Strength and infrared assessment of spot-welded sheets on ferrite steel
Ding Min
a,b,⇑, Liu Shi-sheng
a, Hao Hong
c, Peng Tao
d, Zhang Pei-lei
eaCollege of Material Science and Technology, Taiyuan University of Technology, Taiyuan 030024, China
bKey Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China
cSchool of Environment and Safety, Taiyuan University of Science and Technology, Taiyuan 030024, China
dWuHu Xinxing Ductile Iron Pipes Co., Ltd., Research Center, WuHu 241000, China
eSchool of Materials Engineering, Shanghai University of Engineering Science, Shanghai 201620, China
a r t i c l e i n f o
Article history:
Received 30 March 2013 Accepted 24 May 2013 Available online 5 June 2013
Keywords:
Resistance spot welding Ferrite steel
Nugget growth Infrared assessment
a b s t r a c t
This paper addresses the mechanical properties of ferrite steel resistance spot welds during quasi-static tensile test. The mechanical properties are described in terms of peak load. It was shown that the fusion zone size is the most important. The fusion zone size can control the solidification of the grain which con- trolling factor of spot weld peak load. The dendritic grain and equiaxial axial grains occurred in the micro- structures of the welded specimens joined at various welding currents and electrode forces. The failure mechanism of resistance spot welds during tensile test was studied with the aid of thermography. The thermography gives visible data of temperature changes on the surface of specimen. In light of the failure mechanism, the simple model is proposed to ensure pull out failure mode.
Crown CopyrightÓ2013 Published by Elsevier Ltd.
1. Introduction
Ferritic stainless steels (FSS) are used as material for production of home appliances, furniture, laboratory equipment, where the material does not come into the contact with the aggressive media and where welded joints are not dynamically loaded[1]. The FSS have a ferritic structure during solidification without any transfor- mation in the entire temperature range. These steels are consid- ered as difficult-to-weld due to their grain growth [2–9]. A crucial issue in this regard is an industrial joining means of FSS components, such as resistance spot welding for joining sheet materials[10]. Resistance spot welding has many advantages over other joining means, and a significant amount of knowledge has been accumulated by the industry and academia on welding con- ventional sheet materials. However, such knowledge on resistance welding materials cannot be directly applied to welding ferrite steel.
Resistance spot welding is a welding technique that joins two or more metal sheets through fusion at the contact area of electrode tips[11]. This process basically uses two copper electrodes to com- press the sheets together and supplies huge amount of current (typically 15 kA) through the contact area of electrodes. The flow
of current against the base metal resistance causes heat develop- ment between the sheets and gradually melts the concerned areas.
Once the current flow is stopped the melted area will be hardened, then. The melted and solidified areas of base metals are thereafter called as weld nuggets and it consists of three zones. They are named as fusion zone (FZ), heat affected zone (HAZ) and base met- als (BM). The proper joints between sheets are usually created at the fusion zone due to the thermal expansion of materials. The fol- lowing, heat affected zones are appeared due to the thermal con- ductivity (54 W m 1K 1min) of base metals and the other part of base metals remained unchanged. The weld nugget’s growths are therefore determined by the basic controlling parameters; pri- marily the welding current, welding time, electrode pressing force and welding energy[12–14]. Relatively few investigations were carried out to determine the effect of spot welding on microstruc- tures and mechanical properties of ferrite steels.
Generally, there are three measures for mechanical qualities evaluation of resistance spot welds including physical weld attri- butes (e.g. weld nugget size, electrode indentation, etc.) [11], mechanical properties[12]and failure mode[13]. Strength assess- ment can be the link that connecting physical weld attributes mechanical properties and failure mode. Failure mode of the shear can reflect the strength easily. Failure of spot welds may affect the vehicle’s stiffness and noise, vibration, and hardness performance.
The failure characteristics of spot welds are very important param- eters. In the interfacial mode[13], failure occurs via crack propaga- tion through fusion zone; while, the other one, failure occurs via nugget withdrawal from one of the sheets. The failure mode under which RSWs fail, can significantly affect their carrying load capac- 0261-3069 Crown CopyrightÓ2013 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.matdes.2013.05.076
⇑ Corresponding author at: College of Material Science and Technology, Taiyuan University of Technology, Taiyuan 030024, China. Tel./fax: +86 03516010076.
