and GTAW, the net heat input is reduced approximately forty percent.

Description:
GOVERNMENT INTEREST STATEMENT 
       [0001]    The present invention was made with government support under agreement KSTC-184-512-08-048 as the matching fund from the Kentucky Cabinet for Economic Development (CED) Office of Commercialization and Innovation for contract N00024-08-C-4111 awarded by the Department of the Navy. The government has certain rights in the invention. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to arc welding, and more particularly to plasma arc welding and its variants. 
       BACKGROUND 
       [0003]    Plasma arc welding (PAW) introduced by Gage in 1957 [1] offers certain advantages. As an extension of gas tungsten arc welding (GTAW), PAW uses a constricting nozzle/orifice to create a plasma arc that has a higher heat density. This increased heat density not only provides higher arc temperatures but also a much stronger arc force. In general, its most widely used configuration is the transferred arc PAW, in which the plasma arc is formed between the tungsten electrode and work-piece as shown in  FIG. 1 . Its welding current is generally set to be DCEN (Direct Current Electrode Negative) in order to provide better control of the energy release [2]. In addition, since the plasma arc is highly constricted compared with electrical arcs in GTAW process, the arc length has an excellent linear relationship with the arc voltage under the same welding current. As a result, a measurement of arc voltage may indicate the arc length more accurately to better reflect the penetration. 
         [0004]    The transferred arc PAW process typically operates in either keyhole or melt-in (conduction) mode [3]. The special torch used in PAW has a constraining orifice [4] designed to deliver a highly constrained plasma jet. Keyhole mode can obtain much deeper penetration compared with other arc welding processes. In this mode, the plasma jet melts the work-piece and displaces the molten metal to form a keyhole or deep narrow cavity [5]. By doing this, the plasma jet is able to heat the work-piece through the whole thickness, giving keyhole PAW high penetration capability [6]. On the other hand, melt-in mode, with reduced penetration capability, is suitable for joining thin sections (0.025-1.5mm or 0.001-0.060 inch), making fine welds at low currents, and joining thicker sections (up to 3 mm or 0.125 inch) at high currents. The operation of melt-in mode is similar to that of GTAW process. 
         [0005]    Repeated experiments show that weld beads made by keyhole PAW typically have relatively large and irregular ID (inner diameter) reinforcements associated with considerable amount of spatters. On the other hand, those made by melt-in PAW show large ID weld beads that may cause excessive convexity around 12 o&#39;clock and concavity around 6 o&#39;clock. To resolve these issues, the double stage PAW method is invented to combine keyhole and melt-in mode into a single welding procedure. 
       SUMMARY OF THE INVENTION 
       [0006]    The method of the present invention to plasma arc weld uses a controlled plasma arc welding system as shown in  FIG. 2  which at least includes: (1) a plasma arc welding power supply  211  or an equivalent; (2) a plasma arc welding torch  222  or an equivalent; (3) a device to measure the plasma arc voltage  221 ; (4) a computer  203  or equivalent with necessary accessories to read the measured arc voltage  221 , to process the measured arc voltage  221  and send the control parameter signals 220 to control the output amperage of the plasma arc welding power supply  211 ; and (5) the computer accessories that include but not limited to the input isolation modules  204  and output isolation amplified modules  201 . 
         [0007]    The method of the present invention is to use this controlled plasma arc welding system to first apply a relatively high plasma arc current, that is high enough to establish a keyhole on the work-piece being welded if the application time is sufficiently long, for an appropriate period without establishing a full penetration; then apply the plasma arc current at a reduced level that is not sufficient to establish a keyhole through the work-piece; then process the plasma arc voltage measurements to determine if the criterion for the full penetration establishment has been met; then further reduce the amperage of the plasma arc current to start the base period. The criterion for the establishment of the full penetration constitutes another key of this invention and will be discussed further. 
         [0008]    An embodiment of the controlled plasma arc welding system is illustrated in  FIG. 3 . The computer  203  is now an embedded system  303 . The plasma arc welding torch  312  is a Thermal Arc PWH-3A plasma welding torch. The power supply  311  is in constant current (CC) mode and operated in DCEN. The torch is water-cooled by the coolant recirculator inside the power supply. Pure Argon is used as the shielding gas, plasma gas, as well as backside purging gas for the pipe in case a pipe is welded. The torch can be carried by a motion system, either a welding robot or any other mechanical motion systems. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  (Prior Art) shows the principle of the conventional plasma arc welding known to one of ordinary in the art. 
