J. Cent. South Univ. (2016) 23: 1633-1642

DOI: 10.1007/s11771-016-3218-6

Performance analysis of a pneumatic to servo converted system for electrode actuation in resistance spot welding using 304L austenitic stainless steel

Nachimani Charde

Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

Central South University Press and Springer-Verlag Berlin Heidelberg 2016

Abstract: A high current,AC waveform controller with C-type body frame of spot welder (75 kVA), was examined for the electrode actuating system whose pneumatically driven system has been redesigned and refitted for the servo based system without any vertical spring assistance in the 50 mm movable distance. Moreover, the pressing mechanism was carefully handled during the entire pressing tasks as to ensure that no catastrophic reaction happens for the electrodes’ caps, electrodes’ holders as well as the other part of mechanical assembly. With the mechanically originated-pneumatic and also the converted-servo systems, the stainless steels are welded for both systems to characterize the improvements. While the welding processes were carried out, the electrical signals have been captured to compute the signals’ representation of entire sequences. Later, the welded samples were underwent the tensile shear test, metallurgical observation and hardness test. The analytical results show distinct changes in the force profiles which has led to profound changes in mechanical properties of welded specimens.

Key words: force profiles; electrode actuation system; servo and pneumatic system

1 Introduction

An excellent weld formation basically relies on the proper setup of process-tuning parameters in any resistance spot welding technique. The weld formation is established due to its process-tuning parameters; specifically the electrical current, current flowing time, electrode pressing force and the electrode cap’s contact areas [1-3]. To understand how the tuning of parameters works within the welding lobe diagram, two out of four parameters are kept unvaried while changing the other two parameters concurrently to determine the proper window (see Figs. 2(a) and (b)). There are numerous discussions available to support either pneumatic alone or servo alone separately but there is inadequate information available about the electrode actuating system that converted from either pneumatic to servo or vice versa. These two systems have dissimilar characteristics in terms of force stimulations as far as the pressing mechanism is determined [4-6]. Force distributions are usually measured from the squeezing moment, passing through the welding moment and finally ended up after the solidification process ends. In this experiment 1-mm stainless steels were welded using pneumatic based and re-designed servo based (National Instrument’s Lexium 23 category) electrode actuation system respectively. During the entire welding process, the inherent process behaviors such as the electrode terminal voltage, the welding current and electrode pressing force are recorded with signals detectors [7-8]. Based on these measured signals, the static and dynamic resistance, heat generation distribution and force profiles are computed for both systems. Finally, the welded samples underwent the tensile shear test to compare the weld diameters’ increment and observed for metallurgical alteration and measured for the hardness changes [9-11].

2 Experimental

Welding samples were made of 304L stainless steel, with a rectangular size of 200 mm length by 25 mm width, utilizing the 1-mm thickness materials. The original chemical elements that found on stainless steel sheets were: w(C)=0.048%, w(Cr)=18.12%, w(Ni)= 8.11%, w(Mn)=1.166%, w(Si)=0.501%, w(S)=0.006%, w(N)=0.053% and w(P)=0.030%. The original hardness of the stainless steel was HRB81.7 and tensile ultimate strength was 515 MPa. The test specimens were welded at one side of the end, with an alignment of 30 mm lap joint from one end, as shown in Fig. 1. The electrode tip applied to weld was with 5 mm diameter, manufactured as flat-round-face but truncated type, basically selected from the class two classifications of RWMA’s class two- welding electrode category. The welding procedures are as common as how the other welding process takes place.

Fig. 1 Test sample (Unit: mm)

Welding-lobe diagrams determine the proper working region of process-tuning parameters for any thickness of weldable materials in RSW. The electric current, welding time, electrode force and electrode tip’s size are the tuning elements that establish the relationship among them. Usually, two parameters out of total four parameters are varied in an attempt as to establish proper welds’ regions or simply working window. Electrode force and welding current are gradually increased to form a relationship in this experiments and based on such activity, Figs. 2(a) and (b) were drawn. From Figs. 2(a) and (b), that the proper weld regions are marked with green boxes and enclosed with continuous line. Detailed explanations about the colors’ representation and boxes are given in Figs. 2(a) and (b). With the use of these welding lobes’ limits that marked with “X”, a weld schedule was correspondingly tabulated (Table 1) therefore.

