final project report: weld strength · 2018-10-10 · 4 this project will be looking at the...
TRANSCRIPT
Final Project Report: Weld Strength
ME 306 - Professor Ryan
D. Armor Harris, Chris Hayes (U71722867), Kam Tabattanon
4/28/2014
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Table of Contents:
Abstract…………………………………………………..………………………………………………………………………………………………2
Introduction………………….……………………………………........……………………………………………………………………………2
Theory………………………….…………..……………………….……………………………………………………………………………………2
Methods…………………….…………..……………………….………………………………………………………………..…………………….4
Results……………………….……………………….……………………………………………….………………………………………………….5
Discussion and Analysis…………………………..…………………………………………….………………………………………………10
Conclusion………………..……………………….……………………………………………….…………………………………………………12
Acknowledgments……………………………………….………………………………………………………………..………………………13
References……………………….……………………….…………………………………………………………………..………………………13
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ABSTRACT
This project is designed to test the weld quality of the Scientific Instrument Facility (SIF) aluminum welds
on BU Rocket Propulsion Group hardware. 6061 Aluminum thin walled tubes that had been v-groove
and fuse welded were tensile tested along with non-welded control samples up to failure for strength.
One of either welded sample was axially cut at the weld site and this was mounted, polished, and etched
to observe grain sizes in the regions of tempered aluminum and weld zones. These samples then
underwent Knoop microhardness testing in the same regions.
The results showed the fuse and control samples breaking at the welds and scores respectively as
expected. The fuse samples failed at low loads (5 kN compared to 20 kN for the not welded samples).
The V-Grooves failed at an average of 0.81 in away from the weld. This result suggested that the welds
were strong and either improved or did not affect material strength in the region. Etched images
showed that for all sample types the grain sizes did not change in the regions outside the weld zones
where they did not exist. Hardness testing showed that an overall decrease in hardness exists as the
material tested moves further from the welded region. This is also true for both welded sample types.
This results supports the strength results for the V-Groove samples.
INTRODUCTION
The hybrid suborbital rockets developed by the Rocket Propulsion Group rely on welds made by the
Scientific Instrument Facility in pressure vessels and primary structure. There are knockdowns for weld
design in literature, but the high stress high precision nature of the group's components requires a more
detailed understanding of the strength of the welds. Many of the welds are circumferential welds on
cylinders, so this geometry was chosen for testing. The objective of the project is to quantitatively
characterize the strength of the weld zones on 6061 aluminum thin wall tubing to better inform designs
using similar welding.
THEORY
Weld Zones:
When aluminum is welded, high heat is applied that de-tempers the material. The extent of this effect
depends on the quality of the weld and is critical to quantify. Shown in Figure 1 is a diagram of a V-
Groove weld in 6061-T6. The weld material shown in red fills in the groove and may be ground flush
with the surface afterwards. Adjacent to the weld material there will be a zone of material with the
equivalent strength of untempered (T0) strength aluminum. This is a result of the high heat from the
weld diffusing into the surrounding material. This zone, shown in green, is about 1/8” thick on average.
Adjacent to this is a zone with the equivalent strength as T4 aluminum, which is about 1/8” to ¼” thick.
Once beyond this region, the material is back to T6 temper.
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Figure 1: V-groove weld diagram
When a weld breaks, it usually either forms cracks along the weld or will break in the surrounding
material. Thus the expectation for this experiment is for the welded tubes to break right next to the
weld beads on the Instron machine.
Stress-Strain:
When loaded in either tension or compression, a sample will deform up to a certain point before failure.
For brittle materials, the amount of deformation before failure may be very low. A stress-strain curve
may be obtained from these tests and used to observe material properties such as ductility and
strength. In this project, the strength will be the property of concern.
For engineering stress-strain curves, the stress σ is defined as
[Equation 1]
where F is force in N and Ao is the initial cross sectional area of the sample being tested in m2. This
cross sectional area is the area on the surface over which the force is applied. As deformation occurs,
this area changes, however, it is difficult to measure this change in area as the test proceeds. The
resulting engineering stress-strain curve is therefore different from the true engineering stress. The true
stress-strain curve lays above the engineering curve. For both curves, strain ε is defined as
[Equation 2]
where ΔL is the change in length from the initial length . Strain may be related to stress in the linear
region of the stress-strain curve by E, the Young's modulus of the material. Past the linear region, the
region of elastic deformation, the material deformation becomes plastic. The highest stress in this
region is the ultimate tensile stress, and past this point necking will occur until failure.
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This project will be looking at the strength of the welded thin wall tubes, with the not welded tubes
being used as controls. Stress will be induced in tension using an Instron machine.
