- Welding and Joining of Advanced High Strength Steels (AHSS)
- Properties and automotive applications of advanced high-strength steels (AHSS)
- Properties and automotive applications of advanced high-strength steels (AHSS) - DRO
In DP steels this behaviour can be attributed to the martensite in the ferrite matrix. As the martensite is formed during the transformation from austenite, it is accompanied by volume expansion. The hard martensite increases the ultimate tensile strength [Waterschoot et al. In TRIP steels deformation during tensile loading causes the retained austenite to transform to martensite. This transformation is also accompanied by volume expansion, resulting in a localised increase of the strain hardening coefficient.
This delays the onset of necking and leads to higher uniform and total elongation [Garcia-Gonzalez, ]. Other kinds of AHSS have been developed, all of them having a microstructure consisting of two or more different phases, of which at least one adds strength and hardness to the materials whilst the other s provide additional formability see table 2. Hot Forming steels often referred to as Boron steels, though boron is not necessarily part of the chemical composition have a chemical composition that is much more comparable to that of AHSS.
These steels need to be heated into the austenitic temperature region before forming. They are austenitic when formed, ensuring good formability, whilst the subsequent quenching step ensures a martensitic microstructure that gives the final product very high hardness. As their application is completely different from DP and TRIP steels, as well as any of the other steels listed in table 2. Advanced High Strength Steels are not just classified by their microstructural composition.
Depending on application they can be classified according to their chemical composition, the thickness of the material and their mechanical characteristics. This specifies technical delivery conditions. Automotive manufacturers have their own standards [GM, ] which specify the chemical composition and mechanical characteristics that are demanded.
Additionally they may specify surface conditions and some other properties, including microstructural characteristics typically grain size et cetera. Weldability is often not specified in these documents, however automotive manufacturers require certain sets of tests that have to be passed for a material to be accepted, and these include requirements concerning weldability.
Efforts are made to standardise these tests within the industry, e. Nowadays several thousand spotwelded joints are present in cars. The exact number differs between brands and models see table 2. If data is available for updated models e. Ford Fiesta , the data of the latest model are presented here. Whatever the considerations that will lead to the selection of a certain joining process, it is the performance of the joint itself that will be the most important selection criterion.
If a joining method is not suitable to achieve the desired performance concerning strength, fatigue, crash, corrosion, et cetera , another process will be selected. In the most basic set up of resistance spot welding, two sheets of metal are placed between two electrodes see figure 2. The current runs between the electrodes and heats up the materials. The heat generated is not uniform, as the bulk resistivity of the electrodes and the metal sheets, the contact resistance between the sheets and the electrodes, and between the sheets, differ from each other and vary with temperature.
The electrodes are water cooled [Davies, ] and therefore serve as heat sinks. Additionally heat will be lost to the surrounding material of the metal sheets.
This will lead to a temperature profile with the highest temperature between the electrodes. In the most basic set up, both sheets will be of the same material same electrical resistivity, thermal conductivity and thickness and heating will be most prominent at the interface between the two sheets.
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If the current is high enough the material will melt at that interface and a molten weld pool will start to form. The weld is formed in the centre between the two electrodes. As soon as the current stops flowing the weld pool will start to cool losing heat to the surrounding material and the electrodes and eventually will solidify, forming a joint between the materials see figure 2.
Welds made with a weld current of 8,45 kA, applied during 0, s. Electrode force 3,1 kN. Within the group of resistance welding, a distinction is made between processes using pressure and those that do not. There are even more subdivisions e. Here resistance spot welding is classified in the last group as is friction welding.
Whatever the system, it is important to note that it is not just the current in combination with the material resistance that is a defining feature of the resistance spot welding process, but also the pressure applied by the electrodes. The pressure applied by the electrodes clamps the materials together. The pressure determines the contact resistance between electrodes and sheets and between the sheets. A low electrode force may lead to a reduction in actual contact between the sheets of material and the contact resistance will be high.
If the electrode force is increased the contact resistance is decreased. There is a limiting pressure above which the contact resistance remains uniform [Kearns, ]. If the pressure is too low, expulsion of molten material will occur before the weld is fully formed. If the pressure is too high indentation of the sheet material will occur as the material softens [Kearns, ].
Too much indentation is undesirable for cosmetic reasons and because stress concentration points may be generated as the sheet material deforms. The electrode lifetime, i.
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The electrodes are usually made from material copper alloys that is softer than the sheet material. The electrode pressure causes the electrodes to deform, leading to an increase in the contact area between the electrodes and the sheet metal. As the area increases, the current density decreases. A decrease in current density leads to a decrease in heat generation.
Additionally an increase in contact area leads to an increase in cooling capacity of the electrodes. Combined, the accumulation of heat will decrease, making it harder to produce welds. DC causes the weld pool to grow continuously. AC causes the weld pool to grow in steps as the weld pool is cooled somewhat when the current switches direction. The current can be varied during welding. The idea behind changing current profiles during welding is based on the belief that weld formation and possible expulsion of liquid material depend not only on the heat input, but also on how the heat is applied.
