Mechanical Joining
Examples of mechanical joining in automotive manufacturing are clinching and self-piercing riveting. The process steps and typical equipment for both processes are shown in Figure 1. A simple round punch presses the materials to be joined into the die cavity. As the force continues to increase, the punch side material is forced to spread outward within the die side material.
Figure 1: Process steps and equipment for mechanical joining in automotive industries.
This creates an aesthetically round button, which joins cleanly without any burrs or sharp edges that can corrode. Even with galvanized or aluminized sheet metals, the anti-corrosive properties remain intact as the protective layer flows with the material. Table 1 shows characteristics of different mechanical joining methods.
Table 1: Characteristics of mechanical joining systems.T-12
In a recent study conducted to assess the feasibility of clinch joining advanced HSST-8, it was concluded that 780-MPa DP and TRIP [link to steel material grades] steels can be joined to themselves and to low-carbon steel (see Figure 2). However, 980 DP steel showed tears when placed on the die side (Figure 3). These cracks were found at the ferrite- martensite boundaries (Figure 4). However, these tears did not appear to affect the joint strength. More work is needed to improve the local formability of 980 MPa tensile strength DP steel for successful clinch joining.
Figure 2: Cross sections of successful clinch joints in 780-MPa tensile.
Figure 3: Clinch joint from DP 980-MPa steel showing tears on the die side.
Figure 4: SEM views of tears found in DP 980-MPa steel clinch joints.
Circular clinching without cutting and self-piercing riveting (existing half-hollow-rivets) are not recommended for materials with less than 40% hole expansion ratio (λ) as shown in Figure 5. Clinching with partial cutting may be applied instead.
Figure 5: Balance between elongation and stretch flangeability of 980-MPa tensile-strength class AHSS and surface appearance of mechanical joint at the back side.N-1
Figure 6: Example of DP 300/500 with a self-piercing rivet.G-2
Warm clinching and riveting are under investigation for material with less than 12T total elongation. As with any steel, equipment size and clinch/pierce force are proportional to the material strength and tool life is inversely proportional to material strength.
The strength of self-piercing riveted AHSS is higher than for mild steels. Figure 4.P-6 shows an example of a self-piercing rivet joining two sheets of 1.5-mm-thick DP 300/500. AHSS with tensile strengths greater than 900 MPa cannot be self-piercing riveted by conventional methods today.
Self-piercing rivet joints are typically similar or slightly weaker in strength when compared to spot welds. It is largely dependent on the punch-die size, design, and rivet size. However, self-piercing rivets usually perform better in fatigue loading compared to spot welds because there is no notch effect such as what exists in spot-welded joints Figure 7 (left)]. Although clinch joining is being used in several automotive applications, their performance is lower as shown in Figure 7 (right). Thus, they are not recommended for critical joints in automotive manufacturing.
Figure 7: Strength performance for various mechanical joining methods.
Hybrid Riveting Adhesive
Self-piercing rivets can also be combined with adhesives to result in increased initial stiffness, YS, failure loads and fatigue strength when compared to spot welds using adhesives (Figure 8).
Figure 8: Tensile shear strength of SPR with adhesives and RSW with adhesives.
Arc Welding
Gas Metal Arc Welding: Introduction
Gas Metal Arc Welding (GMAW) (Figure 1), commonly referred to by its slang name “MIG” (metal inert gas welding) uses a continuously fed bare wire electrode through a nozzle that delivers a proper flow of shielding gas to protect the molten and hot metal as it cools. Because the wire is fed automatically by a wire feed system, GMAW is one of the arc welding processes considered to be semi-automatic. The wire feeder pushes the electrode through the welding torch where it makes electrical contact with the contact tube, which delivers the electrical power from the power supply and through the cable to the electrode. The process requires much less welding skill than Shielded Metal Arc Welding (SMAW) or Gas Tungsten Arc Welding (GTAW) [LINK TO SECTION] and produces higher deposition rates.
Figure 1: GMAW
The basic equipment components are the welding gun and cable assembly, electrode feed unit, power supply, and source of shielding gas. This set up includes a water-cooling system for the welding gun which is typically necessary when welding with high duty cycles and high current.
GMAW became commercially available in the late 1940s offering a significant improvement in deposition rates and making welding more efficient. Deposition rates are much higher than for SMAW and GTAW, and the process is readily adaptable to robotic applications. Because of the fast welding speeds and ability to adapt to automation, it is widely used by automotive and heavy equipment manufacturers, as well as a wide variety of construction and structural welding, pipe and pressure vessel welding, and cladding applications. It is extremely flexible and can be used to weld virtually all metals. Relative to SMAW, GMAW equipment is a bit more expensive due to the additional wire feed mechanism, more complex torch, and the need for shielding gas, but overall it is still relatively inexpensive.
