Weld Defects and Material Properties - Paper

Brett Leary
ME 296R Spring 2015
Weld Defects and Material Properties
The causes, effects, prevention methods, and detection
techniques for common weld defects, including inclusions,
cracks, and incomplete fusion/penetration.

1 ABSTRACT
The causes, effects, prevention methods, and detection techniques for common weld defects,
including inclusions, cracks, and incomplete fusion/penetration, are researched. No perfect
materials or flawless weld processes exist and engineers must evaluate the effects that defects
have on their designs. Global sources from general weld experts, as well as aerospace and
marine applications, are consulted, with emphasis on aluminum and steel welded joints. The
works examined are focused on gas metal arc (GMA), gas tungsten arc (GTA), electron beam
(EB), laser, and friction welding processes. Detection techniques are concentrated on non-
destructive test (NDT) methods. Discussion includes purpose, findings, and conclusions.
1.1 KEYWORDS
Weld defects; Inclusions; Porosity; Slag; Cracks; Incomplete fusion; Incomplete penetration;
Effect on material properties; Weld defect detection
2 INTRODUCTION
2.1 MOTIVATION TO RESEARCH
Working at Tri-Tool Inc. as an engineering assistant, I was paired with the welder to gain
practical, hands-on education with welding, pipefitting, machining, and other manufacturing
processes. Mike worked all over the world on oil, gas, and nuclear construction projects and
taught me the lessons he learned in those environments. This was my first exposure to weld
processes.
My professional background today, lies in the aerospace industry at Aerojet Rocketdyne.
Welded joints are critical in many rocket designs, as they are in the oil, gas, and nuclear
construction businesses, though the materials are bit different. As quality engineer, I have a
reasonable understanding of how weld defects relate to material properties, I am by no means
an expert on the subject. I rely on weld, materials, and NDT engineers for their expertise. This
research affords me the opportunity to learn more about these topics.

2.2 RELEVANCE TO MATERIALS AND MECHANICAL ENGINEERING
In the absence of perfect materials and flawless weld processes, engineers must evaluate the
effects that defects have on their designs. Often these are dealt with in the design phase with a
safety factor intended to cover any anomalies or discontinuities, but for some products, that
just is not good enough. Weld defect allowance requirements are absolutely necessary to
maintain the integrity of certain tools, pipelines, wings, or pressure vessels. Failure of such
products presents a safety risk. Engineers may be trying to stretch the limits to maintain weight
or other requirements.
Understanding the causes, effects, prevention methods, and detection techniques for some of
the most common weld defects, helps engineers working to more strict design conditions. Not
simply for special cases, this knowledge aids engineers in selecting appropriate weld processes,
acceptable joint design, and interpretation of weld failures.
3 TYPES OF WELD DEFECTS
3.1 INCLUSIONS (POROSITY, SLAG, AND TUNGSTEN)
3.1.1 TYPES AND CAUSES
Porosity describes the gas holes or voids found within a weld, as shown in Figure 1Error!
Reference source not found.. These pores are caused by gases, which are lighter than the
molten metal, that do not escape from the weld as it cools and are trapped within the weld as it
solidifies. According to the American Weld Society, the root causes for porosity are dirt and
moisture on the base metal, in the electrodes and gases, and in the weld equipment [1]. The
oxide layer on the surfaces of aluminum alloys also contributes to porosity defects. The risk of
porosity is largest for manual metal arc weld processes with coated electrodes due to the
moisture in the coating and thicker oxide layers in the weld wire [2]. There are four main
categories of porosity: uniformly scattered, clustered, linear, and worm [1].