A constant current profile, which is commonly used in resistance spot welding, provides an approximately constant heat input or a constantly increasing heat input as the resistance increases. The pressure in the liquid weld pool rises with increasing temperature or the amount of heat input. Reducing the heat input at the end of welding may reduce the risk of expulsion. Weld current schemes with decreasing current levels at the end of welding have therefore been developed. A rapid decrease in the current magnitude and a large associated heat loss, may have a negative impact on the weld quality.
A more gradual decrease of the weld current magnitude and associated heat loss seems more logical to achieve large welds whilst reducing the risk of expulsion. Reducing current magnitudes affects process times and complicates welding operations. These considerations may limit the application of complex weld current profiles in manufacturing. If the current magnitude is low, then welding times need to increase. If the current level is high welding times need to be reduced to avoid expulsion of liquid material. For the formation of a high quality weld, welding times need to be long enough to allow the material to melt and the weld pool to grow to its desired size.
Welding time is an important factor in manufacturing as it directly relates to costs. Especially when thicker sheets are welded it is often desirable to use multi pulsed welding schemes to form high quality welds of sufficient size. These schemes lead to increased welding times and therefore may be undesirable compared to welding schemes that lead to the formation of a weld in a single pulse.
Multi-pulsed schemes may also be used to perform a post weld heat treatment on the weld nugget. In post weld heat treatment schedules, the weld is subjected to an additional current pulse to heat up the material to temper the weld. This is generally done to soften the weld, to increase its mechanical performance.
After the current stops flowing an electrode force is still applied. During this time no more heat is generated, but heat is still conducted away from the weld pool to the water cooled electrodes. As the heat in the weld pool is reduced, the weld starts to solidify. The duration of the cooling time should be long enough to allow enough molten material to solidify to give the weld sufficient structural strength for it not to fail under thermal stresses after the electrodes are released.
When post weld heat treatments are used, the weld should be completely solidified as molten material cannot be meaningfully heat treated. To temper the weld nugget the material should be allowed to cool sufficiently for it to transform to martensite, which can then be heat treated. As martensite formation starts at temperatures considerably lower than the solidification temperature, cooling times before heat treatments can be quite long compared to the welding time and the post weld heat treatment time.
In practice there are many complications. The first is splash welding. In production, manufacturers want to ensure that a joint has been formed. This may lead them to increase welding times, to ensure sufficient material has been melted [Juettner, ], causing molten material to be expelled, resulting in a splash weld.
Although splash welds may not be a problem, they lead to smaller weld nuggets and increased indentation. Splash welds are especially undesirable if the material is coated, as the material expelled may damage the coating, thus leading to decreased corrosion protection and appearance. Splash welds caused by too long welding times or too high electrode pressure are often calculated risks caused by the desire to be sure of the integrity of the joint formed. They can also be caused by misalignment of the electrodes in which case they are unintentional.
Shunt welding forms a second complication. Again, when a single weld is formed between sheets the current will flow straight between the electrodes. In assembly, many welds will be made in the same sheets, and previous welds will function as short circuits between the electrodes. Part of the current will run via previously formed welds, requiring the current to be higher or the time that the current flows to be longer to form a weld of the same size [Houldcroft, ].
In its most basic form two sheets of similar thickness are joined together. If the thickness of the materials is not the same, the heat conducted away to the electrodes will not be equal for both sheets. This will cause the thermal profile to deviate and melting will start to occur in only one of the materials. If not enough heat is generated this may lead to a weld nugget forming in only one of the materials.
Because the contact resistance between the materials is usually very high, the weld nugget will usually form at the interface, but if the difference in thickness between the materials is big enough this may not be the case. Generally both sheets are made of the same metal. However in some instances the materials may differ considerably. The difference in thermal and electrical characteristics of the materials may lead to significant variations in the thermal profile.
Thus leading to similar effects as with the variations in sheet thickness. If the materials differ in thickness and thermal and electrical characteristic, this may be even more pronounced [den Uijl, ]. Although two sheets are generally welded, stacks of three or more sheets may also be joined. This may complicate weld nugget formation even more [den Uijl, ]. The issue of springback also plays a role. If the springback of a flange after forming is too big, it is hard to press the flanges together. The sheets need to be pressed together by the electrodes to enable a current to flow, otherwise a joint cannot be formed.
There may be additional complications if the strength of the sheets becomes too high. Welds are usually formed at the interface between the sheets, because the contact resistance is higher than the bulk resistivity of the materials. The surfaces of sheet material in automotive applications are often coated. The coatings change the contact surface resistivity.
If the coating has a lower melting and evaporation temperature than the melting temperature of the metal of the sheets, the thermal profile will show discontinuities. In automotive manufacturing, steel sheet is often coated with zinc. Due to the higher thermal and electrical conductivity of the zinc coating compared to an uncoated surface, and because the softer zinc surface conforms better to the electrode tip, higher current needs to be applied for a longer period of time when welding coated sheets [Madsen et al.