GMAW is “self-regulating”, which refers to the ability of the machine to maintain a constant arc length at all times. This is usually achieved using a constant-voltage power supply, although some modern machines are now capable of achieving self-regulation in other ways. This self-regulation feature results in a process that is ideal for mechanized and robotic applications.
Figure 2 provides important GMAW terminology. Of particular importance is electrode extension. As shown, electrode extension refers to the length of filler wire between the arc and the end of the contact tip. The reason for the importance of electrode extension is that the longer the electrode extension, the greater the amount of resistive (known as I2R) heating that will occur in the wire. Resistive heating occurs because the steel wire is not a good conductor of electricity. This effect can become significant at high currents and/or long extensions, and can result in more of the energy from the power supply being consumed in the heating and melting of the wire, and less in generating arc heating. As a result, significant resistive heating can result in a wider weld profile with less penetration or depth of fusion. The stand-off distance is also an important consideration. Distances that are excessive will adversely affect the ability of the shielding gas to protect the weld. Distances that are too close may result in excessive spatter build-up on the nozzle and contact tip. Various gases are being used for shielding the in GMAW process. The most common ones include argon (Ar), helium (He), and carbon dioxide (CO2) and combinations of these. Figure 3 illustrates the effect of the shielding gas on the weld profile.
Figure 2: Common GMAW terminology
Figure 3: Effect of shielding gas on weld profile
AWS A5.18 is the carbon steel filler metal specification for SMAW, and includes both filler metal for both GMAW and GTAW. A typical electrode is shown on Figure 4. The “E” refers to electrode and the “R” refers to rod which means the filler metal can be used either as a GMAW electrode which carries the current, or as a separate filler metal in the form of a rod that could be used for the GTAW process. The “S” distinguishes this filler metal as solid (vs. the “T” designation which refers to a tubular GCAW electrode or “C” for composite electrode), the number, letter, or number/letter combination which follows the S refers to a variety of information about the filler metal such as composition, recommended shielding gas, and/or polarity.
Figure 4: Typical AWS A5.18 electrode.
In summary, the GMAW process offers the following advantages and limitations:
- Advantages:
- Higher deposition rates than SMAW and GTAW
- Better production efficiency vs. SMAW and GTAW since the electrode or filler wire does need to be continuously replaced
- Since no flux is used there is minimal post-weld cleaning required and no possibility for a slag inclusion
- Requires less welder skill than manual processes
- Easily automated
- Can weld most commercial alloys
- Deep penetration with spray transfer mode
- Depending on the metal transfer mode, all position welding is possible
- Limitations:
- Equipment is more expensive and less portable than SMAW equipment
- Torch is heavy and bulky so joint access might be a problem
- Various metal transfer modes add complexity and limitations
- Susceptible to drafty conditions
GMAW Procedures and Properties
Despite the increase alloying content used for Q&P 980, there is no increased welding defect type or rate compared with mild steel Gas Metal Arc Welding (GMAW) welds. Figure 5 is the microhardness profile of 1.6-mm Q&P 980’s GMAW weld joint. Both welded seam and HAZ are all less than 500 HV, and there is no obvious softened zone in HAZ.B-4
Figure 5: Microhardness profile of 1.6-mm DP 980’s GMAW weld joint. B-4
GMAW was used on three steels studied under a range of conditions. The left represents the FZ location and the middle is the HAZ. The figures show various degrees of HAZ hardening and softening depending on material grade and other conditions. The highest hardness occurs in the near HAZ, while the softest point is in the far HAZ. DP 980 [LINK TO THE MATERIAL IN METALLURGY] shows the greatest degree of HAZ hardening and softening. The nominally high CR condition is a combination of low heat input and heat sink. The plots show that CR tends to have the largest effect on the DP steels, with the TRIP steel being somewhat less affected. Pre-strain has the largest effect on the TRIP Base Metal (BM) , increasing the BM hardness by about 25%. The hardness of the softest location of the TRIP 780 HAZ is also increased by pre-strain, although degree of softening (about 20%) is not significantly changed. Pre-straining increased the DP 780 BM hardness by only about 10%. Pre-straining did not affect the peak HAZ hardness for either material. Post-baking did not appear to have a significant influence on the HAZ hardness profiles of the DP 780 material or the TRIP 780 material, regardless of pre-strain condition (Figures 6 through 8).