The entire welding process was accomplished in accordance with the combinations of process-tuning parameters, which is summarized in Table 1. Seven welded pairs were developed for each weld schedule as the first five pairs were used to the tensile test, sixth one was used for hardness test and the final one was used for metallurgical observation. As for the hardness test and tensile test, the Rockwell hardness tester using scale B and 100 kN tensile test machines were engaged to complete the experiments. The V2A etchant which contains 100 mL water, 100 mL hydrochloric acid and 10 mL nitric acid was applied to etch the well prepared and weld polished bakelite-samples.

Fig. 2 Welding-lobe diagram of welding current against electrode force:

Table 1 Process-tuning parameters

3 Results and discussion

3.1 Welding lobe diagrams and working windows

It is found that the servo based system has improved the working regions of welding lobe for 1 mm thickness of stainless steel as the working window has been widened up for good welds and pulled towards initial startup. Look at the green colored boxes in the welding lobe diagrams of both systems (Figs. 2(a) and (b)), with respect to its columns and rows, in pneumatic based system, the welding current against electrode force sets has produced 18 good welds whereas in the servo based system, the welding current against electrode force sets have produced 22 good welds. This is noticeable change for good welds in servo based system as it increases by 37% of the pneumatic based system’s performance. Moreover, the light expulsion region starts at 5.50 mm for pneumatic based system but not same for the servo based system. In servo based system, the light expulsion starts at 6.00 mm of diameter. It should be noted here that the critical weld growth of stainless steel starts at; where t stands for the thickness of base metal [12-13]. Although the critical weld growth of stainless steel starts at 4.00 mm for 1 mm thickness of two sheets, the servo based system has successfully maintained the light expulsion until the weld growth reaches about 6.00 mm of weld diameter. This factor has also to be seen in another aspect because the electrode tip’s original diameter was 5 mm without any mushrooms or any other electrode deformation during welding process. If the electrode tip is with 5 mm of diameter and then the possibility of weld indentation has to be high if the molten area can reach up to 6 mm. This factor has been predominantly handled in the servo based system due to force profiles’ consistencies.

3.2 Static resistance of both systems

Accessing the static resistance, it leads to the prediction of mismatch or misalignment of electrode caps as the path has sandwiched contacts of resistive components. Hence, the static resistance (R₁+R₂+R₃+ R₄+ R₅) has to be measured for both electrode actuating systems as to understand the contact resistances’ changes because the force and the resistances are inversely related to each other within the good working region of welding lobe diagram (Fig. 3(a)). The contact resistances are measured from upper electrode cap towards all the way to the lower electrode cap with base metal placed- in-between [14]. It consists of five layers, where R₁ represents the contact resistance between upper electrode and top base metal, R₂ represents the bulk resistance of upper base metal, R₃ represents the resistance between upper and lower base metals, R₄ represents the bulk resistance of lower base metal and R₅ represents the contact resistance between lower electrode and lower base metal (Fig. 3(a)). Literally, when there is an electric current flow across these resistive components, the sum of contact resistances is all designated as dynamic resistance; otherwise, it remains just as static resistances.

Fig. 3 Various resistive components in static resistance (a) and static resistance of pneumatic based system (b) and static resistance of servo based system (c)

Initially, the static resistance of pneumatic based system is measured and the results are plotted for various force levels against total resistance. See Fig. 3(b) as it was measured from 1 to 6 kN of electrode pressing force. The calculated values of static resistance are ranging from 3 Ω to 120 μΩ, in this analysis for 1-mm thickness. However, the total resistance affected from 1 kN to 6 kN has inverse relationship with regard to the force increments. Similarly, the measurement was done for the servo based system and the results are shown in Fig. 3(c). Widely scatted resistive points are obviously noticed in the pneumatic based system whereas the servo based system has very accurately-aligned resistive points. This shows that the pneumatic based system has severe problem of force inconsistencies as far as the static resistance is concerned.