           [0010]      FIG. 2  (This Invention) shows the principle of the system for the controlled plasma arc welding in this invention. 
           [0011]      FIG. 3  (This Invention) illustrates a specific system for the controlled plasma arc welding in this invention. 
           [0012]      FIG. 4  (This Invention) uses the current and voltage waveforms to illustrate the principle of the double-stage plasma arc welding in this invention. 
           [0013]      FIG. 5  (This Invention) gives a specific flow chart of a specific control algorithm to further illustrate the principle of the double-stage plasma arc in this invention. 
           [0014]      FIG. 6  (This Invention) shows a specific system for the controlled plasma welding when an optional filler is added in this invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Double Peak Stage Procedure 
       [0015]      FIG. 4  shows the principle of the double peak stage procedure that characterizes the present invention. The method of this invention for plasma arc welding (PAW) also has two periods, peak period  400  and base period  450 , similarly as in conventional PAW of the prior art. However, this invention divides the peak period  400  into the first stage  401  and second stage  402  with the current in the first stage, i.e., I p1    403 , significantly greater than that in the second stage, i.e., I p2    404 . 
         [0016]    The first stage is to use a keyhole mode type of operation to penetrate the work-piece rapidly. However, this stage stops before the keyhole fully penetrates to the backside of the work-piece in order to prevent the problems in a normal keyhole mode aforementioned. Then a melt-in mode type of operation follows as the second stage to continue and finish the establishment of full penetration but in a smoother and slower manner with a much lower penetration capability. The weaker penetration capability similar as in a GTA can produce smooth full penetration welds eliminating the geometrical irregularities. That is, the first stage achieves a penetration depth with a minimal heat input and the second stage finishes the full penetration establishment process using an arc similar to a GTA. Smooth and relative narrow welds may thus be produced in the second peak stage. The base period further reduces the heat input to freeze the liquid metal before the next peak period begins. 
         [0017]      FIG. 4  uses reducing the peak current from I p1  to I p2  as an example to reduce the penetration capability. While reducing the current is an effective method to reduce the penetration capability to switch from the keyhole to melt-in mode, the essence of this invention is to switch from the keyhole to melt-in mode before the work-pieces is fully penetrated. Hence, this invention does not exclude the use of other means to change the penetration capability for PAW including but not limited to (1) changing the actual flow rate of the plasma gas to switch from the first stage  401  to the second stage  402 ; (2) changing the effective diameter of the orifice where the plasma gas exits from the plasma torch; (3) using any electrical, magnetic, and/or mechanical ways to change the distribution of the plasma arc to change its penetration capability; (4) using any ways to change the setback of the electrode. 
       Adaptive Second Peak Stage 
       [0018]    In conventional PAW of the prior art, the amperages of the peak current and base current and their durations are programmed based on the applications. In this invention, the amperage during the first stage in the peak period and its duration and the amperage during the second stage, i.e., I p1    403 , T p1    401 , and I p2    404 , are pre-programmed but the duration of the second peak, i.e., T p2    402  is determined using the arc voltage  460 . Further, the determination is made using the slope of the arc voltage  460  and the criterion to end the second peak period  402  to switch to the base current period  450  is to judge if the positive slope of the arc voltage has become below a sufficiently small positive number ε≧0. In addition, because the arc voltage measurements are noisy, the slope of the arc voltage needs to be determined using filtered arc voltage signal. The filtering of the arc voltage requires consecutive measurements made at consecutive sampling instants such as t 1 , t 2 , . . . t 4    470 . 
       An Example 
       [0019]    A detailed example procedure to realize the PAW method of this invention can be given below with reference to  FIG. 4  and  FIG. 5 :
       (1) Initialization of process, including welding parameters and control parameters;       
 