Other coatings may decrease the electrical conductivity between the sheets e. AlSi coatings on boron steels. As the temperature rises, the zinc will melt and evaporate, which may not cause too much complications for the contact resistance between the two sheets, but at the electrode tip, molten zinc will be in contact with the electrodes which are usually made of copper. Alloying of the copper with the zinc will lead to the formation of brass which increases the resistivity at the electrode face even more.
The point where weld dimensions and quality are not acceptable any more determines the electrode lifetime. Redressing can be done by removing all of the affected material, but also by restoring just the side of the electrode. The electrode lifetime is an important parameter, because the need to redress the electrode tips adds costs to the manufacturing operations.
Conversely, measures to counter electrode degradation e. Traditionally, one of the methods to counter the challenges faced by resistance spot welding has been to use multi-pulsed welding [Pfeifer, ]. Instead of one single pulse to melt the material, two or more pulses are used, with intermediate pauses, in which the material is allowed to cool down via the electrodes that are still in contact with the sheets.
After the last pulse, there is usually a last holding stage to allow part of the molten weld pool to cool down sufficiently to solidify and form a joint. The duration of the different stages is given in cycles for alternating current with the duration of a cycle dependent upon the frequency used and milliseconds for direct current. Finally, even the field of resistance spot welding does not escape the need to look into power consumption.
With rising energy prices and possible future scarcity, optimisation of energy consumption has become a topic of interest in automotive manufacturing. Not just for the products produced, but also for the processes used. Resistance spot welding faces an extra challenge here as operations tend to interfere with the supply offered by the power grid e. Other factors that may play a role in future developments of resistance spot welding in automotive manufacturing are health and safety related [Boyer, ].
Bentley et al. Using a similar approach Gould  compared simulation results with experimentally determined weld nugget sizes. One-dimensional numerical models lack the ability to accurately simulate local thermal gradients due to variations in current density. Two dimensional axisymmetric heat transfer models for analyzing resistance spot welding using finite difference methods were developed later.
Still a finite difference approach poses limitations to the model due to rigidity in geometry. A basic finite difference model requires that key aspects of the geometry, most notably contact areas, are pre-defined. For accurate simulation of resistance welding this is a drawback. The use of the finite element method to simulate welding processes also dates back to the s [Ueda et al. A thermomechanical model using the finite element approach was developed by Friedman in In recent years finite element analysis has become the mainstay in spot weld simulations. Using commercial finite element software packages such as Ansys, Abaqus and LS-Dyna as well as purpose built finite element models, researchers have investigated various aspects of resistance spot welding.
Adjusting process parameters such as electrode geometry and material characteristics often requires manipulation of the model set up by an operator with extensive knowledge of finite element method theory. Most of the early work concerned either purely academic exercises or purpose built models to investigate a specific problem.
The conference has been held biannually since and has grown into the leading event in the field [Cerjak, ]. The conference proceedings present a comprehensive state of the art at the time of the conference. Murukawa et al. Robin et al. It was reported that there was very little difference in using a weakly coupled approach, simulating first the complete thermal response to welding operations followed by a mechanical simulation compared to a strongly coupled approach, simulating alternatively the thermal response and the resultant mechanical effect for a set number of time steps.
This work was further elaborated upon in the next conference where the results of finite element simulations using Sysweld were transferred to another finite element software package, Pamcrash, for analysis of the behaviour of the welded joint under dynamic loading [Robin et al. It was reported that the results were promising, but the calculation effort is very high due to the use of solid elements in crash simulations. In crash simulations dedicated resistance spot weld descriptions are required for simulation of crash behaviour of parts with welded joints.
Prerequisite for this and similar approaches is a high level of predictive and repetitive behaviour of the welded joint under crash loading e. The investigation was focussed on welding of aluminium. It was reported that the simulation of thermo-mechanical elasto-plastic behaviour was limited due to lack of mechanical material properties at elevated temperatures. Sprikunwong et al. However the thermal history at the beginning of the process was not as well modelled; the temperatures predicted in the first half of the welding process were lower than the temperatures actually reached experimentally.
Again the importance of accurate material data at different temperatures was stressed. It was stated that enthalpy, bulk thermal and electrical conductivity play a large role. Adjusting these parameters may help to improve simulation results, but the physical justification is unclear. Even more important is contact resistance.
This parameter is hard to model accurately as it is less well known and hard to measure with varying temperature. No finite element simulations of resistance spot welded joints were reported, but the issues concerning the representation of welded joints for large structural simulations were discussed. This local-global approach allows the use of strongly coupled electrical, thermal, metallurgical and mechanical phenomena to compute local effects of welding. The results of such a simulation is used as a single input parameter in assembly simulations.
Inserting these results in a mechanical assembly model allows for prediction of the direction of distortions caused in assembly by the spot welding process and the welding sequence used. The approach requires less calculation effort than a complete 3D simulation of welding. Effects such as shunt welding and degradation of the electrode tip during subsequent welding steps are not taken into account. This more practical approach has been the focus of much work reported in another series of seminars. In the first International Seminar on Advances in Resistance Welding provided a platform for applied work simulation of resistance welding as well as experimental developments, mostly in the automotive sector.