Grains and Microhardness
Crystalline solids may be polycrystalline and consist of a collection of small crystals or grains. Grains may
be viewed on a mounted sample through polishing and etching techniques that reveal the grain
boundaries. The grain sizes for the constituent phases can be seen under a microscope. This project will
observe the grain sizes in the different zones around the weld.
The hardness of these different zones will also be observed. The hardness of a material may be tested
using methods such as the Rockwell hardness test or the Knoop microhardness test. For mounted
polished samples, the Knoop microhardness test is used. This test uses a diamond pyramidal shaped
indenter that presses into a level sample.
Figure 2: Image of pyramidal diamond Knoop Hardness Indenter
The Knoop hardness number (KHN) is the ratio of the applied load to the uncovered area. The resulting
indentations much be symmetrical to be valid; an asymmetrical indentation is an indication that the
sample was not level or that the indentation was caused by a non-equibiaxial residual stress state. The
KHN will be recorded for the different regions along the length of the aluminum weld and control tubes.
METHODS
There were three types of 6061-T6 aluminum tubes tested. Tubes were chosen because most RPG welds
are circumferential welds on cylinders as opposed to flat or linear welds on plates. All tubes started out
5.75” long x 0.50” diameter 0.650” wall thickness and were modified as follows:
Controls: 3 tubes were not cut and welded, the only modification to the stock material was to
score the mid section on a lathe to ensure that the tubes broke at the midsection and not at the
Instron clamps
V-Groove samples: 6 tubes were cut at their midpoint, a 30 degree v-groove was added, and
they were TIG welded back together with 1000 series aluminum filler material.
Fused samples: 3 tubes were cut at their midpoint and fused back together with heat and no
filler material.
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The control pieces were used to calibrate the Instron machine and verify accuracy of the results
compared to known strengths of T6 aluminum.
A total of 10 samples were tested on the Instron machine: 3 control tubes, 2 fuse welded tubes, and 5 V-
cut tubes. One fuse tube (f3) was cut at the weld for mounting, and one V-groove tube (v6) was cut
both at the weld and away from the weld for mounting. The cut made away from the weld served as
the control sample for the grain viewing and hardness testing.
Figure 3: Images of the test samples; (Top Left) No weld tube; (Top Right) V-Groove tube; (Bottom) Fuse weld tube
Each test piece was individually loaded on to the Instron Machine. Loads were applied in tension until
failure, and the stress data was recorded. Measurements post failure were also made to determine
where the sample failed.
For each the V-groove weld, fuse weld, and no weld zone sample, a sample was cut axially across the
weld area and was then mounted, polished, and etched. These mounted samples were viewed under
the microscope and images of the grain boundaries were taken in regions ranging from far away from
the weld to into the weld zone. These samples were also tested for the Knoop hardness number (KHN).
Figure 4: Mounted, polished, and etched samples for (From Left to Right) the Fused, V-Groove, no weld samples.
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From the collected data the strength of the weld zones were determined, and these values were
compared to published strength knockdowns. The weld strength of the material was also compared to
the material strength of the control samples. The weld quality was evaluated in order to determine the
strength RPG welded joints should be designed for.
RESULTS
Prior to testing, the outer diameter above and below weld zones were taken for all samples. The control
samples were scored in the middle; only one measurement was made for these samples.
Table 1: Measurements made from untested tubes.
Region Measurements (in)
Type of Weld Sample #1 #2
Fuse Weld
f1 0.5000 0.4995
f2 0.5000 0.5002
f3 0.5000 0.4950
V-Groove
v1 0.5020 0.5025
v2 0.4995 0.5010
v3 0.5030 0.4990
v4 0.5000 0.5025
v5 0.5000 0.5005
v6 0.4995 0.5050
No Weld
c1 0.4835 -
c2 0.4880 -
c3 0.4885 -
Raw data from the Instron machine was collected in stress over time. Strain measurements could not be
made for tube samples because the Instron is only set up to read strain from flat dogbone samples.
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Figure 5: Ultimate tensile strength of samples as determined from the raw stress curves obtained during Instron testing.
Figure 6: Fracture strength of samples as determined from the raw stress curves obtained during Instron testing.
The location of failure was also noted in all stress tested samples. For the V-Groove, the point of failure
did not occur at the weld zone except for v5; the Fused sampled failed at the weld zone and at very low
applied force; the control samples failed in the scored area as expected.
0
5
10
15
20
25
0 2 4 6 8 10
Ult
imat
e T
en
sile
Str
en
gth
(kN
)
Sample
Ultimate Tensile Strength of Samples
No Weld
V Groove
Fused
0
2
4
6
8
10
12
14
16
18
20
0 2 4 6 8 10
Frac
ture
Str
en
gth
(kN
)
Sample
Fracture Strength of Samples
No Weld
V Groove
Fused
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Figure 7: Different points of failure on the tested samples. (Leftmost and Middle) V-Groove samples; (Rightmost) Fused
sample. Failures at the weld were only present in the fused samples.