3.3 Force profiles of both systems

Electrode actuators plays important role in the resistance spot welding as it establishes the force profiles before, during and after the welding process takes place. Escalation or de-escalation of mechanical forces during or after the welding process is fundamentally known as the mechanically generated forging forces [15-17]. However, the inherently existing or electrically generated forging force in AC waveform spot welder is yet, undefined and therefore it has to be introduced at the first place before analyzing the force profiles. Apart from the well-known mechanically generated forging forces (MGFF), the electrically generated forging forces or electrically ignited vibrating forces (EGFF) are originated from the total collapse of the electromagnetic bonds between atoms in crystalline structure. Thus, when the high amount of electric current is forced to pass through a metal, the naturally bonded atomic electromagnetic-ties break up due to the enormous pressure and consequently the conduction grows. The heat is initially generated at the place where the conduction finds difficulties in its flow. This behavior intensifies the heat accumulation at that particular spot and disintegrates the total atomic clusters (crystalline structure) through fusion. While it breaks the electromagnetic ties between neighboring atoms, the process unleash electromagnetic waveform and this phenomenon can be often observed from the acoustic sensors or through the vibrating patterns of electrode levers. Once the area of concentration is completely fused, the electric current flow has to be stopped immediately and allow the molten area to solidify itself. The solidification process happens in natural way by reforming the electromagnetic ties between atoms and hence, no additional mechanism is required to accomplish such processes. The atomic turbulence happens in bidirectional way for an AC spot welder and consequently generates forging effects at atomic levels. So by analyzing the electrically generated forging effects in the force profiles, the force escalation is easily predictable. Let’s look at Figs. 4(a) and (c) for force profiles which are also escalated by the electrically generated forging forces. The force escalation is simply marked with circled capital letters such as A, B and C. The corresponding dynamic resistive patterns are shown in Figs. 4(b) and (d) for the pneumatic based system which represents the similar characteristics of heat generation as well the heat distribution in spot welding.

It was traditionally believed by welding experts that the force profiles may influence the dynamic resistive patterns and this assumption was proven in this experiment with the conversion of pneumatic system into spring-free servo based system. This assumption becomes a reality in two different electrode actuators as the electrically generated forging forces (EGFFs) are very low in servo based system in this case. By diminishing the EGFFs using servo based system, the dynamic resistive patterns have been improved and thereby more heat accumulation is achieved for the same duration of which the pneumatic based system had. Let’s look at the force profiles (Figs. 4(e) and (g)) measured for the servo based system. The markings (A, B and C) that represent the individual EGFFs growth are significantly suppressed. Better dynamic resistive patterns are therefore gained from these EGFFs suppression (see Figs. 4(f) and (h)).

3.4 Weld growth computation due to variation of process-tuning parameters

Weld growth is an important factor in resistance spot welding as it establishes the bounding strength between the welded parts [18]. Literally, if the bigger the fused areas are, the better the bounding strength will be. The increment of electric current will generally enlarge the molten areas as high amount of heat (Q=I²Rt) will enhance the fusion across the faying surfaces of the materials. However, the increment of electrode pressing force will lead to the drop of resistive components and also will influence the heat generation reversely. Hence the process-tuning parameters (electric current versus electrode pressing force) are both gradually varied within the welding lobe limits (see Figs. 2 (a) and (b)) to characterize the weld growth for the pneumatic and servo based systems, accordingly.

Fig. 4 Force profiles of pneumatic based system for various levels of electric current (a), dynamic resistances of pneumatic based system for various levels of electric current (b), force profiles of pneumatic based system for various levels of electrode pressing force (c), dynamic resistances of pneumatic based system for various levels of electrode pressing force (d), force profiles of servo based system for various levels of electric current (e), dynamic resistances of servo based system for various levels of electric current (f), force profiles of servo based system for various levels of electrode pressing force (g) and dynamic resistances of servo based system for various levels of electrode pressing force (h)