E-mail addresses:[email protected],[email protected](D. Min).
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ity and energy absorption capability. It is believed that vehicle crashworthiness, the main concern in the automotive design, can be dramatically reduced if spot welds fail via the interfacial mode.
Interfacial weld failures at low loads can adversely affect the load distribution, causing buckling and reducing crushing energy absorption of structural members. As a result, it is needed to adjust welding parameters so that the PF mode is guaranteed[13]. Many researchers studied the characteristics by analytical calculation, numerical simulation, and metallurgy. Chao based on the competi- tion between shear plastic deformation in nugget circumference (i.e., nugget pullout) and crack propagation in weld nugget (i.e., interfacial failure mode), derived an equation for critical weld nug- get size (DC) in the cross-tension (TS) test. Although his model re- lates the critical weld size to fracture toughness of the weld nugget and fracture strength in shear of the HAZ, he tried to show that his model is not material dependent. In fact, the suggested formula for cross-tension loading is variational with material. Although mea- surement of tensile shear strength and nugget diameter is widely used, another method which is more efficient is required for time and cost savings in practice.
The investigation on the effects of parameters on microstruc- tures and mechanical properties of spot welding ferritic steel is very limited. Moreover, the strength mechanisms of the nugget are still an open question. The objective of the research is the fail- ure characteristics of spot weld. The infrared thermography is a non-intrusive instrument. Indeed, it forms part of the techniques of non-destructive testing and can be carried out on installations in service. Therefore, in this article, Infrared camera was selected to investigate the mechanical properties of spot welding ferrite steel joints.
2. Experimentation
The base metals were rectangular in shape with equal size (60 mm10 mm0.4 mm) as shown in Fig. 1and its chemical properties are tabulated inTables 1 and 2.
A pair of water cooled copper electrodes with tip diameters of 4 mm was used to join these base metals. The test samples were initially placed on the top of lower electrode (tip) of the welder as overlaying 10 mm on each other and then the initiating pedal was pressed. The weld process was started right after with squeez- ing cycles and; once the squeezing force is reached the welding current is delivered in accordance with the given preset values.
There after the electrode pressing mechanism (pneumatic based) consumes some time for cold work and eventually return to the home position of electrode. These process controlling parameters (Table 3) are set before the welding process starts. Based on some combination of values; a welding schedule was developed to con- duct the entire experiment to understand the basic parameter changes that cause the weld growth in 0.4 mm ferrite steel.
The welded samples of base metals were undergone common strength tests that of the tensile shear tests according to ASTM:
E8 M-11 in this experiment. InfraTec VarioCAM hr, FLIR Systems, was used for the thermograms recordings. The camera resolution is 384288 pixels. Camera was positioned at a distance of 0.5 m from the surface of the sample. The sensitivity of the camera is 0.08k at 30°C, the field of view is 2418, minimum focus distance
is 0.3 m, the spatial resolution is 0.65 mrad, recording frequency 50 Hz, the electronic 1–8zoom continuously. The hardness test was carried out to understand the hardness changes.
The results of these tests were insufficient to understand the nuggets characteristic and therefore the metallurgical study was carried out to complete the analysis. Samples for metallographic examination were prepared using standard metallography proce- dure. Optical microscopy was used to examine the macrostructures and microstructures and to measure the weld fusion zone (weld nugget) size. A microhardness test was used to determine the hardness profile of the joints, using a 1000 g load on a microhard- ness tester. The microhardness traverses were performed on a diagonal covering microstructural zones in both sheets.
3. Results and discussions 3.1. Microstructure
Fig. 2shows the dendritic grain and equiaxial axial grains oc- curred in the microstructures of the welded specimens joined at various welding time and electrode forces. BM microstructure in Fig. 2a shows the typical finely dispersed particles (white) sur- rounded by a ferrite matrix (grey), which are characteristic of fer- rite steels. The CG–HAZ microstructure consists of blocky ferrite grains about 35 to 50
l
m in diameter as shown inFig. 2. The FZ shown in figure is characterized by the columnar nature of solidi- fication mainly consisting of ferrite.It is seen fromFig. 2that colour difference is seen among the grains due to intergranular orientation and that the structure con- sists of columnar grains. Also crack and porosity were not observed around to weld. The thermal conductivity of ferritic stainless steel is being higher, and the heat flow in the direction perpendicular to the weld would be maximum. Therefore, the grains tend to grow in that direction, giving rise to columnar grains. Axial grains start in the center regions grow along the length of the weld, blocking the columnar grains growing from the interface.