         [0021]    (2) Output the base current I b    451  for the base period T b    450 ; 
         [0022]    (3) Output the first peak current I p1    403  for the first peak period T p1    401 . Both I p1    403  and T p1    401  are empirically determined; 
         [0023]    (4) Switch to the second peak current I p2    404  and then wait for a short period (typically less than 50 ms); 
         [0024]    (5) Sample the arc voltage and calculate an average each 10 ms as a sampled arc voltage measurement V p    460 ;
       (6) For each four consecutive V p  measurements (e.g. at t 1 , t 2 , t 3 , t 4 ), a linear model is fitted by the least squares method; extension can be easily to more consecutive measurements higher order models;   (7) The slope of the fitted curve, dV p /dt, is then computed from the model and compared with a pre-determined criterion threshold ε≧0. If dV p /dt&gt;ε, it is judged that the desired penetration is not reached. If this is the case, the control program goes to step (5); otherwise, to step (2) to start the next control pulse period.       
 
       Mode Switch 
       [0027]    If the welding is performed using a single (either keyhole or melt-in) mode, the plasma torch configuration and welding conditions can be set-up in advance and then be kept unchanged during the entire welding process. However, for this invention, the PAW process needs to switch from keyhole to melt-in operation mode in real-time. The challenge here is how to switch from keyhole to melt-in mode with a torch configured for keyhole mode. This question can be simplified as how to reduce penetration capability of plasma arc during welding operation. To find an acceptable solution, several key factors affecting penetration capability should be considered. 
         [0028]    The physical configuration of plasma torch is one of the most important factors in determining penetration capability. Smaller orifice diameter can provide better mechanical constriction. Larger electrode setback can achieve similar effects. However, during welding operation, it is not practical to change any of them. Hence, the torch configuration is so determined that the penetration capability is just sufficient for keyhole operation. 
         [0029]    The plasma gas flow rate is another key factor determining the penetration capability. PAW process is sensitive to this parameter. A simple adjustment of plasma gas flow rate from 2.0 scfh to 1.0 scfh can considerably reduce the penetration capability and change the operation mode from keyhole to melt-in. It is technically possible to use an adjustable flow control valve, and the gas flow rate can be controlled by an external electrical signals. However, this flow rate control mechanism has a relatively large time delay compared with the needed pulse period of welding current. The valve reaction to the control signal and the flow rate change from the gas supply to the torch end both take time. Therefore, similar to torch configuration, the plasma gas flow rate is set to a level that just sufficient to for keyhole operation. 
         [0030]    The welding current controls the penetration capability and heat input of plasma arc. With a reduced welding current, the heat input may become insufficient to achieve full penetration if a single melt-in mode is used. However, since the full penetration is almost achieved in the first stage, the establishment process for full penetration may still be able to continue and finish with the reduced heat input. This operation status is considered a quasi-melt-in mode. As an electrical parameter, the welding current can be easily adjusted by the control system in real-time. Therefore, the transition from keyhole to quasi-melt-in mode is switched by adjusting the welding current. 
         [0031]    There are other welding conditions and parameters that also affect the penetration capability, such as coolant recirculation rate, overall torch size and rating, distribution of plasma gas, etc. However, in comparison with the parameters/variables aforementioned, their real-time adjustments are even more difficult. Hence, this invention switches from the keyhole mode to the melt-in mode by reducing the current from the first peak to a second peak. 
       Mode Switch with a Second Electrode 
       [0032]    When the peak current is changed from I p1  to I p2 , an additional current may be provided into the work-piece by adding a second electrode. This current can provide sufficient heat input for the melt-in mode operation. To this end, a parallel circuit can be established as shown in  FIG. 6 . Because this added current is not constricted by the orifice of the plasma torch, the ability to penetrate the work-piece is not much increased by this added current. The penetration capability and heat input may thus be separately controlled. This would allow I p2  to be further reduced while the full penetration may still be established due to the controlled heat input. 
       Torch Travel and Wire Filler 
       [0033]    The welding torch can travel in a continuous mode or stepwise mode. In both modes, the optional arc length control and optional filler wire addition are implemented during the base period. 
         [0034]    The stepwise mode torch motion is preferred. If the continuous travel model is used, the work-to-tungsten  distance during the second peak current period must be minimized or the distance slope is added as additional information to analyze the vertex. 