The seminar has been held biannually since and focuses heavily on applications, There is room for all kinds of simulation work [Fukui ; Ikeda et al. This is less of a drawback than it may seem. Westgate  at the beginning of this century mentioned that a number of systems had been devised and were continually improved. Spotsim, developed by the Tula State University, Russia, and the Aachen Welding Institute, Germany was designed for analysis of weld formation in resistance spot welding of low- carbon, non-alloy and CrNi steels with thickness of 0, mm [Spotsim, ].
It seems to have made virtually no impact. No scientific or engineering publication has been found using Spotsim to simulate resistance spot welding, although it may be possible that Spotsim has made some impact on the Russian market. The reason for this is probably the availability of better solutions provided by Sorpas and Sysweld.
Spotsim is no longer available [Spotsim, ]. This approach requires quite some understanding of the operation of programming languages and finite element theory to vary process parameter settings and finite element parameters. Combined with the cumbersome and less than user friendly graphical user interface of Sysweld, this made it less suitable for the welding engineer on the work shop floor. Added to that, changing the geometry of the electrode, form the given electrode geometry required even more hands-on work for the operator. In later versions a graphical user interface was supplied that made it possible for the operator to simulate process variation with more ease, but restricted the number of parameters that can be varied.
These are important capabilities for the engineer and designer, but not of primary concern for the weld shop floor. Sorpas is designed in a way that it can be used by anyone that is familiar with resistance spot welding equipment. The parameters that need to be set are the same that need to be set in real life. The software comes supplied with a large database of materials, electrode geometries and equipment characteristics. It is fairly straight forward to adjust the values in the databases to simulated differences in electrode geometry and material characteristics.
The software is equipped with standard options to simulate weld growth curves for different process parameters. A drawback is that the results cannot be automatically used as input in other software platforms. In fact this requires manual intervention to feed the results of Sorpas simulations e. Performance: the quality of the welded joints, which will be discussed in section 2. Weldability tests are a phase in every new car project. Larsson et al. Welding characteristics are an important part of these standards [Larsson et al. The main focus for the welding engineer is to find welding parameters which result in a robust production process [VDEh , ].
To evaluate a welding window a weld growth curve is made. A weld growth curve gives the weld nugget diameter as a function of the welding current, while the welding schedule and the electrode force are kept constant see figure 2. The welding range for resistance spot welding is often defined as the range in welding current from the point where the weld nugget reaches a required size minimum weld size and the onset of splash. Table 2. In this example from BMW Group Standard GS Category A spot welds are joints that can endanger human life, as well as the function or safety, in the event of their failure.
Category B spot welds are joints whose failure make the product unusable for its intended purpose or result in a loss of property, and Category C spot welds are joints whose failure has only little negative impact on the product in terms of its intended use [BMW, ]. Also it is possible to acquire perfectly good welds after the first splash welds have occurred. But as a rule of thumb splash welds can be expected as soon as the size of the weld nugget exceeds the electrode diameter though they do not need to occur [Den Uijl, ]. The width of the weld growth curve referring to the welding range gives an indication of the anticipated tolerance of a particular welding schedule in production, the aim being to maximise the welding range to achieve the greatest safety margin on weld quality [Westgate, ].
Thus, the process reliability depends on the selection of welding parameters and on additional influencing factors. Important influencing factors are the shape and material of the electrode caps, the base metal and the coating of the steel sheets as well as the static and dynamic mechanical machine properties of the welding equipment [Weber, ].
Another quality aspect of a weld is penetration, describing the extent of through thickness melting during welding. A small penetration may mean insufficient heating and indicate a cold weld. In general large penetration is preferred. But as penetration is directly related to the amount of heating, large penetration means softening of the material and possible large indentation by the electrodes. As mentioned before a very important parameter of weldability is the electrode lifetime, i. Welding engineers face challenges to establish robust process windows for AHSS applications [Subramanian, ].
The lower limit for the welding current results from the requirement of a minimum spot weld diameter. The upper limit of the welding current is given by the physics of the welding process. Commonly used lower quality limits are spot weld diameters of 3.
A slightly lower welding current is required for the high strength steel, because of higher electrical resistance, but weld splash occurs earlier [Westgate, ]. Early work on high strength steels indicated that simply by increasing the electrode force, the welding range can be opened up to give a similar performance to low carbon steel [Rivett et al,; ]. Material suppliers often recommend the force levels required for different steel types and thicknesses. The increased yield strength of advanced high strength steel compared to low carbon steels increases the total resistance across the electrode during the welding process and generates more heat that causes the material to melt much earlier.
Tumuluru  cautions that with the use of high force, the electrode indentation into the base material should be monitored because higher electrode force causes deeper indentation, but generally indentation at a given force will be greater in a low than in a high- strength steel. Scotchmer  mentions that the weld growth curve widens as the welding pressure is increased, and the current required for a given weld nugget diameter increases.
