EnglishRDCBooklet

Reversing DC and The Benef i ts of Joining Lightweight Mater ials

ABSTRACT

With the need to produce lighter weight vehicles, the switch from conventional steel applications to aluminum is beginning to take hold in the automotive industry. The aluminum applications have been primarily centered in closures. However, the auto industry is looking to expand the usage of aluminum into the Body in White (BIW). There will be a predicted increase from 4% of BIW as complete aluminum to 18% by 2025 [4]. While Resistance Spot Welding (RSW) of aluminum is nothing new to the auto industry, until now, it has been used in lower production rate vehicles or batch build closure applications. To meet the demand of CAFÉ standards, there is need to use it on high production lines. This need will, right or wrong, amplify the areas of concern that have plagued aluminum RSW in past applications. And while the ability to reverse polarity with special AC systems (frequency conversion) is used in many aerospace aluminum spot welding applications today, the type of system used in aerospace applications is too large for robotic and automated type equipment and therefore, not conducive to high production processes [3]. It has been proven through prior studies that having the ability to change polarity when using DC to weld aluminum is helpful in increasing electrode life [2]. It is also helpful in obtaining proper weld nugget growth when challenged with dissimilar alloys, and multiple 3T and even 4T thicknesses. However, with robotic welding applications in high rate production, it is unrealistic to expect that robot programs/tooling can be modified to insure proper polarity for each spot weld to resolve the RSW aluminum difficulties—robotic weld gun geometry and the assembly process present a big challenge. The solution to this problem is provided when the welding transformer switches polarities by a simple signal from the weld controller. The signal system functions within the transformer and can be easily incorporated into existing welding controllers.

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INTRODUCTION

TEST DESIGN

Factors The equipment selected was a pneumatic press welder outfitted with a RoMan RDC transformer. The weld electrodes were a long radius Class 1 Zirconium. To understand the principals of being able to alternate polarity on consecutive welds, we established a base range while utilizing only a single polarity (A polarity). This represented current welding practices without the ability to switch polarities. Then, the experimental weld electrode life consisted of switching polarity after each weld (A/B polarity). We utilized the same materials to eliminate the variable of heat transfer between dissimilar sheet thicknesses. This was not researched in the previous studies of polarity to stack-up orientation study [2]. Table 1 shows the materials selected and the starting parameters.

The major inhibitor of the widespread use of aluminum RSW applications has been cap life [5]. In general, DC systems may have lower electrode life, and a greater tendency than comparable AC systems of electrode-to-sheet sticking due to the increased erosion of the positive (anode) electrode [1]. With the decrease in electrode life, manual interaction with tooling increases (electrode changes), this in turn decreases uptime of the machine and overall jobs per hour (JPH). With the increase in applications using aluminum, different series of aluminum plus applications of 3 thickness (3T) welding are possible design options. However, with different series aluminum, polarity sensitivity can be present, especially when welding 3T applications. Therefore, change in the resistance and heating characteristics can make 3T welding very difficult in production applications. Past studies have shown an increase of 30%-60% electrode life when selecting the proper polarity to stack-up orientation [2]. Now, a test study conducted by RoMan confirms the increase in electrode life by being able to control polarity within the weld schedule by means of a RoMan Reversing DC power supply (RDC). The test study also shows the effects of alternating the Anode and Cathode, and proves the ability to control nugget migration in dissimilar series aluminum welds.

TABLE 1 Life Test Materials and Base Parameters

To establish the Weld Range IMin - IMax the following method was utilized. Begin the weld coupons at 70% of the above mentioned weld currents. Each amperage was welded for 3 welds and then increased 500 amps until minimum prescribed nugget diameter was measured on the 3 welds (IMin). To establish IMax we continued the weld increase of 500 amps until one of the following occured: Excessive indentation, expulsion or sticking (IMax).

