2004: A robot MIG weld report:

This report is applicable to most
carbon / stainless robot
applications < 4 mm.

 

2006: This report could be applicable to any robot steel / stainless applications in the gage range of 1.5 to 4 mm. This report is partially based on a weld evaluation I carried out for a tier one supplier of MIG robot welded steel frames and the required sub assemblies.

ROBOT WELD RISKS:

In evaluating specific robot welding issues, it's important that the unique robot weld risks be understood for the parts under discussion. With robot MIG welds, the primary weld concerns with common parts 2 to 6 mm is the attainment of "acceptable consistent weld fusion". On parts > 2 mm, as long as the welded part dimensions and robot TCP tolerances are maintained, the robot, the weld process and consumable choices should be simple and the resulting weld quality / productivity issues should be minimal.

The auto industry is full of unqualified process
confidence and inflated unwarranted egos:

At many USA and Canadian plants where the robot welded parts are >2 mm, thanks to the low weld burn through risk you will often find that the managers, engineers and technicians have a false weld process weld manufacturing confidence, a confidence that has little to do with weld process expertise, Best Weld Practices, or the implementation of Robot Weld Process Controls. Along with the unwarranted confidence you can always find egos and attitudes in which the workers with two years expertise decide they don't need to further their narrow weld education.

You will find many of the robot weld issues are caused before the robots get to the plants. It's not uncommon to find weld issues that result from the selection of oversized MIG weld consumables. These consumables are typically selected by some project engineer (mechanical engineer) or by a purchasing manager both lack an understanding of the MIG process. Other issues will arise from the selection of erratic pulsed weld equipment selected again by an inexperienced project manager. Add the weld consumable, equipment and people issues together with the inexperienced plant manufacturing manager. This is that individual who typically lacks the ability to recognize the root cause of his plants robot weld production / quality issues. This is a common team found in the global auto / truck manufacturing industry.

 

In many Canadian and USA auto / truck manufacturing plants that utilize MIG welding robots on the easy to weld > 2 mm steel parts, you will find numerous confident, young technicians / engineers who have gained a little weld process expertise and overnight turned themselves into highly opinionated weld process control experts.

 

ROBOTS AND WELD RISKS

In contrast to the steel parts > 2 mm, when welding parts < 2 mm, the primary robot weld concerns are the "avoidance of weld burn-through". With these applications the robot weld risks are much greater and the need for real world weld process control expertise and management implemented process controls is much greater than those thicker parts.

The plant in review here today provides many robot welded steel parts that are 1.8 to 4 mm. As with too many plants the mechanical engineers at this plant lack the ability to create parts to the as specified design dimensions. The part dimension issues at this plant and robot TCP deviations along with a general lack of weld process expertise will compound the potential for weld burn and extensive weld rework.

As the robot weld production ramps up in this new plant, it's too late to make changes to the robot welded parts, changes that typically would reduce weld issues. Many plants in this industry daily have to compensate for the part "designers weld process ignorance". The stamping issues and resulting weld part variations along with the lack of effective manufacturing production controls that are typically applied to robot welds will result in major part dimensions and weld issues.

Combine the variable robot welded part dimension problems with;
[] the project engineers lack of expertise in not ordering the optimum weld equipment, the optimum weld consumables and the available automated TCP controls necessary for each robot cell,
[] add in the general lack of weld process expertise that prevails with too many engineers and robot technicians,
[] process apathetic manufacturing management that lack an understanding of Best Weld Practices or Process Controls and you can be sure that the robot weld issues will increase daily in proportion to the weld production attained.

BEST PRACTICES OR THE
MISSING LINK OF THE AUTO / TRUCK INDUSTRY

To minimize the potential robot weld issues that can occur before the robot line is purchased, a logical engineered approach is required in the establishment of "Best Robot Weld Practices". The big five auto manufacturers in the USA get most of the welded parts from their tier one suppliers. Most tier one suppliers grant manufacturing autonomy to each of their plants, and boy do they like to do things in their own unique ways. At each plant you will find many different weld practices to weld similar parts. The inability to establish best weld practices with a two control, fifty year old process points to a major corporate manufacturing weakness in the understanding of weld automation.

In the auto / truck industry the selection of inappropriate MIG weld consumables and the use of inferior weld equipment / process modes has for decades created major weld issues and added billions to the cost of robot / manual MIG welds. It's obvious that many of the people who today make the robot weld recommendations are simply not qualified and as we all know, many of the robot weld problems we find today are influenced by decisions made at corporate offices.