When three levels of electric current increments (6, 7 and 8 kA) are initially analyzed, then the weld growths were proportional in both electrode actuating systems but the servo based system had little larger size of weld growth as compared to the pneumatic based system. This merely happened due to the rigid hold of force actuator in servo based system. Figures 5(a), (b) and (c) represent three levels of increments of electric current in servo based system while Figs. 6(a), (b) and (c) represent the similar levels of increments of electric current in pneumatic based system. The nugget growth is absolutely little higher in servo based system for three electric current levels as compared to the pneumatic based system. On the other hand, the electrode pressing forces are analyzed for three increments (3, 4.5 and 6 kN). It was very obvious that the three increments of electrode pressing forces have reduced the process resistances and consequently drop the heat generations during welding process. Although this behavior is very common in RSW, it is still unclear between two different electrode actuators. As such both electrode actuators are subjected to the equal amount of force increments. The results show that the decreasing coefficients are slightly lower in servo based system as it handles proper force executions. These phenomena can be easily understood by looking at the macrographs of the welded zones. Let’s compare Figs. 7 (a), (b) and (c) with respect to Figs. 8(a), (b) and (c). Even the nugget growth reductions are fairly executed in servo based system as these have been clearly shown in macrographs.

3.5 Tensile shear test and results

Shearing the spot welded specimens is very common mechanical test in welding research [19]. Hence all the welded specimens are tensile-sheared in this experiment using ordinary testing procedures without any additional supporting instrument during loading effects. The test was conducted for all the 45 speciments with standard setups. Five pairs of specimens were welded for single weld schedule with which the process- tuning parameters were selected according to the marks given in Figs. 2 (a) and (b) and there are 9 weld schedules were followed. At last, the average of five welded specimens is accounted as the shearing strength for the particular weld schedule. Figure 9 shows the average tensile shear force used to crack the welded specimens. The results show that when the welding current is increased gradually in steps, weld growth is also propotionally increased as how witnessed in weld growths. Bonding strength becomes astonishing factor here and the dynamic resistances are revealing the changes caused by the heat variations. Similarly, the electrode force increment decreases the weld growth and low level of force is required to crack such cases. The factor now is that two different electrode actuators behaviours are subjected for the current and force increments and the results show that servo based system has better output than the pneumatic one. Thus, the servo based system has produced slightly higher weld growth from which more tensile shear forces are drawn for crack initiation. Likewise, the drop of weld growth does not deviate widely but somewhat in relative way when the force was increased in servo based system. This margin was largely established in the pneumatics based system due to its rapid fall of dynamic resistances during the reduction of electrode pressing forces.

Fig. 5 Weld growth for servo based system:

Fig. 6 Weld growth for pneumatic based system:

Fig. 7 Weld growth for servo based system:

Fig. 8 Weld growth for pneumatic based system:

Fig. 9 Tensile shear test reults:

3.6 Crack initiation and propagation of failure mode

Conventionally, the spot weld crack initiation and crack propagation are distinguished by two distinctions during tensile tests [20-22]. The premature cracks are categorized as interfacial failures in which a separation at the welded zones is physically witnessed. This phenomenon occurs due to inadequate ties at the weld formation area and it is basically known as poor weld joints. Perfectly broken cracks are categorized as button pullout in which the neck breaking and complete button pullout conventionally appeared. This sort of explanation finds difficulty in distinguishing the better weld joints against the best weld joints. Hence, a post crack propagation mode is proposed here to primarily differentiate the better and best weld joints. Thus, the interfacial failure (IF, see Fig.10(a)) remains as poor weld’s representation as before, but the neck breaking occupies a new category as partial failure mode (PF, see Fig.10(b)). In this partial failure modes, either side of the base metal will fracture after facing the post crack pulls. In order to monitor the post crack propagation, the tensile pulling methodology has to be continously carried out until the welded joints are utterly separated from each other. Likewise, the button pullout category will be examined for the post crack propagation mode in which the metal trails will be seen on both sides of metal sheets or simply a perfect button pullout (TF, see Figs.10(c) and (d)). A best weld joint will never fracture at the heat affected areas like neck breaking or something, but pave the way for either a tear from both sides of the metals as trails follow weld zones or complete button pullout.