Figure 6: Hardness profiles of DP 780, TRIP 780, and DP 980 lap welds produced with the nominally high CR, no pre-strain or post-baking.P-7
Figure 7: Hardness profiles of DP 780 and TRIP 780 welds
produced both with and without pre-strain for the high CR condition.E-1
Figure 8: Hardness profiles of DP 780 and TRIP 780 welds produced both
with and without post-baking for both pre-strained sheet and not pre-strained
sheet for the nominally high CR condition.E-1
TRIP 780 lap joint static tensile results for different filler metal and CR conditions are shown in Figure 9. The results are expressed in terms of joint efficiency and the strain at peak load. The data indicates joint efficiencies ranged from about 50% to about 98%. Strains at peak load ranged from less than 3% to nearly 8%. Fracture occurred either in the far HAZ or at the weld fusion boundary. Filler metal strength had no discernable effect on the tensile properties. Figure 9 shows static tensile test results of the TRIP 780 butt joints. All the welds failed in the softened region of the far HAZ with joint efficiencies in excess of 89%. On average, welds made using higher CR experienced higher strains during loading than those made using lower CR. As was the case with the lap welds, filler metal strength did not appear to influence the static tensile properties. The abbreviations of high and low “CR” indicate high and low CR used for each weld.
Figure 9: Static tensile test results of TRIP 780 lap and butt joints.E-1
The static tensile test results of the DP 780 butt welds are shown in Figure 10. All welds failed in the softened region of the far HAZ. As shown, the high CR welds had joint efficiencies in excess of 90%. The high CR welds also appear to have slightly greater strains at peak load.
Figure 10: Static tensile test results of DP 780 butt joints.E-1
Figure 11 (left) shows the TRIP 780 lap joint dynamic tensile results for different filler metal and CR conditions. UTS ranged from 372 to 867 MPa (54 to 126 ksi) and strain at peak load ranged from less than 1% to over 5%. The high CR lap joints had lower strengths and strains at peak load. These welds failed along the fusion line presumably due to porosity present at the root. All the low CR lap welds produced with the ER70S-6 wire failed in far HAZ of the bottom sheet. Of the low CR lap joints produced with the ER100S-G wire, two dynamic tensile specimens failed in the softened region of the far HAZ, and one failed along the fusion line of the top sheet without the presence of porosity at the weld root. Analysis of Figure 11 (left) indicates that filler metal strength did not have a distinguishable effect on the dynamic tensile test results. Figure 11 (right) shows the dynamic tensile test results of the TRIP 780 butt joints. All failed in the softened region of the far HAZ. The UTS of the butt joints ranged from 840 to 896 MPa (122 to 130 ksi), and strain at peak load was generally between 3 and 4%. The figure indicates that neither filler metal strength nor CR condition had a distinguishable effect on the dynamic tensile test results of the butt joints.
Figure 11: Dynamic tensile test results of TRIP 780 lap joints and butt joints.E-1
The dynamic tensile test results of the DP 780 butt joints are shown in Figure 12. All failed in the softened region of the far HAZ. UTS ranged from 841 to 910 MPa (122 to 132 ksi), and strain at peak load ranged from 2.25% to less than 4.0%. It should be noted that similar UTS were obtained for the DP 780 and TRIP 780 butt joints. On average, TRIP 780 butt joints had slightly higher strain at peak load. Neither filler metal strength nor CR condition appears to have a distinguishable effect on the dynamic tensile properties.
Figure 12: Dynamic tensile test results of DP 780 butt joints.E-1
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High Energy Density Welding
The word “laser” is an acronym for “light amplification by stimulated emission of radiation.” Lasers produce a special form of light (electromagnetic energy) consisting of photons that are all of a single coherent wavelength. Light of this form can be focused to extremely small diameters allowing for the creation of the high-energy densities used for welding. The laser beam itself is not useful for welding until it is focused by a focusing lens.
Figure 1: Laser beam welding.
Lasers vary in the quality of the beam produced. A high-quality beam will diffract less when focused, providing for the creation of a smaller spot size. Reflective lenses are important to lasers as well since they are used in the optical cavity where the beam is generated, as well in the beam delivery systems for some lasers. For these reasons, optics play a major role in laser beam welding.