Slag describes oxides and non-metallic solids trapped within a weld. Slag forms from chemical
reactions that occur between the metal and the electrode, when not fully dissolved into the
molten metal. This slag can lead to more porosity within the weld [1].
For gas tungsten arc welding (GTAW), a non-consumable tungsten electrode creates the weld
arc between the tungsten and the part. Incidental contact between the electrode and the
molten metal may transfer tungsten particles into the weld.
3.1.2 EFFECTS
These inclusions negatively impact the material by acting as initiators for cracks that rapidly
lead to fatigue failures [5]. Pores nearest the surface of the weld are expected to initiate fatigue
cracks before pores further from the surface. “As this crack becomes the dominant singularity,
no other cracks initiate” [3]. “Excessive concentration near one surface of the bead (most likely
the top) can impose a bending component which amplifies the stress” [4]. Pores can join
together to increase the stress concentration, from one pore to another, as the crack
propagates; see Figure 2. Clustered pores can grow together to behave like a single larger pore.
Increased total area of the pores relative to the weld cross section reduces strength; many
Figure 1 - Weld Porosity [1]

small pores have the same effect as a few large pores with the same total area [4]. According to
a marine weld study, “the single pore (is the) least detrimental in fatigue followed by co-linear
porosity, and uniform porosity. Cluster porosity is predicted to be most detrimental” [3]. In
static tensile tests, porosity affects “fillet welds far more adversely than butt welds,” reducing
tensile strength by up to 60% [2]. The yield strength of the material is insignificantly affected by
pores in the weld, while the ultimate tensile strength is reduced by increased porosity, as
shown in Figure 3Error! Reference source not found. [5]. “Ductility of the aluminium weld
metal is sharply reduced by the presence of small amounts of porosity” [5]. Slag and tungsten
inclusions are often evaluated in the same manner as the more common porosity defects [1].
According to Thielsch, static tensile and fatigue tests on aluminum alloys indicated that
specimens with substantial tungsten inclusions did not cause any failures of the joints [2].
Rework of inclusions can be a time consuming, expensive process. When the weld contains a
considerable number of pores with respect to the size of the weld, due to contamination, no
amount of rework may be able to correct them.
Figure 2 - Clustered Porosity Grows to Behave Like a Single Pore [2]

3.1.3 PREVENTION
Prevention of porosity depends somewhat on the weld process being utilized, but for all of
them, the cleanliness of the base metal, shielding gas, and filler metal are critical. Hydrogen
porosity can form from moisture in the air. Aluminum is especially susceptible to oxides on the
part surface, which must be etched or scraped to remove them. If the shielding gas flow rate is
too high or low, then porosity can result [5]. The filler metal must be stored properly to prevent
contamination by dust and dirt [7]. Store at the same temperature as the weld cell to prevent
condensation from any temperature change prior to weld [7]. The welder and fitter should
wear gloves to inhibit potential dirt, oils, and moisture from touching the part or filler wire [7].
Certain weld processes contain greater risk for porosity, slag, and tungsten than others. There is
a greater likelihood to find porosity and tungsten in a manual GTAW over an automated
electron beam or laser weld. Aluminum GTAWs often look like Swiss cheese when sectioned as
they contain many small pores; aluminum EB welds often have many fewer pores, but the
pores they do contain tend to be larger in size than the average pore in the aluminum GTAW.
3.1.4 DETECTION
Radiography techniques including x-ray and gamma can detect inclusions below the surface of
the weld. Traditional 2D shots make it difficult to realize the spacing between pores that are
actually in different planes within the weld. With computed tomography (CT), the weld can be
Figure 3 - Stress Strain Curve for Aluminum Tensile Samples with Porosity [5]

rotated to more clearly understand the position of the pores within the weld. Tungsten is easily
spotted and will illuminate bright white within the weld. These methods are more expensive
than other methods, but provide a permanent record of the weld inspection that be review
later as necessary. Ultrasonic testing will pick up slag and tungsten, but is not useful in finding
pores. Visual inspection, liquid penetrant, and eddy current can detect surface discontinuities.
These methods are inexpensive compared to radiography do not typically provide a permanent
record that can be re-checked later. Where possible a proof and leak test of the weld joint can
detect inclusions that extend through the weld from the inner to the outer surface [1].
3.2 CRACKS
Cracks describes a broad spectrum of indications that “results from localized stress that at some
point exceeds the ultimate strength of the material.” The residual stresses, caused by the weld
joining process, due to shrinkage, create these cracks. Cold cracks refers to cracks that occur
below 400F (usually at room temperature) and hot cracks refers to cracks that occur above
400F [2]. “With narrow, deep grooves a crystallisation takes place (on) all sides of the bead,
entrapping the remaining melt in the bead centre. With the occurrence of shrinking stresses,
hot cracks may develop” [8]. The chemical composition of the metals can lead to an increased
cold crack susceptibility [8]. Some alloys contain sulfur or phosphorus that cause solidification
cracks and some alloys have a high solidification temperature that contributes to cracking [6].
The weld joint design and weld method are also contributors to weld cracks [6]. Hydrogen
within the weld filler and humidity around the weld joint increase the risk of cold weld cracks;
the longer the materials are stored, the higher the potential water content of the electrode
coatings [8]. “Cracking is more likely to occur if the metal is either hard or brittle,” while a
ductile material may better withstand the stress concentrations, because it can yield locally [1].
There are two main categories of cracks described in the AWS weld inspection handbook (all
cracks noted in Figure 4):