3.7 Micro structural orientation

Analyzing the microstructural orientation is another way of looking at the changes that happened for the welded zones against the unwelded regions [23-25]. The original austenitic stainless steel had horizontal dendrites made of austenites with ferrites as grain boundaries (Fig. 11). The welding current flow has changed the horizontally-aligned dendrites into vertically-aligned dendrites. Besides, the ferritic orientation seemed to be randomized at the weld zones. Generally, the solidification of the austenitic steel welds causes minor changes in the dendrites arrangement or sometime in recrystallization. In pneumatic based electrode actuating system, the primary austenitic dendrites and inter- dendrites of ferrite are randomly scattered at the welded nucleus (Fig. 12). The entire weld zones are now seen as recrystallized austenite surrounded with tilted thin ferrite independently. From the randomized orientation of austenites and ferrites phases (pneumatic based system) to oblique-columnar austenite with widmanstatten ferrites (servo based system) are what absolutely witnessed in this process. Figures 11-13 are compared for the micro structural changes.

Fig. 10 Post crack propagation mode in tensile shear test:

3.8 Hardness of welded and unwelded areas

Basically a solidification process alters the hardness of any materials, particular at the fusion zones how it was supposed to be happened due to the cooling act of electrode caps [23-25]. The austenitic stainless steel has very lower rate of hardness changes as compared to other materials that were commonly found in resistance spot welding. However, the hardness changes have been noticed approximately in similar patterns for both electrode actuating systems though it sounds less importance in this experiment (see Fig. 14). The original hardness of the stainless steel was HRB81.7 and the welded zones have produced approximately HRB95 in overall average. The distributive patterns seemed to be fluctuating at the fusion zones. Figures 14 (a) and (b)represent the pneumatic and servo based systems, respectively.

Fig. 11 Original austenitic stainless steel’s micro orientation

Fig. 12 Welded areas in micro orientation (pneumatic actuator)

Fig. 13 Welded areas in micro orientation (servo actuator)

Fig. 14 Hardness distribution in welded zones against unwelded zones:

Another concern involved in the hardness changes is the phase transformation. The original horizontal- austenitic dendrites have been vertically aligned with randomized austenites for pneumatic based system and with single phase austenites for servo based system. This is the absolute cause for the slight hardness increment at the fusion zones regardless of the electrode actuating systems.

4 Conclusions

1) The welding lobe of servo based system has been slightly shifted towards the beginning portion of welding window and 18:22 was the ratio for good welds between the pneumatic actuator and servo actuator.

2) The static resistances are obviously scatted in the plotting diagram for pneumatic based system, leaving noticeable deviations between one another. In contrast, the servo based system has properly aligned scattering points in which more stable patterns of resistive changes have been noticed.

3) The dynamic resistive patterns have been partially improved for the servo based system as compared to the pneumatic based system. These phenomena led to the proper heat accumulation and diffusion during welding process.

4) The electrically generated forging force (EGFF) is a new factor that influences the welding process. This EGFF is tremendously reduced in the servo based system due to the firm hold of the servo based-pressing arm.

5) The welded areas of servo based system show slight increment in terms of diameter growth as compared to the pneumatic based system. The results also show that the decreasing coefficients of weld growth are slightly lower in servo based system as it handles proper force executions.

6) In tensile shear test, the servo based welded samples draw more pulling force to initiate the cracks due to the increments of welds diameter. So, three types of breaking failures are introduced to distinguish between them as interfacial failure (IF), partial failure (PF) and button pullout (TF).

7) From the randomized orientation of austenites and ferrites phases (pneumatic based system) to oblique- columnar austenite with widmanstatten ferrites (servo based system), is absolutely witnessed in this process for the micro structural changes.

8) The original horizontal-austenitic dendrites have been vertically aligned with randomized austenites for pneumatic based system and with single phase austenites for servo based system. This is the root cause for the hardness increments at the fusion zones regardless of electrode actuators.

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(Edited by YANG Hua)

Received date: 2015-06-12; Accepted date: 2015-09-22

Corresponding author: Nachimani Charde, Researcher; Tel: +60379674497; Fax: +60379675317; E-mail: nachicharde@yahoo.com