In many auto / truck manufacturing plants, weld consumable selection may have more to do with a "purchasing cost reduction decision" rather than with a logical weld engineering decision, this again is a corporate issue. With the majority of the robot lines installed in the last five years, the selection of the weld equipment and process utilized typically has had more too do with salesmanship than with practical robot weld process considerations, again this is a corporate management issue. Hopefully for those that give a dam, the following weld data will assist them in dealing with gage parts.

 

PRESENT WELD PRACTICES.

The manufacturing plant in this article has many weld issues. This plant purchased Fanuc robots with National Standard E70S-6, 0.045 1.2 mm weld wire. For the welds they of course selected that state of the art, new Lincoln, Pulsed Power Wave F355 power source. Over 100 robots were purchased for this plant, none of the robots were ordered with automated TCP controls. Also the 0.045 weld wire size selected is not appropriate for parts < 2 mm, and of course the pulsed power source used was a poor choice at unnecessary high cost.

The power source selection added an additional $400,000, 00 of unnecessary cost to the robot weld lines.

BEST WELD PRACTICES AND PART DIMENSIONAL DEVIATIONS.

Part dimension and TCP control is critical when welding gage applications. Also on these parts we typically have to be able to place oversized welds without concern for weld burn through. Aggressive weld weaves may also be necessary to compensate and its great if we have the option of "touch sensing or through arc tracking" for pin point weld accuracy.

Typically on parts > 2 mm we would deal with weld fusion issues concerns, weld profile concerns, and weld start stops concerns. In contrast with the thin gage welds < 2.0 mm, we know the weld burn through potential will be high and that a weld burn through will disrupt production as the weld arc is lost in the hole made during the weld burn-through. Thinner gages means smaller welds and a high degree of weld / joint accuracy, also concern for heat / distortion issues. It's imperative at this facility which has many welds on parts less than 2 mm that an automated robot tool center point (TCP) control be added to the robot cells and the TCP be checked daily at the commencement of the shift or when weld / gun changes are made.

 

 

BEST WELD PRACTICES AND THAT GMAW PULSED POWER SOURCE.

For almost two decades the MIG equipment manufactures have been developing and promoting pulsed MIG equipment for "steel applications". The latest Lincoln / Miller or Japanese pulsed equipment purchased for today's robot weld lines will have undergone years of development and numerous E Prom changes, yet here we are in 2004 and every pulsed power source I have tested still provides no weld benefits for steels > 1 mm. (Note my view of pulsed for carbon steels changed in 2006)

With these robot applications and the Lincoln equipment, the pulsed "arc length sensitivity" is a serious concern especially on welds in which high weld speeds > 40 ipm are required. It's ironic that the Lincoln pulsed weld equipment purchased purchased by consumers who should know better can also cost 100 to 300% more than the available Lincoln or Miller superior, stable, traditional CV equipment. The pulsed weld equipment purchased will also likely cost three times as much to repair and you will need to keep more spare equipment than normally would have been required with traditional, more durable CV equipment.

To attain stability on your parts when high weld speeds are required the pulsed mode should be replaced with the more stable, less arc length sensitive spray transfer mode. While the spray mode runs hotter than the pulsed (not a concern > 2mm) the weld heat can be reduced with the selection of a smaller wire diameter such as an 0.040 1.1mm or 0.035 1mm wire.


The day the majority of global auto / truck plants will have selected the optimum MIG power source or wire diameter for their robot MIG applications is the day I will be retire, or hell will have frozen over.

My Management and Engineers Guide to MIG book was written nine years ago. It spelled out the pulsed instability and arc length sensitivity issues of the pulsed MIG process and the importance of robot weld wire selection for a specific thickness. Nothing has changed in that time except the pulsed bovine fecal matter has increased and we now have a generation of robot weld decision makers who simply don't know the capability of a traditional low cost CV MIG power source. Its unfortunate that few of the individuals who make important weld decisions in auto plants seem to want to read a weld process control book.

In the majority of plants that I visit it's rare to find the key weld decision makers have subscribed to the "free" welding magazines that are available here in North America. Hey why read about welding when you already have all the answers, why read about welding when all you have to do is ask that Lincoln or Miller salesman, after all he will provide you with all the info you require on the latest state of the art pulsed power source. I guess it will take another decade and hundreds of millions of dollars wasted on unnecessary costly weld equipment, lost weld production and weld rework before the weld industry wakes up to a weld reality.