Laser beam welding (Figure 1) does not require additional filler metal and shielding gas is optional. When the beam hits the workpiece, it melts and vaporizes metal atoms, some of which are ionized by the intense beam. This creates what is known as a plume (or plasma) over the weld area that can sometimes interfere with the beam. In these cases, shielding gas may be used to deflect the plume.
The choice of laser type depends on cost, the type and thickness of material to be welded and the required speed and penetration. Lasers are distinguished by the medium used to generate the laser beam, and the wavelength of laser light produced. Although there are many types of lasers, the common lasers for welding include the Nd:YAG, fiber, disk solid-state lasers, and the gas-based CO2 laser. The lasing medium in solid-state lasers are crystals (Nd:YAG and disk lasers) or fibers (fiber laser) that have material added (doped) that will “lase” when exposed to a source of energy, whereas the lasing medium in the CO2 laser is a gas blend consisting of CO2, He, and N2 gas. In all cases, “lasing” occurs when the atoms/molecules of the medium are excited to a higher energy state through the introduction of additional energy (known as pumping). When this occurs, photons are emitted, which, in turn, excite other atoms/molecules. This results in a cascade of photons that travel in coherent waves of a single wavelength, the two properties for which laser light is known.
CO2 lasers produce wavelengths of 10.6 µm, while the wavelength of the solid-state lasers is 1.06 µm. CO2 lasers are generally less expensive, but the longer wavelength of light does not allow its beam to be delivered through fiber optic cables which reduces its versatility. Its light is also more reflective, which limits its use with highly reflective metals such as aluminium. The solid-state lasers are generally more compact and require less maintenance than the CO2 laser. They are more conducive to high-speed production since their beams can be delivered through long lengths of fiber optic cable which can then be attached to a robot. Some of the solid-state lasers such as the fiber laser produce beams of outstanding quality. However, the shorter wavelength of these lasers requires additional safety precautions regarding eye protection.
Figure 2: Focusing of the laser beam.
The choices of focus spot size, focus spot location in the joint, and focal length are all important considerations when laser beam welding. Usually, a small focus size is used for cutting and welding, while a larger focus is used for heat treatment or surface modification. As indicated in Figure 2, the location of the beam’s focal point can also be varied based on the application. When welding, it is common to locate the focal point somewhere near the center of the joint. But cutting applications benefit from placing the focal point at the bottom of the joint. Weld spatter onto the focusing lens can sometimes be a problem, especially when there are contaminants on the surface of the parts being welded. Approaches to minimizing the spatter problem include choosing a long focal length lens which keeps the lens a safe distance from the weld area, or the use of an air “knife” to protect the lens. A-11, P-6
In summary, the advantages and limitations of laser beam welding are as follows:
Advantages:
- High energy density process allows for low overall heat input which produces minimal BM degradation, residual stress, and distortion.
- Fast welding speeds.
- No filler metal required.
- Relatively thick (¾ in.) single-pass welds can be made.
- Concentrated heat source allows for the creation of extremely small weld sizes needed for small and intricate components.
- Easily automated, especially with lasers that are conducive to fiber optic delivery.
- Since there is no bulky torch as with most arc welding processes, laser beam welding is capable of welding joints with difficult accessibility.
Limitations:
- Equipment is very expensive
- Portability is usually low
- Requires very tight joint fit-up and accurate positioning of the joint relative to the beam
- Metals that are highly reflective such as Al are difficult to weld with some laser beam welding processes
- High weld CR may create brittle microstructures when welding certain steels
- Laser plume may be a problem
- Energy efficiency of lasers is poor
- Some lasers require special (and expensive) eye protection
- Laser beam welding is complex and requires significant training and knowledge
High Energy Density Welding
Fundamentals and Principles of High Energy Density Welding
High energy density welding processes are those that focus the energy needed for welding to an extremely small size area. This allows for very low overall heat input to the workpiece, which results in minimal BM degradation, residual stress, and distortion. Welding speeds can be very fast. The two main processes known for extreme energy densities are laser (Figure 1) and Electron Beam Welding (EBW).
Figure 1: Laser welding.
As shown in Figure 2, energy densities of focused laser and electron beams can approach and exceed 104 kw/cm2. These energy densities are achieved through a combination of high power and beams that are focused to an extremely small diameter. Diameters as small as a human hair (0.05 mm) are possible. PAW[KH1] offers greater energy density than conventional arc welding processes and is sometimes referred to as the “poor man’s laser”.
Figure 2: Power densities of various welding processes.
