Welding and Testing Tubes for Hydroformed Applications
By: Menachem Kimchi, Edison Welding Institute Inc, USA
This paper describes quality control methods and test procedures in welded tube manufacturing that can vastly improve processes and weld quality in seamed hydroforming tubes. Prior art relied on the use of standard tests that can fail to identify defects that will be magnified by the hydroforming process. A bulge test is shown to be a useful tool to improve weld quality and can greatly assist in improving tube quality and formability.
Current hydroformed tube applications are typically frames and chassis components fabricated mostly using mild steels. New-generation vehicle designs will likely require the use of high-strength steel (HSS) hydroformed components. One common barrier in the use of any hydroformed components is the difficulty in producing tubes with satisfactory seam welds. This difficulty is particularly true in HSS applications.
The problem of developing quality tube-stock for hydroforming applications is evident regardless of whether the seam welds are made using high-frequency (HF) welding, laser, or other methods. The fundamental problem is that the weld will invariably have slightly anisotropic material properties that vary from the base material. For small strain deformations, especially of low strength materials, seam welds pose little difficulty. However, by its very nature, the hydroforming process seeks to induce high-strain. Thus, the ability to produce a weld with properties that lend themselves to quality hydroformed tube fabrication is paramount to success.
Any manufacturing process requires control of process parameters, with the obvious goal being to produce parts more and more alike, closer and closer to dimensional intent. Simply fabricating the part and verifying that it meets dimensional tolerance leaves a number of variables to chance. In order to guarantee quality that cannot be defined with a dimensional specification, manufacturers typically require performance tests and adherence to various government and industry standards.
Each new specification, dimensional tolerance, or verification procedure adds cost to the process and introduces one more degree of difficulty in finding an optimized set of process parameters.
Standard Methods No Longer Work
Prior to the advent of hydroforming applications, manufacturers specified a variety of simple tests to ‘prove’ the quality of tube seam welds. For example, typical performance specifications for seam-welded tubes include expansion, flare and flattening tests, see Figure 1.
Figure 1: Examples of expansion, flare, and flattening tests of seamed tubes
However, expansion, flare, and flattening tests load the seam approximately uniaxially, i.e., stress is applied along only one axis. Either a tensile load from stretching or a bending load from flattening is applied. While giving the appearance of being able to withstand large strains, the unixial character of these tests actually limits their ability to detect certain types of problems in welds.
Recently, a ‘bulge test’ has been introduced at OSU’s Engineering Research Center for Net Shape Manufacturing that loads the hydroformed tubes biaxially. A biaxial load is much closer to the type of loading experienced by hydroformed tubes during typical forming operations. The results of bulge testing revealed that a significant number of tubes capable of passing an expansion, flare or flattening test actually fail a bulge test.
Far from being a detriment, the use of a bulge test can actually improve the quality of hydroformed components and can provide a criterion for improving process design. A bulge test can provide a convenient method of reducing the range (tolerance) of process variables that can be tolerated, thus reducing variability in finished components.
Figure 2: Typical process window for controlling the welding process of HF seam-welded tubes
Optimization of Process Parameters
Process variables to be considered when developing an HF tube welding process, for example, include:
Relative position of edges
Position of supports
Degree of upset
Variability of material properties
From a welding optimization perspective, the variables listed above can be divided into independent and dependent variables. The independent variables used in the optimization process include: welding speed, welding temperature, and squeeze roll setting. The dependent variables are: degree of upset, force, and weld quality.
Figure 3: Weldability window for type 439 stainless steel at 1.0-T displacement setting (A 1.0-T displacement setting is a relatively low degree of upset)
A typical process window is shown in Figure 2. The variables listed above combine to determine the location within the process window. Actual measured data are shown in Figures 3 and 4 comparing the effect of displacement setting for a 439 stainless steel application.
Figure 4: Weldability window for type 439 stainless steel at 1.8-T displacement setting (A 1.8-T displacement is a relatively high degree of upset. Note the larger acceptable operation window compared to Figure 3)
In the development of such data, the appropriate instrumentation for measuring weld temperature, force in the squeeze roll box, and displacement should be employed. Figures 5, 6 and 7 show typical instrumentation needed for obtaining these data.
Figures 5, 6 & 7: Typical instrumentation of the tube welding process: equipment includes pyrometer for temperature measurement, laser vision camera and laser vision displacement sensor
These techniques can be further applied to compare operating parameters for two different welding processes. Figure 8 shows a preliminary comparison between two welding processes.
Figure 8: Comparison of bulge test results of two different welding processes for a 304 stainless steel material [photograph on left is laser welded, figure on right is HF welded. Weld line is vertical on left side of both figures. In the right-hand photograph, compare the difference in bulge from left (weld line) to right side (base material). Further optimization of process parameters is required of the HF process.
The anisotropy of the weld is magnified by the bulge test, and in this case, indicates that further optimization of process variables is required for the HF weld to perform as well as the laser welded sample. The bulge test greatly improves the ease of developing an improved process window.
Process optimization for manufacturing seam-welded tubes used in hydroforming applications is quickly becoming a requirement as quality levels are pushed to new heights. The use of bulge testing of seam-welded tubes can greatly enhance the quality and help identify problems that might otherwise not be detected until later in the manufacturing process.
This work was sponsored by an EWI Cooperative Research Program. EWI acknowledges Yingyot Aue-u-lan and Tylan Altan at The Ohio State University’s Engineering Research Center for Net Shape Manufacturing for their contributions to this program.