A higher electrode force can enable larger weld sizes to be achieved before splash and help to reduce internal porosity or shrinkage imperfections. The disadvantages are the need for higher capacity guns. Dupuy  carried out several welding range tests using different spot welding machines. It was found that all machines gave a similar response in spot welding, producing the same nugget diameter with a given current, although the amount of scatter varied. The welding range did vary depending on the stiffness of the construction.
Increased stiffness enabled larger welding ranges as larger welds could be produced before splash occurred. It is difficult to be precise about electrode force levels to be used, as it also depends on the weld time. Higher forces are required particularly at shorter weld times, if short sequence times are required for high production rates. However, longer weld times can also be beneficial in expanding the available welding range [Westgate, ]. Subramanian  also noticed that nugget formation was spontaneous with high strength grade steels.
Significant variation in nugget size was found at low weld time. However, when the weld time was sufficiently large, the nugget reached its maximum size which was almost the same for all investigated grades. This could be attributed to similar high temperature stress-strain behaviour of various grades. Even more complicated schedules combining variations in welding time and electrode force adaptive welding have been proposed [Ikeda et al. Some of the longer welding schedules would influence production rate but it might be possible to tolerate special procedures where a limited number of welds are to be made on the more difficult steels [Westgate, ].
The electrode shape has also been found to influence the weldability. The increased contact area leads to reduced current density. As a result, dome shaped electrodes increase the current required to produce a large weld. This, in turn, increases the welding current range [Chan et al. But as mentioned before, the biggest influence of coatings is on the electrode lifetime. As the electrode surfaces alloy, they become more susceptible to deformation [White, ].
This effect becomes even more pronounced if the electrode force is increased to weld advanced high strength steels. Such solutions are not preferred as electrode geometries are well defined in standards [ISO , ] and therefore there may be reluctance to use speciality electrodes. Several categories can be distinguished as described in section 2. It is for that reason that minimum weld nugget diameters are set for certain categories of application see table 2. Apart from the weld nugget size the performance of RSW joints is primarily a function of the chemical composition and microstructural composition of the materials and the thermal profile during welding especially the maximum temperature reached during welding and the subsequent cooling rates.
Three aspects in particular are important when assessing the weldability of the materials [Den Uijl et al. Microstructure The resultant microstructure after welding is different from the microstructure before welding, which is the key characteristic determining the strength and formability of the material.
The weld nugget consists of material that has melted during welding. After welding the material solidifies and subsequently undergoes phase transformations. Whether the resultant microstructural composition will be martensitic, bainitic or a mixture of both is dependent upon the chemical composition and cooling rate. At increased levels of alloying with carbon being the primary alloying element and sufficiently high cooling rates the resultant microstructure will be mostly martensitic.
Note that the voids at the centre line are caused by shrinkage due to cooling and solidification. The direction of the grains in the weld nugget runs along cooling lines; vertically where cooling is directed to the water cooled electrodes and horizontally along the centre line where cooling is directed to the surrounding material [Den Uijl, ]. Material heated above the austenisation temperature transforms to austenite during welding.
After the material has been transformed to austenite the grains will start to grow. The amount of grain growth in the HAZ is determined by the maximum temperature reached and the time it has been heated above the austenisation temperature. Upon cooling the material will transform into martensite or bainite. Material that has been heated below the austenisation temperature will be tempered during welding. This can cause significant softening of the material [Blondeau et al. Hardness Hardness is measured in a variety of ways e. Vickers hardness and Rockwell hardness , but all methods measure the response of the material to indentation.
For each microstructural phase there is a relationship between the chemical composition and the cooling rate that combine to make up the post weld hardness. The main element determining post weld hardness is carbon. To avoid high post weld hardness levels the amount of carbon in steels is often limited. This limitation also affects the strength of the base material. Therefore the steels are alloyed with other elements that increase the strength. Other elements are added to achieve the microstructural composition of the base material.
These elements e. Failure mode; a qualitative measure of weld quality. Fatigue; a quantitative measure of the number of cycles to failure under a certain repeated loading pattern. Procedures for testing are described in standards. Steel manufacturers use standardised test procedures to supply material data [Smith et al. Automotive manufacturers use standards to ensure that materials comply with the minimum requirements they have set for different applications [e.
To streamline differences in standards, general standards have been developed. These may vary regionally e. VDEh standards for Europe [e. AWS D8. Although a wide variety of standards is available, they tend to address similar issues and set similar demands, but variations do occur. Shop Floor Practice To asses welding operations, welds are tested at assembly lines. Because of the limited availability of testing facilities and time, weld quality testing at the work shop floor is usually limited to evaluating the weld failure mode and measuring the weld button size.
In the latter case care must be taken that these coupons are welded using the same conditions as are used for production parts. The material should be the same both in chemical composition and in thickness. Preceding operations, such as surface cleaning should be identical and care must be taken to prevent misleading effects from current shunting through adjacent welds [Kearns, ]. The main aim is to detect brittle weld failure and the occurrence of cold welds joints without fusion between the sheets.