The first category is weld metal cracking with three types of cracks. Transverse cracks occur
perpendicular to the weld axis and more likely to occur when the weldment is highly restrained.
Longitudinal cracks form most often at the center of the weld. These may initiate during the
weld process and propagate out to the surface as the weld cools. Crater cracks can develop at
the start and stop locations of the weld bead. They are typically star-shaped and may be
initiation points for longitudinal cracks [1].
The second category is base metal cracking with two types of cracks. Transverse cracks occur
perpendicular to the weld axis, occurring more regularly with high strength steels. Longitudinal
cracks run parallel to the weld are more commonly found than transverse cracks. “For fillet
welds, longitudinal base metal cracks may be divided into two types: (a) toe cracks, which
proceed from the toe of the fillet weld into the base metal, (b) root cracks, which proceed from
the root of the fillet weld and progress into the base metal” [1]. The root cracks can sometimes
be found on the opposite side from where welding occurred. Base metal cracking occurs when
the weld and base metal cool quickly due to temperature of parts, processes, and environment.
Arc strike cracks are a little different. They are formed by an unintentional arc outside of the
weld area, usually an error by the weld operator. They occur as the localized area is rapidly
heated, melted, and cooled, causing localized embrittlement leading to a crack [1].
Figure 4 - Types of Weld Cracks [8]

3.2.2 EFFECTS
These cracks create stress concentrations in the weld and in the base metal. The significance of
those cracks certainly depends on their type, size, and location. In addition to typical fatigue
crack propagation issues, the weaker heat affected zone properties may accelerate the growth
compared to a crack in the base metal outside the heat affected zone [1]. Stress corrosion
cracking (SCC) may also accelerate from weld cracks on high alloy metals at the weld toe [10].
Microcracks that were 5% of the cross section of a 2 3/8” wide by 1/2” thick fatigue test sample
reduced the fatigue strength in 10 million cycle by about 60% [2]. If an improper weld filler is
chosen relative to the base metal, the mismatch of the properties can drive cracks more rapidly
in fatigue [7]. “Solidification cracking susceptibility is exacerbated if there is mid-section bulging
in the weld cross-section profile due to locally increased strain” [6]. Rework of cracks, like
inclusions, can be a time consuming, expensive process. For particularly large or deep cracks, no
amount of rework may be able to correct them.
Figure 5 - Weld Cracks Create Stress Concentrations [1]

3.2.3 PREVENTION
The risk of formation of weld metal cracks can be reduced by adjusting the electrode position
and movement or electrical conditions, decreasing the electrode travel speed, preheating the
parts to reduce thermal stresses, sequencing the welds to balance the unintended shrinkage
loads, and avoiding conditions that allow rapid cooling [1]. When the welder must compensate
for a large weld groove, widening the weld to fuse the metal, the likelihood of cracks in the
weld. “Proper part fit-up and good joint design are both key in preventing” cracks. For the weld
joint, “make the depth (ratio) 5:1 to 2:1 the size of the width” [7]. “Post-weld heat treatment
should be regarded as essential to obtain maximum SCC resistance from welded joints in high
strength steels” [10].
For base metal cracking, “in the case of low carbon, medium carbon, and low alloy steels,
hardness and the ability to deform without rupture” depend on the alloy composition and their
rate of cooling after the weld. Generally, the temperature inputs during weld can be controlled
better with automated versus manual weld processes. The base metal can be preheated and/or
the local temperature of the environment surrounding the weld can be raised to decrease the
cooling rate [1]. A thick base material tends to increase the rate of cooling and should be
avoided if possible [1]. The addition of copper or other heat sinks that increase the rate of
cooling can lead to cracks. High alloy steels are more susceptible to base metal weld cracks due
to hardening and/or embrittlement during cooling. Hardenable steels have “variations in the
microstructure of the heat affected zone (cause differences in mechanical properties), which
can occur with variations in the cooling rate” [1]. For hardenable steels, the AWS weld
inspection handbook, recommends preheating the base metal, controlling the heat input during
weld, and ensuring the correct electrode and weld materials are used [1]. Filler metal and base
metal should have the same or similar compositions; differences increase the risk of cracks in at
the edge of welds [1]. Hobart Brothers recommends filler wire with a low hydrogen content
where possible. Filler metals that “contain high levels of hydrogen scavengers, including
fluoride, sodium and calcium that can combine with hydrogen to remove it from a cooling
weld” can reduce the risk or cracks [9].