Measurements were made for all samples that did not fail as expected at the weld.
Table 2: Measurements of point of failure from the midpoint of the weld for the V-Groove welded tubes
Sample v1 v2 v3 v4 v5
Distance (in) 0.9205 0.8350 1.0290 0.8520 0.4305
Polished and etched samples were viewed under a digital camera microscope. Images of interest
started away from the weld zone and moved into the weld zone.
Figure 8: Series of Fuse weld images under a digital camera microscope. Scale to 0.01 mm as indicated. (From Left to Right)
Region away from the weld zone; Region approaching the weld zone; Region at the weld line; Inside the weld zone.
Figure 9: Series of V-Groove weld images under a digital camera microscope. Scale to 0.01 mm as indicated. (From Left to
Right) Region away from the weld zone; Region approaching the weld zone; Region at the weld line; Inside the weld zone.
The breaks occurred between the first two images on the left.
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Figure 10: Series of no weld images under a digital camera microscope. Scale to 0.01 mm as indicated. (From Left to Right)
Region starting at one end of the score moving towards the opposite end in equidistant steps.
Grains size across the samples appeared to be roughly the same. The presence of bubbles throughout
the samples made it difficult to locate a region of pure grains; this was particularly an issue in the
control tube. The grain size does not appear to change as the images approach the weld line and
become indistinguishable passed the line in the Fuse and V-Groove tubes. In the no weld tubes, no
change in the grain size or presence was observed.
In Knoop microhardness testing, it was discovered that the metals were hardest at the weld with
hardness varying with an overall downwards trend as testing moved away from the weld. The not
welded sample had insignificant variance in hardness along its length as expected.
For all hardness tests, the point denoted at the zero distance is located within the weld zone. The
second set of points is located at the transition line into the weld zone, and every point thereafter is a 2
mm shift from the previous moving away from the weld. The overall decrease in hardness is shown by
the solid trend lines. There was, however, significant variance in the harness over this decrease, as
shown by the dotted trend lines.
Figure 11: Hardness data collected from the Knoop microhardness testing for the cut fused sample. The overall decrease is
represented by the trend line (Hk.3) = -0.9635x + 79.37 , where x is the distance in mm; R² = 0.2115.
0
20
40
60
80
100
120
140
0 5 10 15 20
Har
dn
ess
(Hk
.3)
Distance (mm)
Fused (Sample f3)
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Figure 12: Hardness data collected from the Knoop microhardness testing for the V-Groove sample. The overall decrease is
represented by the trend line (Hk.3) = -1.1203x + 76.759 , where x is the distance in mm; R² = 0.3127.
Figure 13: Hardness data collected from the Knoop microhardness testing for the no weld sample. The overall decrease is
represented by the trend line (Hk.3) = -0.0141x + 89.905 , where x is the distance in mm; R² = 0.0001.
Though harness varied from 40-100 Hk .3 in the V-weld and 50-120 Hk .3 in the fused weld, both welded
samples showed an average hardness drop of 20 across the 18mm tested length.
DISCUSSION AND ANALYSIS
The breaking of the control samples at the scored region was expected; the control samples were of the
same material and treatment along its length and therefore had the same fracture strength throughout.
By decreasing the cross sectional area of the sample via scoring, following Equation 1 and keeping the
same stress σ, the force F required is lower. The Fused samples were bonded weakly at the welded
region and failed there at low applied loads (5 kN compared to the 20 kN load for the control samples)
as expected.
The V-Groove samples failed the strength test outside of the weld zone at an average of 0.81 in away.
This suggested that the material strength was either unaffected or improved in the welded region.
0
20
40
60
80
100
120
0 5 10 15 20
Har
dn
ess
(Hk
.3)
Distance(mm)
V Groove (Sample v6)
0
20
40
60
80
100
120
0 5 10 15 20
Har
dn
ess
(Hk
.3)
Distance (mm)
No weld
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However, the fracture strength for the V-Groove samples were lower than the fracture strength of the
control samples by an average of 6 kN. This results suggest that the material in the welded samples
were weaker than standard material outside the weld. This could be explained by the varying tempers
in the material as shown in Figure 1.
Etching was difficult for these samples due to the impurities present in the stock aluminum used as
samples. The etching process resulted in a large number of bubbles being present in the final product.
These bubbles made it difficult to see complete grains. The trade off with etching was that the longer
the etching process lasted, the more clearly defined the grain boundaries would be but the number of
bubbles present in the samples would also increase. The amount of bubbles were greatest in the
control sample.
Grain sizes did not change outside of the welded region. In addition, grain sizes for all sample types
appeared to be similar. This suggested that there was no significant difference in the aluminum strength
itself outside of the weld zone for the fused and control samples.