The weld nugget may be measured to ensure that a set minimum is achieved. Peel roller test In a peel roller test the sheets are separated by a roller applied to one of the sheets much like the way cans of sardines are sometimes opened. As the roller rolls over the weld the sheet is torn off at the weld and the weld is torn out if the joint is ductile, or the sheets are separated without much effort, if the joint is brittle.
The test is not always suited to measure the weld nugget size, as the shape of the button can be highly irregular especially if the base metal is torn out. Unlike the chisel test, the peel roller test is conducted on coupons and cannot be used for manufactured parts. Instrumented testing Steel manufacturers use several standardised tensile tests to assess the weldability of materials, such as see figure 2. The test is also informative because the majority of resistance spot welds will be located on flanges in an automotive structure.
The test configuration provides information on the weld strength and failure mode, but not on the energy absorption because the deformation of the flanges generally absorbs most of the energy. Cross Tension Tests This test is designed to load a weld in the direction normal to the weld interface. Overlap Tensile Testing Overlap shear tensile testing provides data on the ultimate strength of the resistance spot welded joint and the failure mode. Compared to cross tensile tests and peel type tensile testing the results are less dependent on the exact location of the weld.
Not many joints in critical areas of an automotive construction are subjected to static shear tensile loads, but because of the simplicity of the fabrication of test specimens and the test itself combined with the limited amount of experimental scatter compared to cross tensile and peel type tensile testing it is the most commonly used test configuration.
The main drawback of the test is that the test specimens are asymmetric and deform during testing see figure 2. The amount of rotation is dependent on the thickness and the size of the spot weld. Failure strength is primarily dependent upon the weld diameter [Den Uijl et al. Although the standardised test configurations supply a lot of information on the weld failure mode and failure strength of the spot welded joints, these coupon tests cannot completely simulate the complex loading conditions that occur during crash testing [Kearns, ].
They are commonly used to test resistance spot welded joints in industry. Static tensile tests allow for clear comparison of resistance spot welded joints. But resistance spot welded joints are seldom subjected to fatal static tensile loads in automotive applications. Therefore the mechanical properties of resistance spot welded joints in dynamic loading are recognised as important to assess the performance of welded structures.
Dynamic weld tests reveal trends, but the results are often difficult to quantify [Khan et al. Dynamic tests are complex requiring specialised equipment [Den Uijl, ] and tend to be not very reliable and repeatable. It can be seen that there is a positive relationship between the increase in strength in static tensile testing and the increase in dynamic tensile testing. Dynamic tests were performed using a standard drop weight tower [Den Uijl et al.
Three types of weld failure mode are commonly defined to classify weld failure of RSW joints [Den Uijl et al. There are gradations of partial plug failures which can be detailed in standards [e. Full plug failure occurs when the RSW joint itself does not fail see figure 2. The test specimen fails either in the HAZ or in the base material. Partial plug failure occurs when the RSW joint is affected by failure, but part of the weld nugget has remained intact see figure 2. Interfacial plug failure occurs when the weld fails along the centre line, essentially separating both sheets without leaving any part of the weld nugget intact.
If full plug failure occurs especially when failure occurs outside of the HAZ the weld process did not affect the ability of the material to resist the load. Partial plug failure indicates that the materials ability to resist loading has been affected. The joint has become the weakest link.
It is often hard to quantify the results, and therefore partial plug failure should be avoided. The influence of weld failure mode on the overall performance of test specimens with RSW joints can be seen in figure 2. It can be seen that the overall deformation of the test specimen is dependent on the failure mode of the RSW joint.
The specimen showing interfacial failure failed at low deformation, indicating decreased absorption of energy during loading. The specimen showing full plug failure failed after much more deformation of the base material, indicating increased absorption of energy before failure occurred. The safety critical parts of cars depend principally upon their assembled properties and, as a result, welds have to meet the expected design specifications in respect of the application considered.
The weld failure mode is important for design of automotive parts. Ideally the weld should not fail in crash testing. The joint should be able to resist the load. As the mechanical characteristics of the base material are often known, or can be easily determined, full plug failure allows for design based upon these characteristics of the material. With the increased use of simulation to design automotive parts, the ability to use the mechanical characteristics of the base material allows designers to accurately predict the mechanical performance of parts with RSW joints.
The practice on the shop floor during spot welding quality checking is to refer to the failure type as a quality criterion: plug failure is accepted, interfacial failure is rejected [Bouzekri et al. Left interfacial failure, centre partial plug failure and right full plug failure. Generally, spot weld failure occurs in two modes: interfacial and partial plug failure [Marashi et al. In interfacial mode, failure occurs through the nugget, while in plug failure, failure occurs by complete or partial withdrawal of the nugget from one sheet see figure 2.
