Groover/Introduction to Manufacturing Processes

Welding Case Study: Steel Wall Support Panels in North Carolina

A small start-up company has developed in central North Carolina to take advantage of the relatively strong market there for low-cost, manufactured homes. The company has researched and prototyped several types of structural wall support panels made from cold rolled, medium-carbon steel that can outperform wood construction methods in terms of performance and cost. The prototype panels are made of two separate steel components joined along one side. The prototype assemblies were constructed using threaded fasteners since they are cheap and the designers frequently needed to disassemble the panels as the design requirements changed.

Now, however, the panel design has matured to a well-defined product. To produce dozens of identical panels, the group has planned an arc welding process to add fillet welds along both sides of the overlapping components. Although the design of the steel components includes a number of pockets and cutouts to reduce weight, a simplified cross-section of the assembly arrangement is shown in the figure below.

A manual stick welding (SMAW) process is used for the welds. The time to complete one 2-meter long panel section is determined by the 100 A power supply (operated at 20 V), the steel alloy used (its melting factor is typically approximated by 0.55), and the size of the components (each is 12 mm thick). Each of the two weld seams has approximately 72 mm2 cross section. The company welders typically achieve an arc time proportion of 25% of total work time.

Production numbers for the panels are expected to ramp up into hundreds or even thousands in the near future. A more automated welding solution is proposed in the form of flux-cored arc welding (FCAW). With the continuously fed electrode of FCAW, a higher current of 200 A can be used, and mechanized control of motion will increase the arc time to 50%. The automated solution, of course, requires a higher initial investment.


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1. What is the purpose of the slag that covers the weld area in an arc welding process?

2. Why do you suppose that tungsten is used for non-consumable electrodes in arc welding?

3. What conductive material is typically used for the electrodes in resistance welding?

4. What type of welding common in automotive body fabrication uses resistance heating to produce one single weld nugget at a time?

GO TO THE TEXT: Chapter 23

5. Hot, melted metal oxidizes rapidly in an oxygen environment. Describe at least three ways that fusion welds are protected from oxidation. See Section 23.1.

6. What kind of welding is MIG welding, and how is it more productive than SMAW? See Section 23.1.

7. Not all welding involves melting. Describe in general how friction welding and ultrasonic welding work. See Section 23.5.

SOLVE

8. Estimate the arc welding time to complete the two triangular weld seams needed for each wall support panel assembly using the manual SMAW welding process. Use the heat balance method described in Section 22.3.2 in the text. Note also the heat transfer factors given in Table 23.1

9. Consider the arc time proportions and the extra current in FCAW to estimate the total time to complete the welds for both the SMAW and automated FCAW cases. Note that the remaining factors in the calculations of Problem 8 can be assumed the same for the two processes.

10. The SMAW equipment in the case study can be purchased for a fixed cost of $12,000, and it costs $30/hour to run the process. The automated FCAW equipment costs $25,000 to purchase and $36/hour to run. At what level of planned production volume would it make sense to implement the FCAW process rather than SMAW?