3.2.4 DETECTION
Visual inspection, liquid penetrant, magnetic particle, ultrasonic, and eddy current can detect
surface cracks easily; however, the depth/severity of the crack cannot be assessed clearly.
These methods do not typically provide a permanent record that can be re-checked later.
Where possible a proof and leak test of the weld joint can detect cracks that extend through
the weld from the inner to the outer surface. Radiography techniques including x-ray and
gamma can detect subsurface cracks in the weld. Traditional 2D shots make it difficult to realize
the full dimensions of the crack within the weld. With computed tomography (CT), the weld can
be rotated to more clearly understand the dimensions of the crack within the weld [1].
3.3 INCOMPLETE FUSION/PENETRATION
Incomplete fusion describes “the failure to fuse together adjacent layers of weld metal or weld
metal and base metal” occurring “at any point in the welding groove” [1]. The part may not
locally fuse because the temperature of the weld process was insufficient to cause the surface
of base metal to melt. This lack of fusion may be caused by a layer of oxides or dirt not removed
during cleaning, that does not adequately dissolve when heated during the weld [1]. This
phenomena is often confused with incomplete penetration. Compare the incomplete fusion
defect examples shown in Figure 6 to the incomplete pentration examples in Figure 7.
Figure 6 - Lack of Fusion Defect Examples [8]

Incomplete penetration “describes the failure of the filler metal and base metal, or the base
metal alone if no filler metal is used, to completely fill the root of the weld” [1]. There are two
main causes: failure of the weld to melt to the full depth desired of the weld, insufficient filler
metal to reach the root of the weld joint [1]. However, like lack of fusion defects, a layer of
oxides or dirt may prevent full penetration of the weld, though this is less common [1].
3.3.2 EFFECTS
Incomplete fusions of the whole length joint or most of the joint means that the part is not
fused together and may be obvious if the part is handled after weld. The part may behave as if
it were never welded at all. For localized lack of fusion, the strength is reduced because the
metal is not fully fused as intended for the design [1]. Service failures due to lack of fusion are
Figure 7 - Lack of Penetration Examples [1]

not common; in such cases, the incomplete fusion more than 10-30% of the effective wall
thickness [2].
Incomplete penetration creates detrimental stress concentrations as the parts are not welded
at the root of the weld. These are affected by tension and bending loads. Shrinkage and
distortion can lead to cracks in areas of the weld bead with inadequate penetration [1]. The
cracks add to the stress concentration that already exists from the lack of penetration on the
back side of the joint. Over time these can cause fatigue failures [1]. “Mechanical fatigue tests
on pipe butt welds with lack of penetration on the inside surface” reduce fatigue strength by
more than 50% [2].
3.3.3 PREVENTION
Melt rate and weld travel speed must be properly balanced or the risk of inadequate fusion
increases. For example, if the melt rate is slow and the travel speed is fast, then incomplete
fusion will occur due to low performance, but if the melt rate is fast and travel speed is slow,
then incomplete fusion will occur due to poor pre-flow. The orientation of the weld head is
crucial to prevent lack of fusion. Lack of fusion can be reduced with a 45 degree angle of the
weld head relative to the parts for fillets [8].
The melt rate versus the weld speed also plays the same role in incomplete penetration defect.
The two must be balanced properly to achieve melt through the entire weld depth desired and
to allow the filler metal to adequately fill the root of the weld joint. The orientation of the weld
head is also critical to prevent lack of penetration. Incomplete penetration can be reduced with
a 90 degree angle of the weld head relative to the parts for butt joints [8].
3.3.4 DETECTION
Sectioned weld samples can verify the weld parameters before parts are weld to ensure there is
no lack of fusion or penetration. Visual inspection, liquid penetrant, ultrasonic, and eddy
current can detect lack of fusion, like they do for surface cracks. Radiography techniques
including x-ray and gamma can detect lack of penetration in the weld, but are do not work well
to find lack of fusion [1].