Knoop Microhardness test results showed that there was a downwards trend in hardness as the material
moved further from the weld and that this trend was the same in both the Fused and V-Groove cases.
The control hardness was steady at roughly 90 Hk while the welded samples both ranged from about 80
– 60 Hk over the same distance. Where the breaks occurred in the V-Groove samples, the hardness was
around 60-70 Hk. 6061-T6 has a nominal hardness of 120 Hk, while untempered 6061 has about 40-50
Hk.
In the V-Groove sample, the hardness at the points furthest away from the weld increased in hardness.
This is therefore the end of the weld zone at around 15mm or 0.6 inches. From the theory, the samples
would be expected to break inside of the weld zone (so within 0.6 inches on either side of the weld).
However, only one of the v groove samples actually broke inside of 0.6 inches. This is quite puzzling,
since it appears as though the samples broke in the “clean” material. Unfortunately the sample length
for the Knoop hardness test was not long enough to verify that the hardness levels would return to T6
material conditions, so it can only be postulated that the material returns to T6 properties past the area
analyzed for hardness.
To check to see if the yielding caused by the clamps created stress concentrations in the tubes, an FEA
analysis of the tube constrained as it would be in the Instron machine is shown below. On the right side,
the tube has significant clamp forces applied, and on the left side there are only tensile forces applied.
There is some increased stress in the areas near the break (red jagged line), but nothing that is the
smoking gun as to why failure occurred there.
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Figure 14: FEA analysis of welded tube.
Given the hardness results and the grain images, it would follow that had the fused samples not failed
so quickly at the weld zone, the regions outside the weld zone would be as strong as the V-Groove
samples.
CONCLUSION
The experiment accomplished its purpose of providing more information about the strength of welds in
aluminum. It can be said for sure that welding weakens the structure, and that fuse welding is much
weaker than V-groove welding. However, a conclusive result as to why the V-groove samples failed
where they did was not ascertained.
The control samples failed at loads between 45,000 and 55,000 psi, which is on the upper range of
ultimate tensile strength
Based on the results, fuse welding should generally be avoided. The fused samples were only able to
hold about 1/4 to 1/3 of the load of the control samples, making them virtually unusable for structural
applications.
The v-groove welded samples consistently broke at 2/3 of the maximum load of the control samples,
with the weakest breaking at 1/2 of the maximum load of the control samples. Thus based on the data
set of this project alone, welded parts should only be designed for stresses of ½ the normal yield
strength. The problem is that based on the data, it is not well understood how far away from the weld
material properties are affected and thus how far away from a weld normal 6061-T6 material properties
can be assumed.
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The greatest unknown in the results is why the v-groove samples broke where they did on an Instron,
about halfway between the clamp and the weld. By theory, the material properties should be at full
strength in the area where the samples consistently broke. By hardness analysis, the material should be
close to T6 hardness in the fracture zone. Etching is inconclusive. FEA is also inconclusive but shows
possible slightly higher stresses in the fracture zone. The most plausible explanation is that the heat
from the welds affected the samples all the way up to the fracture zones, but in a very subtle way that
doesn’t show up in the microstructure analysis.
The most likely source of error on the samples is that the person who welded them was undergoing
welding training at the time, so they might not be of the same level of quality as actual rocket flight
hardware welds. For the Instron testing, the lack of useable strain data means that the only point of
comparison is the peak load and the fracture loads. Strain data would provide more information about
the stress states in the material and drive down error. Hardness testing is limited by the sample size-
more samples would average out the variations seen in the hardness data. Etching produced very
inconclusive results because each of the samples came out slightly different, and the multitude of
bubbles obscured many of the grains, making it difficult to determine grain size.
To improve this project, bigger samples should be tested. This would make the welds more
representative of actual geometries and eliminate concerns about scaling. Fiber optic strain
measurement sensors could be embedded axially in the samples so that the strain across the entire
length of the sample is known to check for stress concentrations. A more extensive hardness test matrix
of perhaps the entire length of the sample would also yield more data for comparison.
Overall, this project does provide more insight into how welds affect 6061 aluminum samples, but the
results are too inconclusive to base design decisions on the data. It would be very difficult to predict
where zones with lower material properties would be in welded parts, so part designs have to remain
very conservative. However, an overall strength knockdown of 1/2 to 1/3 yield strength for part design
seems appropriate.
ACKNOWLDGEMENTS
Kara Mogensen. Much love.
WORKS CITED
Callister, William D., and David G. Rethwisch. Materials Science and Engineering: An Introduction.
Hoboken, NJ: John Wiley & Sons, 2010. Print.
"MatWeb - The Online Materials Information Resource." MatWeb - The Online Materials Information
Resource. N.p., n.d. Web. 27 Apr. 2014. <www.matweb.com>.