Why will appear this kind of metallographic structure? Each point of the welding metal at a given distance from the nugget cen- ter line experiences a different peak temperature and cooling rate.
As the distance from the weld center line increase, the peak tem- perature and cooling rate decrease, which influences the micro- structure growth. The problem of extensive grain coarsening and associated brittleness often cited in the welding of ferritic stainless steels by traditional arc processes, which is not seen in spot weld- ing. The phenomenon is due to the relatively high cool rate associ- ated with spot welding. During cooling through the austenite formation ranges, a considerable amount of austenite formed at the grain boundaries, which are determined by the relative amount of ferrite and austenitic stabilizing elements and the cooling rate.
The cooling rate is very great, the austenite has not time to grow, which transformation to austenite was fairly suppressed leading to a predominantly ferritic structure. The steel is not enough al- Fig. 1.The rectangular in shape with equal size (60 mm10 mm0.4 mm).
Table 1
Chemical composition of TTS443, wt%.
C Si Mn P S Cr Cu Ti N
0.01 0.38 0.1 60.02 60.002 21 0.4 0.3 0.012
Table 2
The properties of TTS443.
Rp0.2(MPa) Rm(MPa) A(%) (HV0.1)
330 480 33 153
loyed for the austenite to remain stable down to room temperature.
WhenFig. 2is examined, it is seen that the microstructures of all the welded joins are considerably different from those of the base metal and that the grains were oriented towards heat source.
In addition, new grains were formed in the interface as the result of welding and these grains are seen to be smaller than the original grains due to heat input during the welding. As the thermal con- ductivity of steel is very low, the heat remains in the welding zone for longer period of time and this, in turn, leads to grain growth.
Heat energy used in welding process has a direct influence on the microstructures of both weld nugget and HAZ.
When the microstructure photos given inFig. 2obtained from the specimens joined at 3 KN electrode forces for 20–40 cycles welding cycle or 4 KA for 20–40 cycles welding cycle, it is seen that twins obtained for 40 cycles are more than that obtained for 20 cy- cles at 3 KN electrode forces. Why metallographic structure size can change? At low welding current and high electrode force, the nugget stays overheated. The molten metal in the nugget pool moves from the center out to the nugget edge, where it transmits a portion of its thermal energy. At the weld center temperature is higher than at the edge, so grains grow towards the nugget center.
At high welding current and low electrode force, a larger volume of the nugget pool is obtained. A high temperature maintained at weld pool sides slows down the grain growth from the base metal towards the center and promotes the formation of equiaxial grains at the nugget. The nugget pool starts solidifying in the nugget cen- ter before the grains from the sides reaches the nugget center.
With the resistance welding time increasing, the dimension of
the columnar grains and axial grains can increase. When the resis- tance welding time is less than 25 s, the growth amplitude is very big. The time is more than 25 s, the growth amplitude is slow. With increasing the clamping time, the dimension of the columnar grains and axial grains can increase firstly, and then decrease.
The resistance welding time and clamping time can influence the dimension of the columnar grains and axial grains. The grain can refine on condition of pressure. When the clamping time is less than 35 s, the affect of the resistance welding time can greater than that of the clamping time, so the grains grow. However, when the clamping time is more than 35 s, the pressure can crush grain boundary, the grains can decrease. At the upper and bottom of the nugget, the narrow liquid region near the pool tail would inhi- bit fluid flow there because of boundary layer effects, which may enhance the local solidification velocity. We can find the axial grains at the upper and bottom of the nugget.
3.2. Tensile test results
Hardness measurements were carried out on all the resistance- welded specimens in order to determine the hardness variations in the welding zone (base material, HAZ and weld nugget). The mea- surements were made in two directions; one along the radius of the nugget whilst the other along the sheet thickness. The hardness characteristic of resistance spot welds is one of the most important factors affecting the failure characteristics.
When the hardness values inFig. 3are examined, a decrease in the hardness value is seen as the distance from the weld nugget to the base metal increases. The highest hardness value belongs to the Table 3
Effect of hold time on diameter of nugget and maximum change in temperature at the crack tip.
Sample No.