4 CONCLUSIONS
The causes of inclusions, cracks, and incomplete fusion/penetration can be attributed to
process conditions (like cleanliness, environment, and materials chosen for weld), improper
welder inputs or techniques, or poor joint design.
Though weld defects occur within all types of materials and all types of weld processes, each
has trade-offs that must be recognized. Manual GTAW of aluminum welds will introduce many
small pores to the weld and EB of aluminum welds will reduce the total number of pores, but
they are likely to be larger in size. Nevertheless, defects affect the material properties similarly.
One of the most common effects is the introduction of stress concentrations that reduce
strength. These risers play a significant role in fatigue failures of welded joints. Porosity does
not significantly impact the yield strength of the material. The weld cannot be made perfect,
but good cleaning techniques, correct weld parameters, and proper joint design, can
appreciably reduce the risks of weld defects and their negative effects. The methods for
detection of these weld defects are fairly similar, though some work better for finding certain
defects. This knowledge can be adapted to select only the inspection necessary for a design.
Note that the companion PowerPoint presentation contains additional discussion and images
related to porosity types, causes, effects, prevention, and inspection.
5 REFERENCES
[1] Welding inspection (2nd ed., pp. 44, 47-48, 50-53, 56, 113-121). (1980). Miami, Florida:
American Welding Society.
[2] Thielsch, H. (1967). The Sense and Nonsense of Weld Defects (1st ed., pp. 17-18, 21, 23, 25,
37, 46). Lake Zurich, Illinois: Monticello Books.
[3] Walsh, W., Leis, B., & Yung, J. (1989). Study to Determine the Influence of Weld Porosity on
the Integrity of Marine Structures. SSC-334. Retrieved April 24, 2014, from
http://www.shipstructure.org/pdf/334.pdf

[4] Rudy, J., & Rupert, E. (1970, July). Effects of Porosity on Mechanical Properties of Aluminium
Welds. Welding Journal, 322-S. Retrieved April 24, 2015, from
https://app.aws.org/wj/supplement/WJ_1970_07_s322.pdf
[5] Shore, R. & McCaulay R. (1970, June). Effects of Porosity on High Strength Aluminium 7039.
Welding Research Supplement, 311-s. Retrieved April 24, 2015, from
[6] FAQ: What are the typical defects in laser welds? (n.d.). Retrieved April 24, 2015, from
http://www.twi-global.com/technical-knowledge/faqs/structural-integrity-faqs/faq-what-are-
the-typical-defects-in-laser-welds/
[7] 6 Key Ways to Prevent Weld Failures. (2012, March 7). Retrieved April 24, 2015, from
http://www.hobartbrothers.com/news/111/523/6-Key-Ways-to-Prevent-Weld-Failures.html
[8] Dilthey, U. (2005, January 1). Welding Technology 2 - Welding Metallurgy. Retrieved April
24, 2015, from http://www4.hcmut.edu.vn/~dantn/course/Pdf/WeldingTechnology2-
English.pdf
[9] Understanding Weld Cracking, Its Causes, Consequences and Remedies. (2009, September
9). Retrieved April 24, 2015, from
http://www.hobartbrothers.com/news/48/523/Understanding-Weld-Cracking-Its-Causes-
Consequences-and-Remedies.html
[10] Gooch, T. (1974, July). Stress Corrosion Cracking of Welded Joints in High Strength Steels.
Welding Research Supplement, 287-s. Retrieved April 24, 2015, from

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