Weld Time (cyc)

Hold Time (cyc)

Material A (Top)

Material B (Bottom)

Current (kA)

GMT (mm)

MWS (mm)

Force (lbf)

Test

Cap

Test

Range and A & A/B Life Range and A & A/B Life Range and A & A/B Life

1.00 mm 6022-T4

MWZ- 6149

6457t1

To itself

27.0 1.00 4.0 900

5

5

1.20 mm 6022-T4

MWZ- 6149

6457t2

To itself

30.0 1.20 4.4 900

5

5

2.00 mm 6022-T4

MWZ- 6149

6457t3

To itself

45.0 2.00 5.7 1200

5

5

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5

Nugget Migration Testing Nugget migration testing showed how nugget growth is affected by the materials and polarity of welding. It is worthy of note that, heretofore, very little thought or discipline has been applied in most automotive design to deal with the problem of uneven thermal characteristics and how to process with DC welding currents. During this test multiple scenarios were tested to prove or disprove that polarity can effect welding characteristics of nugget growth. This was a Macro Level study of location and penetration depths. The study tested the following (3) scenarios: 5000-6000 materials, equal thicknesses materials, thin to thick gauge, and a thin-thick-thick 3T stack-up. The weld coupon layout in Figure 2 (previous page) was utilized, and the weld # 3 (previous page) was cut and etched to show nugget growth characteristics. They were be welded in the same orientation with (A) polarity and then with (B) polarity to show the growth tendencies as it related to polarity.

FIGURE 1 Life Panel

With the Weld Range established, the testing continued for the life test of the (2) mentioned polarities (A), (A/B). The desired weld current was the amperage that achieved 5.3Sqrt(T). The testing speed was 10 Welds/Min on panels shown in Figure 1. After each 80 welds made on the main panel there was a peel sample made to verify the nuggets were still sufficient to continue onto the next main panel. Figure 2 shows the layout of the peel sample; welds 3-6 were peeled, and only when all 4 welds fell below Minimum Weld Size (MWS) did the test continue.

TABLE 2 Nugget Migration Stack-up and Weld Parameters

Weld Force (lbs)

Weld Cycles

Weld Current Programmed (Ka)

Weld Current Actual Ka (Polarity)

Stack-up (mm) XXXX Series

(1.0) 6022 - (2.4) 5754

900

5

26.5

27.1 (A) / 27.3 (B)

(1.0) 6022 - (1.4) 6022 - (2.4) 5754

1200

5

31.0

31.7 (A) / 31.8 (B)

(1.3) 5754 - (1.4) 6022

1200

5

27.0

27.6 (A) / 27.7 (B)

FIGURE 2 Peel Coupon

(1.0) 5182 - (1.0) 6016

670

5

35.0

35.0 (A) / 35.0 (B)

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GRAPH 2 Life test results for “A” polarity

GRAPH 3 Life test results for “A/B” polarity

TEST RESULTS

Life Test Results To verify results, two independent labs were used to validate the data. Both caliper method and camera scanning methods (Figure 4 on page 8) were used to determine nugget diameters. All welds were peel tested and optically measured by personnel at the Alcoa Technical Center. The equipment was specifically designed to automate peel testing and button measurement for resistance spot welding. The peel tester can process panels 5” (125mm) by 18” (450mm), containing 48 to 100 welds. The fully calibrated vision system then measures and records each weld on the panel. Prior to peel testing, the forming lubricant is wiped off the electrode sheet side and the imprints are colored with a black marker which provides an enhanced contrast for the vision system. During electrode life testing, panels containing 80 welds were produced. After each panel was peel tested and measured, all the weld diameter data was compiled together to provide 100% inspection of all welds produced.

Another key point established by the tests is the consistency of the welding when switching “A/B” polarity (Graph 4 on following page). The Tables 3 below, and 4 on following page, show the individual weld diameters for all welds made during the life test until end of life was established. The “A/B” polarity did not fall below 4 Sqrt(T) during any of its weld while the “A” polarity began a downward trend in nugget diameters around the 200 weld mark and continued until failure.

TABLE 3 Nugget diameters for “A” polarity Life Test

Weld Range results are shown in Graph 1. For the Life tests, it was determined by the formula of 5.3Sqrt(T) that the 1.0 mm 6022 will be welded at 29.5 Ka.