 

July 30-04. E Mail to Ed,

Ed: Your pulsed description of arc sensitivity with high speed welds is is exactly what I am experiencing with our robots and the new Miller Accu-Pulse process / Auto Axcess. Our new auto bumpers are thin gage, 1/16 1.6mm HSLA and martensite. I tried to weld above 40 IPM with the Accu-Pulse and could not do to weld skipping and arc instability. We went to an .035 1 mm wire and could not get the travel speeds. We changed to .045 1.2 and had to run the pulsed arc with the arc length buried in the part, this resulted in extensive weld spatter also the part could not handle the pulsed weld heat and we would have holes all over the place. With the disappointing pulsed weld results we now use high end short circuit CV with the 045 wires and are attaining 40IPM travel rates. I have no spatter on the part and have no arc stability problems with the short circuit. I hate to admit it but this is is another pulsed failure in my book. I could get these short circuit results CV power source for half the costs..

Regards G S.

THE PROCESS

When using an 0.045 1.2 MIG wire and pulsed welding "lap" welds on parts 1.6 to 2 mm weld travel rates of up to 55 ipm can be attained, less speed for fillets. However to achieve the desired minimum weld travel rate of 50 ipm, on 1.6 mm lap welds, the "pulsed" MIG weld wire has to virtually make contact with the weld surface. The pulsed "wire to work contact" not only causes arc instability, it also can cause extensive weld spatter which will impact the tip life. The pulsed weld spatter will also get on to the fixtures causing part assembly / fit issues.

Note: Remember this is 2004 pulsed equipment, (not quite right). In 2006 the following negative pulsed comments are no longer applicable.

When establishing the pulsed trim voltage (arc length) with the high speed (> 45 ipm) pulsed weld applications on thin steels, if the pulsed weld voltage (arc length) is set to a none weld spatter condition, the pulsed open arc weld transfer from most pulsed equipment will typically provide both arc instability and low energy welds causing "skip" welds (missed welds with weld blobs). Every wheel manufacture in the industrial world has likely suffered from this pulsed MIG condition.

 

<2004: Typically when pulsed MIG welding we should have a sufficient arc length to enable the pulsed weld drops to form and transfer without making a short circuit, (explosive) disruptive contact with both the work and wire tip. Something pulsed MIG manufactures are not likely to proclaim in their descriptions of their so called artificially intelligent, pulsed wave forms or fuzzy wuzzy weld equipment, is the desired "minimum arc length" required for optimum pulsed welds is "detrimental" when trying to pulse weld thin gage parts using an 045 wire at high weld speeds.

Thin gage parts limit the allowed pulsed wire feed rates utilized which also limits the attainable pulsed weld frequency. The moderate frequency pulsed plasma is therefore influenced by the amount of frequency times peak times back ground weld current the thin gage parts can absorb. With the large weld wire diameters and a pulsed weld in which 50% of it's time is at a low background current of typically less than 100 amps we not only have arc sensitivity issues we can have the formation of sluggish welds which are poorly suited to high speed weld speed applications, (this will be more evident with "convex fillet welds" rather than on gage lap welds.)

Note: Remember this is 2004 pulsed equipment, (not quite right). In 2006 the following negative pulsed comments are no longer applicable.

Another ironic fact is when you use the Lincoln or similar pulsed equipment in the traditional spray transfer modes which typically will improve the weld stability high weld speed applications, is the weld decision maker may not be aware that the slope influenced performance of the traditional spray mode from the pulsed MIG weld equipment is typically inferior to the conventional slope out put weld performance from the lower cost traditional CV equipment.

When considering weld best practices one would simply wonder why after two decades of poor pulsed arc characteristics for high speed, steel / stainless MIG welds how the selection and purchase of this sensitive, electronic, costly, inconsistent weld equipment was ever justified. I guess the bottom line is the selection of this ridiculous weld equipment after all these negative experiences is simply a reflection of the prevailing management / engineering weld process expertise.

 

WELD BEST PRACTICES AND WIRE DIAMETER SELECTION.

 

 

As I write this report on the common weld practices, I should note that too many auto plants are using the ridiculous 0.052 1.4 mm MIG weld wires for steel parts 1.8 to 5 mm thick. As the pulsed mode is typically hopeless for high speed MIG welds, and the spray transfer weld current is too hot from the 0.052 wires, the poor weld technicians, weld personnel by the way who typically have nothing to do with the selection of the oversized consumables have only one weld choice. To avoid weld burn through on the thin parts, the weld technicians will set the weld parameters in the globular transfer mode. The globular mode will create excess large globular droplet transfer causing poor weld fusion and extensive weld rework. The globular weld spatter will block the gun nozzles and tips causing wire burn backs and extensive robot down time for tip replacement.