Electrode tip (mm)
Current (KA)
Force (KN)
Time (cycle)
Force (cycle)
Diameter of nugget (mm)
Maximum change in temperature (DTmax) at the crack tip (°C)
1 5 4 3 20 30 4.9 1.95
2 5 4 3 25 30 5.0 3.22
3 5 4 3 30 30 5.1 3.28
4 5 4 3 40 30 5.2 5.00
5 5 4 3 30 20 5.2 5.01
6 5 4 3 30 25 5.2 5.19
7 5 4 3 30 35 5.1 5.41
8 5 4 3 30 40 5.1 4.21
Fig. 2.The dendritic grain and equiaxial axial grains occurred in the microstructures of the welded specimens joined at various welding time and electrode forces.
nugget and this is followed by the HAZ and the base metal. In addi- tion, when the hardness values of the specimens joined at different welding parameters (electrode force, welding current and welding time) are compared, the biggest factor is electrode force.
The obtained results showed that the highest hardness value of about 215 ± 15HVbelongs to the weld nugget. It is considered that rapid melting and rapid cooling of the weld nugget led to the high- est hardness value. In all of the welded joints, heat input increased with increasing welding time and welding current. This, in turn, enlarged the weld nugget. Therefore, the hardness value of weld nugget was found to be higher than those of the base metal and HAZ due to the deformation hardening as the result of electrode force throughout the holding period. Moreover, it is hard to achieve thermodynamic equilibrium.
The tensile test results were shown inFig. 4. The ultimate ten- sile strength (UTS) was taken as the maximum weld strength after which the weld joints have broken. Average strength values from the five samples were taken as the equivalent strength of that par- ticular weld schedules.Table 3shows the diameters changes with respect to current; weld time. These currents increment is found in both set of attempts. The welding time too has increased the
strength as it increases the diameters as well. This fulfils the Joule’s law of heating. By increasing; either current or weld time; the heat supplied at the electrode tip is also equivalently increased and therefore the corresponding diameters increments were obtained.
For the tensile strengths and elongation, the specimens increase with welding current increasing. And the specimens climb up and then decline with welding force. The tensile property of the spot welding specimens can be related to their microstructures.
Fine-grains provide strength and coarse-grains improve uniform- elongation. For example, the fine-grained dominant specimen and coarser grains lead to higher tensile strength and lower elongation.
We have noticed that the breaks happened in accordance with average strength values. A poor weld has interfacial fracture and the force seemed to be falling between. A moderate-good weld has torn from either side of base metal and; the force falls between.
Furthermore a good weld has better bounds between sheets. Often it tears from both sides and button pull out of the base metals as the break does not occur at the welded area but rather at the heat affected areas. The heat affected zones (HAZ) hardness was slightly lower than the fusion zone but higher than the base metals.
3.3. Thermography test results
In the paper, the thermography is used to observe the fracture phenomenon. Thermography is a measurement technique which provides an image of the distribution of the temperature on the surface of the examined object[15–18]. Thermography proceeds by decoding, using an adapted detector, information temperature resulting from the infra-red radiation emitted by any object. The principal advantage of infra-red thermography is its non-intrusive character. Indeed, it forms part of the techniques of non-destruc- tive testing and can be carried out on installations in service. The deformation of solid materials is almost always accompanied by releases of heat. When the material becomes deformed or is dam- aged and fissured, a part of energy necessary to starting and the propagation of the damage are transformed in an irreversible way into heat. The plastic deformation mechanisms are mainly attributed to their various stress conditions. The tensile specimen is under the plane stress condition, where the local plastic defor- mation takes place on the maximum stress direction. It is appar- ently observed that the hot-spot zone expanded during the Fig. 3.The transverse hardness values of the spot welding.
Fig. 4.The tensile test result of the spot welding ((a) Tensile strength and welding time. (b) Tensile strength and electrode forces).
fatigue tests, demonstrating a strongly localized damage phenom- enon. Once the threshold value of the tensile damage was reached, tensile fracture was expected to take place at the hot-spot zone due to the large plastic deformations[16]. Tensile fracture takes place at the boundary of the nugget length due to the maximum stress and the notch effect here[16]. The tensile damage identifica- tions in this part confirm the potential capability of the infrared thermography in detecting the tensile damage of structures, and in monitoring its propagation in situ and in real time on the basis of the hot-spot evolution.