         [0035]    Further, the torch travel can be manual or mechanized/automated. The filler wire addition can be manual or mechanized/automated. 
       ANALYSIS AND ADVANTAGES 
       [0036]    PAW process gives different performances in keyhole and melt-in modes. For keyhole mode, highly constricted arc is capable of reaching full penetration rapidly. However, due to the high penetration capability, the weld bead produced with keyhole mode tends to have large back-side weld reinforcement (convexity on the backside bead). At the same time, there may be spatters blown out of the weld pool by the strong plasma jet. On the other hand, for melt-in mode, its moderate penetration capability resembles that of GTAW process, which is capable of generating smooth weld beads. However, the welding current needs to be increased considerably in order to produce full penetration on same joints. The melt-in mode thus can only be used to weld work-pieces with thickness much less than those can be welded with keyhole mode. In addition, since heat input is increased, the weld pool becomes larges and sometimes collapses may occur. 
       Welds Made in Keyhole Mode 
       [0037]    The weld beads produced by keyhole PAW are sensitive to a number of welding parameters including welding speed, welding current, flow rate and composition of plasma/shielding gas, electrode setback, torch standoff distance, etc. [7, 8]. Extensive studies have been conducted on keyhole PAW and effective methods have been proposed, implemented and tested for the control of keyhole PAW process[9-12]. With the control system developed in [13], welding parameters can be adjusted to generate consistent weld bead in the presence of various disturbances. Its principle is to pulse the welding current to intentionally produce a varying weld pool and associated varying arc voltage and then determine the weld penetration depth from the arc voltage measurements [14]. During the peak current period, the welding torch stays at the same spot to gain accurate measurement of arc length from the arc voltage signal and determine if the desired penetration has been achieved; during the low current base period, the torch moves for a certain fixed distance to the next spot and waits for the next pulsing control period. 
         [0038]    To operate in keyhole mode, a relatively small orifice diameter is needed. A relatively large plasma gas flow rate is also needed to further enhance the penetration capability of the plasma jet. Then the full penetration can be obtained through the strong penetration capability of the plasma jet. The resultant weld beads on both sides are narrow (compared with GTAW). However, strong penetrating plasma jets also cause problems for keyhole mode. Full penetration is obtained by punching a hole with strong plasma jet inside the liquid weld pool such that a small portion of the melted metal inside the weld pool may be blown away as spatters. Immediately after welding, small particles of spatters were found inside the pipe. At the same time, solidified weld beads on the back-side of the work-piece exhibited geometrical irregularities and excessive convexities (over 2 mm reinforcement). 
       Welds Made in Melt-in Mode 
       [0039]    Melt-in mode PAW can be performed using a reduced penetration capability. To this end, the orifice diameter can be increased and the plasma gas flow rate can be reduced. Due to the weakened arc force, a larger heat input had to be used by increasing the welding current in order to achieve desired penetration. After these adjustments, the process could operate in melt-in mode and produce full penetration but it resembled the behavior of GTAW process. 
         [0040]    With fine-tuned welding parameters, full penetration welds can be produced under melt-in mode. The weld bead is smooth without undercut and large convexity, similar to that made by GTAW process. The smooth weld bead meets visual inspection requirements. However, due to the weak penetration capability, full penetration can only be guaranteed in a small welding speed range, which makes it difficult for manual welding practice. Furthermore, the HAZ is large, because the weld penetration is achieved by conduction of heat under melt-in mode. The excessive heat input (compared with keyhole mode) generates a large weld pool, which may occasionally collapse. 
       Heat Input Analysis 
       [0041]    With experimental results in [15], comparisons were made for heat inputs delivered to the work-piece (net heat input) among GTAW process and three PAW process operation modes, i.e. keyhole, melt-in and double stage modes. A number of studies have been conducted to investigate the heat input and arc efficiency in arc welding processes [16-19]. For the double stage PAW in this invention, the primary objective is to produce full penetration with reduced net heat input. Arc efficiency gives a quantitative measurement of the fraction of total arc energy delivered to the work-piece. The total energy generated by the power supply can be easily calculated based on the arc voltage and welding current measurement. Referring to the arc efficiency results from [17], a general comparison is possible for PAW and GTAW process. A comparison has been made for welding of the exactly same pipes and listed in Table 1, i.e., Table 6 in [15]. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Comparison of net heat input in unit weld bead length 
               