The robot weld facility I evaluated is welding thin cradle auto parts. This facility will benefit from using stable spray transfer, however the welds should be made with a smaller wire diameter than 045 (1.2mm). As I have said at least ten thousand times in the last decade, weld wires such as 0.040 or 0.035, (1.1 - 0.9 mm) should be used on parts less than 4 mm.

<2004: ROBOT WELDS AND CURRENT COMPATIBILITY WITH GAGE THICKNESS.

Let's take a moment to look at the MIG weld current compatibility with common gage thickness. With robot single pass "butt" welds on parts 1.2 to 1.6 mm, we typically would require short circuit transfer welds made at 140 to 200 amps. For these thin applications it's logical to use an 035 - 040 wire. Note: In contrast to butt / fillet gage welds, lap welds do allow higher weld current as the two thickness combine to absorb the weld heat. The butt / fillet weld data show here minimizes weld burn through potential.

For any robot weld application that uses a weld current over 200 amps, rather than use the pulsed mode I would advice the use of the high deposition, stable spray transfer mode.

Note: Remember this is 2004 pulsed equipment, (not quite right). In 2006 the following negative pulsed comments are no longer applicable.

With gage parts 1.7 - 1.8 mm, the butt weld current would be increased to approx. 200 to 240 amps, again note that lap welds will allow higher weld current but it's logical to keep the weld current close to the butt max current to avoid weld burn through. Butt welds on 2 mm parts could be robot welded using 240 - 280 amps.

An 045 MIG wire and argon CO2 mixes requires a minimum of 250 - 260 amps to attain stable spray transfer, most 3/16 and ¼ fillet welds are welded between 250 and 350 amps. As many of the welds produced on the gage cradle require less than 260 amps it makes no sense to use an 045 weld consumable that will simply cause weld burn through on many the parts. The bottom line; when welding parts < 4 mm its imperative the plant uses an 035 or 040 wire. The 035 wire will go into spray around 200 amps, the 040 at approx. 230 amps.

A message I have delivered since the 1980s. Is anyone listening? For those interested in weld optimization the world's best MIG wire diameter for all welds less than 4 mm is the 040 1.1 mm wire. As this is the logical wire choice for the auto industry you wont be surprised to find that its rarely utilized.

Note: The 040 wire was used by Dana to weld Ford F-150 Trucks in 2006, see home page.

 

WELD WIRE COSTS.

I can understand why third world countries get uptight about weld consumable costs, however in many North American auto plants weld engineers don't have to worry about the $1 a pound MIG wire costs. In many auto plants purchasing managers rather than engineers may decide on the wire diameter selected. If the purchasing manager does not select the weld wire it may be selected by some big three corporate mechanical engineer who could not do a MIG weld if his life depended on it. By the way a typical auto cradle may today use approx. 1 to 2 lbs of weld wire per cradle. So the purchasing manager can save 20 to 40 cents a cradles by recommending an 0.045 or 0.052 wire instead of the 0.040 or 0.035 wires. In this case changing the weld wire to a smaller wire will dramatically improve both the weld productivity and quality, but hey why go that route when the purchasing manager can save a few cents a per part.

Note: For MIG gas selection for the robot gage welds, vist the MIG gas section:

THIN PARTS WELD WEAVES.

The plant should consider when required the use of weld weaves for gage parts. The weld weaves should comprise of amplitude that creates a narrow, high speed oscillation in the weld center. This weld weave oscillation will not impact the potential weld speeds, it will however cause the fast freeze thin welds to thin out and provide slightly wider weld coverage.

Three important weld benefits are attained from the weld weaves on gage parts;

[1] Reduce weld burn through potential.
[2] Helps compensate for gaps.
[3] Helps compensate for part dimensional deviations.

Hopefully this tongue in cheek weld article will assist some unblinkered auto / truck manufacturers to give a little more consideration to weld choices and process requirements. If you want to spend less time trying to put out the weld shop fires that daily spread through their weld shops your people including your managers, engineers and technicians need process control training.

 

Regards Ed Craig. 2004

 

KEEP SMILING, AFTER ALL YOU ARE PART
OF THE WELDING INDUSTRY