Interfacial failures of spot welds are considered to be brittle and less energy absorbing than plug failures [Mimer et al. Load carrying capacity and energy absorption capability for those welds that fail under interfacial mode are much less than those which fail under plug failure mode [AWS D8. The pullout failure mode indicates that the welds have been able to transmit a high level of force, thus causing severe plastic deformation in adjacent components, and increased strain energy dissipation in crash conditions [Nikoosohbat et al. Steel manufacturers perform a series of static tensile test on resistance spot welded joints before they are subjected to full size crash tests [BMW, ].
Although the standardised tests supply a lot of information on the weld failure mode and failure strength of the spot welded joints, these coupon tests cannot completely simulate the complex loading conditions that occur during crash testing [Den Uijl et al. It has been reported that the failure mode depends strongly on the loading mode. Especially in chisel destructive testing, as used on the shop floor of assembly plants this cannot be carefully controlled [Bouzekri et al.
Post weld hardness Literature reports that the issue of undesirable weld failure modes is primarily related to the relatively high carbon equivalent of advanced high strength steels compared to traditional low alloyed steels, coupled with the fast weld cooling rates observed in resistance spot welding. This can cause high hardness levels and brittleness of the weld, leading to unfavourable fracture modes partial or complete interface failures and low cross-tension strength. For resistance spot welding, plug failure is normally a quality requirement in routine destructive tests.
The interfacial and partial plug failures are considered to be brittle and less energy absorbing than plug failures. Plug failures are considered to be more favourable than interfacial failures because the tensile strength is higher. Gould et al. Pedersen et al.
This is associated with a large degree of martensite in the microstructure. Studies on High Strength Steels in the s attempted to derive a modified carbon equivalent formula to define weldability, in particular the borderline of potential interface failures, for resistance spot welds. While reasonable correlation was achieved, no universal relationship was found. In addition, there is the question whether a maximum weld hardness value could be specified to define the limit of suitable weldability.
Westgate  finds that although hardness levels around HV and above are certainly more likely to give interface failure, there appears to be no ideal answer, as material thickness and material type can also have an effect. Mimer et al. Radaj  even mentions a general desire to aim for degrees of hardness below HV in general in welding. He states that the problem is aggravated by diffusible hydrogen in martensite hardened zones, leading to brittle fracture.
In another report [Gould, ] undesirable failure modes are attributed to solidification-related porosity which can contribute to the formation of critical sized flaws that can eventually propagate down the faying surface. Harder microstructures then allow easier propagation of these flaws into cracks.
Welding and Joining of Advanced High Strength Steels (AHSS)
Carbon equivalence numbers Carbon equivalent numbers have been used now for several decades to compare the weldability of steels with different chemical compositions. Over the years steels and their applications have changed considerably. As far as automotive applications are concerned, much higher strength levels are desired to enable manufacturers to reduce weight, whilst maintaining performance fatigue and crash.
The chemical composition and process routes of steels changed as applications posed new demands on the strength and formability of steels. Carbon equivalent numbers relate the composition of a steel to its weldability, but they themselves are also dependent upon the chemical composition of the steel. Weldability is often defined as the inverse of hardenability [Easterling, ]. Combined with the fact that the load required to cause interfacial failure in very high strength materials e.
Tumuluru  reported that the load required to cause interfacial fracture was almost 90 percent of the load required to cause button pull out fracture. Therefore it was suggested that the fracture mode should not be used as the sole criteria to judge weld integrity in high strength steels.
Instead the load to failure should be considered more important in judging weld quality. For thicker gauges in shear-tension testing of advanced high strength steels interfacial fractures become the expected mode of failure. The strength of a joint is related to the size of the weld nugget [Chao, , Mimer et al. Tumuluru  reported that larger welds fail through button pull out, whereas smaller welds generally fail interfacially.
Plug failure and interfacial fracture are two competing fracture modes and tensile specimens fail by the mode that requires lower load to initiate failure. An oversized nugget requires large scale welding machinery and is thus of higher cost to fabricate. Additionally there is a relationship between sheet thickness and other weld process parameters, such as welding time, weld current and electrode geometry, which are all detailed in standards.
Weld defects Expulsion or splash welding can lead to a decrease in performance of resistance spot welded joints. The ejection of molten material from the weld pool can lead to a lack of material to fill the weld nugget upon solidification. If the electrode force is high enough the weld nugget will be continuous and no cavities or pores will be left after cooling, but the indentation will lead to geometrical deformation. Without expulsion large cavities and pores after the formation of an RSW joint can be caused by the solidification of molten material.
If the electrodes are released before the weld pool has solidified completely, excessive shrinkage of the material during cooling can lead to the formation of pores. These pores are located where solidification occurs last, usually in the centre of the weld [Kearns, ]. It has to be noted that these pores do not need to be detrimental for the weld strength or failure mode. If the joint fails outside the weld nugget, the pores do not contribute to a reduced weld failure strength.
But if the joint fails in the weld nugget the pores can lead to reduced weld strength. Depending upon the chemical composition of the material, hot cracking can occur during welding. Carbon, sulphur and phosphorus are known to contribute significantly to the hot cracking susceptibility of a material. Silicon and nickel increase the hot cracking susceptibility too, but to a lesser extent. For hot cracking to occur tensile stresses are needed.