             
          
           
               
                   
                   
                   
                 Double 
                   
               
               
                   
                 Keyhole 
                 Melt-in 
                 Stage 
                   
               
               
                 Welding Process 
                 PAW 
                 PAW 
                 PAW 
                 GTAW 
               
               
                   
               
             
          
           
               
                 Base period time (ms) 
                 800 
                 600 
                 800 
                 N/A 
               
               
                 Base period current (A) 
                 20 
                 30 
                 20 
                 N/A 
               
               
                 Base period voltage (V) 
                 18 
                 19 
                 18 
                 N/A 
               
               
                 First stage peak period time (ms) 
                 250 
                 350 
                 200 
                 N/A 
               
               
                 First stage peak period current (A) 
                 110 
                 125 
                 110 
                 N/A 
               
               
                 First stage peak period voltage (V) 
                 25 
                 26 
                 25 
                 N/A 
               
               
                 Second stage peak period time 
                 N/A 
                 N/A 
                 300 
                 N/A 
               
               
                 (ms) 
               
               
                 Second stage peak period current 
                 N/A 
                 N/A 
                 60 
                 N/A 
               
               
                 (A) 
               
               
                 Second stage peak period voltage 
                 N/A 
                 N/A 
                 21 
                 N/A 
               
               
                 (V) 
               
               
                 GTAW welding current (A) 
                 N/A 
                 N/A 
                 N/A 
                 120 
               
               
                 GTAW arc voltage (V) 
                 N/A 
                 N/A 
                 N/A 
                 17 
               
               
                 Travel speed (mm/s) 
                 1.22 
                 1.22 
                 1.22 
                 1.22 
               
               
                 Total heat input (J/mm) 
                 999 
                 2021 
                 1246 
                 1672 
               
               
                 Arc efficiency (%) 
                 ~47 
                 ~47 
                 ~47 
                 ~67 
               
               
                 Net heat input (J/mm) 
                 470 
                 950 
                 586 
                 1120 
               
               
                 Typical backside bead width (mm) 
                 3~4 
                 ~10 
                 5~6 
                 9~11 
               
               
                   
               
             
          
         
       
     
         [0042]    From net heat input data in this table, it can be clearly observed that the four processes under comparison can be divided into two groups. The one with the net heat input around 1,000 J/mm includes melt-in PAW and GTAW process. This explains why the melt-in PAW produces welds similar to those using GTAW process. Although the plasma arc voltage is larger under the same welding current, its net heat input delivered to the work-piece is comparable to that of the GTAW. Keyhole and double stage PAW processes are in the group with a net heat input around 500 J/mm. As a result, the weld beads with smaller backside width were produced. Compared with keyhole PAW mode, the double stage PAW does not increase the net heat input delivered in to the work-piece. The combination of two modes of PAW not only reduced net heat input (compared with melt-in mode), but also significantly improved the weld bead (compared with keyhole mode), with smaller backside reinforcement, moderate backside width and no spatters. 
       REFERENCES 
       [0000]    
       
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