The test results are presented inFig. 5: maximum temperature changes during the testing. The hot-spot zones on the specimen surface are visibly identified through the thermal images obtained by the infrared camera. The temperature at a point on the speci- men increases as the crack tip approaches it, reaches a maximum instantaneously and then falls immediately after the crack tip passes the point. Since the plastic deformation energy is converted to heat, the change in surface temperature measurement at the crack tip corresponds to the amount of plastic deformation, hence can be correlated to the fracture strength of the material. Some characteristic points are illustrated with corresponding thermo- grams: the beginning of elastic deformation, the beginning of plas- tic deformation, reaching maximum force, the homogeneous plastic deformation and finally fracture of the specimen. Tempera- ture measurements in different measurement lines are also pre- sented. Maximum change in temperature (DTmax) at the crack tip at various strength of weldment is given inTable 3.
When a material is subjected to tensile loading in the elastic limit under adiabatic conditions, temperature decreases due to thermo elastic effect. Beyond the yield point, a sudden reversal in the direction of temperature change occurs as thermal energy is re- leased when plastic deformation occurs. Here, the irreversible
work done on the material is converted into heat resulting in in- crease in temperature. Hence the elastic–plastic boundary (plastic zone) is the region, where the temperature rise is zero. Typical plastic zone sizes of the specimens with cracks at various strength of the ferrite steel are shown inFig. 5. The plastic zone size of the specimens is approximate to the size of the nugget.
Measurement values for the selected characteristic tempera- ture: room temperature, maximum and fracture of specimen.
According to the local increase in temperature, the initial plastic deformation was noticed in the edge of the nugget. We can find that one hot zone is located in the bottom of the nugget firstly, then two hot zones are located respectively in the nugget up and down, lastly the hot zone is located randomly until the sample fractured during tensile process. The possible reason for this is the fact that at spot welds this zone is exposed to highest stress.
In combination with the presence of brittle structures and/or pos- sible micro-defects, the edge nugget zone is the critical zone for the appearance of the exceeding local limits of plasticity, and the appearance of initial cracks [19,20]. Possible explanations [20–
22]for the behavior of temperature and nugget appear to lie in three areas: the area fraction of larger grains, the tensile stress at the nugget circumference, the hardness behavior of the nugget.
The area fraction of larger grains can arrange the resultant hard- ness duplicity. The more the area fraction of larger grains occupy, the smaller the strength becomes. Therefore, the nugget with low- er area fraction of larger grains generally has a higher tendency to fail in the IF mode. The temperature change can be easier to ob- served in the IF mode than that in the PF mode during the tensile test. The larger grains can get smaller hardness. The ratio of the hardness of the FZ to the (average) hardness of the base metal(s) can determine the tensile stress at the nugget circumference and the hardness behavior of the nugget. The stiffer the sample (i.e. less
Fig. 5.Maximum temperature changes during the testing.
rotation), the susceptibility to the interfacial failure mode is higher.
Therefore, the BM with higher yield and tensile strength generally has a higher tendency to fail in the IF mode. Hence, the higher the stiffness, the lower the tendency to fail in the PF mode there is.
Either of these features by itself could enhance the strength. In this investigation it was not possible to establish uniquely the role of each on the strength.
4. Conclusions
The paper has researched on the effects of parameters on micro- structures and mechanical properties of spot welding ferrite steel.
The obtained results can be summarized as follows:
(1) The dendritic grain and equiaxial axial grains occurred in the microstructures of the welded specimens joined at various welding currents and electrode forces. The grains tend to grow in that direction, giving rise to columnar grains. Axial grains start in the center regions grow along the length of the weld, blocking the columnar grains growing from the interface.
(2) The highest hardness value belongs to the nugget and this is followed by the HAZ and the base metal. It is considered that rapid melting and rapid cooling of the weld nugget led to the highest hardness value. The tensile property of the spot welding specimens can be related to their microstructures.
Fine-grains provide strength and coarse-grains improve uni- form-elongation.
(3) During the tensile test, the thermography gives visible data of temperature changes on the surface of specimen. The ini- tial plastic deformation was noticed in the edge of the nug- get. The temperature change can be easier to observed in the IF mode than that in the PF mode during the tensile test.
Acknowledgments
This work was supported by Natural Science Foundation of Shanxi Province (No.2013021021-1) and Natural Science Founda- tion of Shanghai of China (12ZR1444500)
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