Stresses occur due to the construction of a work piece and shrinkage due to solidification of molten material. These effects can be countered by the pressure exerted by the electrodes. Early release of the electrodes will increase the possible occurrence of hot cracks [Kearns, ]. Hot cracks appear between grain boundaries.
If the joint fails outside the weld nugget, hot cracks do not need to be detrimental for the weld strength. During tensile testing the cracks can grow and thus increase the likelihood of joint failure in the weld nugget. Additionally the presence of micro cracks can serve as preferential weld failure paths [Gould, ].
Reported process issues concern the welding range and the electrode lifetime. Reported performance issues concern the post weld hardness and the failure mode of the joints. It has been reported that the welding range of advanced high strength steels is limited due to the increased strength levels and hardness making it more difficult to join materials together before welding and form a sufficiently large weld nugget. Apart from the high strength of the materials, issues have been reported concerning the electrode force and shape of the electrodes.
The performance of the joints mainly concerns the ability of the welded constructions to absorb energy upon loading. This is evaluated in tensile tests in which the welds are required to withstand a certain level of loading and are required to fail in certain desirable failure modes. Preferably resistance spot welds show full plug failure, i. From the literature it can be concluded that the main cause for the reported decreased ability of resistance spot welds to withstand loads in a desirable way is due to the high post weld hardness, leading to brittle failure.
The development of weld simulation software, specifically when aimed at resistance spot welding, offers tools to investigate the process aspects in more detail. Two software packages especially offer possibilities to support research; Sysweld and Sorpas. Sysweld offers possibilities to investigate the welding process in details such as the metallurgical response of a material on welding operations, but seems somewhat limited when reviewing production aspects such as weld growth curves, the application of multiple materials and geometries.
For the research reported in this thesis, Sysweld and Sorpas complement each other. The thickness, grade and surface condition are given with each experimental result. The variations in chemical composition for the advanced high strength steels used in this research are listed in table 3. Spot welds can fail in any of the following three modes: interfacial failure, in which the fracture propagates through the nugget; pull-out failure, in which the weld nugget separates from the parent metal; and partial interfacial failure, in which the fracture initially propagates through the nugget and then deviates through the sheet thickness, similar to pull-out failure.
Pull-out failure is preferred because it is associated with high-load bearing capacity and high energy absorption. This action might not be possible to undo. Are you sure you want to continue? Upload Sign In Join. Home Books Science. Save For Later. Create a List. Read on the Scribd mobile app Download the free Scribd mobile app to read anytime, anywhere. Introduction to welding and joining of advanced high-strength steels AHSS 1. Introduction 1. Overview of major welding processes for AHSS 2. Properties and automotive applications of advanced high-strength steels AHSS 2.
The automobile body 2. AHSS microstructures and tensile properties 2. Formability and fracture of AHSS 2. Automotive in-service properties 2. Current and future trends in AHSS 3. Manufacturing of advanced high-strength steels AHSS 3. Introduction 3. Key challenges faced in producing AHSS grades 3. Future trends 4. Resistance spot welding techniques for advanced high-strength steels AHSS 4.
Introduction 4. Characterizing welding behavior 4. General considerations in resistance spot welding of AHSS 4. Coating effects 4. Microstructural evolution in welds 4. Weld shear tension strength and cross-tension strength CTS 4. Summary 5. Laser welding of advanced high-strength steels AHSS 5. Introduction 5. Background 5. Laser welding of AHSS 5.
Microstructure of laser-welded AHSS 5. Hardness 5. Performance of laser-welded AHSS 5. Future trends 6. High-power beam welding of advanced high-strength steels AHSS 6. Introduction 6. Back to basics: fundamentals of high-power beam welding 6. Metallurgical phenomena in laser welding of AHSS 6. Body-in-white joining applications 6. Conclusions 7. Hybrid welding processes in advanced high-strength steels AHSS 7. Introduction 7. Laser—arc hybrid process description 7. Laser—arc hybrid process parameters for welding automotive AHSS 7. Applications in the automotive industry 7.
Costs and economics 8.
Properties and automotive applications of advanced high-strength steels (AHSS)
Introduction 8. MIG brazing 8. Friction stir spot welding FSSW 8. Conclusions 9. Adhesive bonding techniques for advanced high-strength steels AHSS 9. Introduction: the exigency of adhesive bonding of high-strength steels 9. Challenges in adhesive bonding of AHSS 9. Boron—manganese steels: anticinder coatings and their influence on adhesive bonds 9. Weldbonding of AHSS 9. Conclusions Mechanical fastening techniques for advanced high-strength steels AHSS Introduction The use of drawn arc welding DAW for attaching studs to metals All rights reserved.
Radaj 3 Which process? Bailey and F. Espie, J. Rogerson and K. Raj, C. Subramanian and T. Zhoa and J.
Properties and automotive applications of advanced high-strength steels (AHSS) - DRO
Houldcrof and J. Lohwasser and Z. Lawrence and J.