GRAPH 1 Weld Lobe for 1.0 mm 6022

The results from the life test are shown in Graph 3 and 4. Graph 3 shows the single polarity “A” welding which end of life occurred at 245 welds into the test. Graph 4 shows the switching polarity “A/B” test which hit the end of life at 480 welds. This has shown that there was a 2 times increase in electrode life when switching polarity after each weld.

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Nugget Migration Results All samples were cut and etched and the results are pictured in Figures 5 thru 7 following. All of the samples showed a growth of 50% > towards the surface of the coupon when the “A” polarity (+) was on the 5000 series material. This shows an increase in temperature in the substrate causing the uneven growth patterns seen in the figures below. Growth has shown in at least one of the samples (Figure 5) that the molten pool would have contacted the weld cap contact surface.

TABLE 4 Nugget diameters for “A/B” polarity Life Test

FIGURE 5 1.0mm 6022 – 2.4mm 5754

FIGURE 6 1.0mm 6022 – 2.4mm 5754

GRAPH 4 Nugget size distribution for Life Test

FIGURE 4 Camera Measuring System

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Conclusions and Discussion As found in the earlier studies, the benefits to polarity selection when welding aluminum are a reality. Without the need of bulky frequency converters, and benefiting from DC welding characteristics, selecting the proper polarity can increase cap life, help with robustness of the weld nugget, and increase confidence in the aluminum welding process. The electrode life increased by a factor of two, and the ability to control nugget migration by matching polarity with stack-up is evident from this study. There are a multitude of variables to consider when welding aluminum and now, polarity direction can be added to the list. This, like many advances in welding technologies, is not the magic bullet to solve all dilemmas—however, the RoMan RDC power supply can now be added as the newest tool to the ever increasing tool belt in Aluminum RSW.

FIGURE 7 1.3mm 5754 – 1.4mm 6022

• Ability to overcome uneven electrode wear

• Ability to position weld nugget

FIGURE 8 1.0mm 6016 – 1.0mm 5182

• Avoids having to physically re-orient welding guns

The nugget location is centered on the weld interface by taking the “A” polarity off from the 5000 series material. This helped distribute the heat, in-turn, creating a more uniform nugget formation in the stack-up.

• Usable by any weld controller

Now, when thinking of the welding process, a determinant can be made of the most important aspects expected out of the weld. It could be life of weld electrodes, it could be understanding the multiple thickness stack-ups, or dissimilar series of aluminum or robustness of the process. Whichever the priority, there are different ways to look at the process—the window of welding capabilities has opened.

Acknowledgements The author gratefully acknowledges the lead of Don Maatz—R&E Automated Systems, for his direction on successfully conducting the tests discussed in this paper. Special acknowledgement is also given to Alcoa Technical Center, and D.J. Spinella, for providing material, testing equipment and test procedures to guide the research.

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References 1. D.J. Spinella, J. Brockenbrough, J.Fridy, “Trends in Aluminum Resistance Spot Welding for the Auto Industry," AWS 2005 2. W. Perez Regalado, A. M. Chertov, D. Dzhuriskiy, V. Leshchynsky and R. Gr. Maev, “Polarity and Stackup Orientation: Effect on Electrode Life in Resistance Welding Aluminum," Sheet Metal Welding Conference XV Sheet 4-5, Oct. 2012, Livonia MI

3. Resistance Welding Aluminum, http://spotweldinc.com/downloads/ Resistance_Welding_Aluminum_shopping_for_equipment.pdf

4. 2015 North American Light Vehicle Aluminum Content Study, https:// www.alcoa.com/car_truck/en/pdf/2014_Ducker_Executive_Summary.pdf 5. S. Fukumoto, I. Lum, E. Biro, D. R. Boomer, and Y. Zhou, “Effects of Electrode Degradation on Electrode Life in Resistance Spot Welding of Aluminum Alloy 5182,” AWS Nov 2003 https://app.aws.org/wj/supplement/11-2003-FUKUMOTO-s.pdf

©2016 RoMan Manufacturing

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Grand Rapids, Michigan | romanmfg.com

4-2017

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