High energy density processes produce weld profiles of high depth-to-width ratio, as compared to other welding processes (Figure 3). As a result, much greater thicknesses can be welded in a single pass, especially with EBW. The figure also illustrates the fact that high energy density processes can produce a weld with minimal heating to the surrounding area as compared to the other processes. However, the high depth-to-width ratio weld profile is much less forgiving to imperfect joint fit-up than the profile produced by arc welding processes.
Figure 3: Comparison of typical weld profiles.
Laser and EBW processes are used in a wide variety of industry sectors. Very high weld speeds are possible and the welds are usually aesthetically pleasing. Laser welding is very adaptable to high-speed production so it is common in the automotive sector. The ability to precisely locate welds on smaller sensitive components with minimal heat input makes laser welding very attractive to the medical products industry.
When welding with high energy density processes, the laser or EB is focused along the joint line of the workpieces to be welded. The extreme power density of the beam not only melts the material, but causes evaporation. As the metal atoms evaporate, forces in the opposite direction create a significant localized vapor pressure. This pressure creates a hole, known as a keyhole, by depressing the free surface of the melted metal. The weld solidifies behind the keyhole as it progresses along the joint (Figure 4). This method of welding known as keyhole welding is the most common approach to laser and EB, and produces the characteristic welds of high depth-to-width ratio. There are some cases where the keyhole mode is not used. This mode is known as conductive mode welding. Conductive mode welds have a weld profile closer to that of an arc weld A-11, P-6
Figure 4: Keyhole mode welding.
High Energy Density Welding
Butt Welds and Tailor-Welded Products
Figure 1: Common automotive applications using laser welding.T-9
AHSS grades can be laser butt-welded and are used in production of tailored products (tailor-welded blanks and tubes). The requirements for edge preparation of AHSS are similar to mild steels. In both cases, a good quality edge and a good fit-up are critical to achieve good-quality welds. The blanking of AHSS needs higher shear loads than mild steel sheets. (see Culling in Blanking, Shearing and Trim Operations)
If a tailored product is intended for use in a forming operation, a general stretchability test such as the Erichsen (Olsen) cup test can be used for assessment of the formability of the laser weld. AHSS with tensile strengths up to 800 MPa show good Erichsen test values (Figure 2). The percent stretchability in the Erichsen test = 100 × the ratio of stretchability of weld to stretchability of BM.
Figure 2: Hardness and stretchability of laser butt welds with two AHSS sheets of the same thickness (Erichsen test values describe the stretchability.B-1)
The hardness of the laser welds for AHSS is higher than for mild steels (Figure 3). However, good stretchability ratios in the Erichsen test can be achieved when the difference in hardness between weld metal and BM is only slightly higher for AHSS compared to mild steels. If the hardness of the weld is too high, a post-annealing treatment (using HF-equipment or a second laser scan) may be used to reduce the hardness and improve the stretchability of the weld.
Figure 3: Improved stretchability of AHSS laser welds with an induction heating post-Heat treatment (Testing performed with Erichsen cup test.T-3)
Laser butt-welded AHSS of very high strength (for example Martensite steels) have higher strength than GMAW [LINK TO 3.2.1] welded joints. The reason is that the high CR in the laser welding process prompts the formation of hard martensite and the lower heat input reduces the soft zone of the HAZ.
Laser butt-welding is also used for welding tubes in roll-forming production lines as an alternative method for HF induction welding.
Assembly Laser Welding
Automotive applications use a variety of welding joint designs for laser welding in both lap joint and seam butt joint configurations as shown in Figure 4. Lap joints and seam butt joint configurations use different characteristics. Seam welds on butt joints need less power from the machine than lap joints due to the smaller weld fusion area, producing less distortion and a smaller HAZ. Butt joint configurations are more cost efficient. However, the fit up for seam welds can be more difficult to obtain than those of lap joints. Also, lap joints tend to provide a larger process window.
Figure 4: Common seam and joint types for laser welding of automotive applications.T-9
When seam welding butt joint configurations, a general guideline for fit-up requirements include a gap of 3-10% the thickness of the thinnest sheet being welding and an offset of 5-12% thickness of the thinnest sheet. A guideline for lap joints can require a gap of 5-10% the thickness of the top sheet being welded (Figure 5). These general guidelines are not absolute values due to the change of variables such as the focus spot size, the edge geometry for butt welds, strength requirements, etc.
Figure 5: Fit-up requirements for butt joint and lap joint configurations in laser welding.T-9
Laser welding is often used for AHSS overlap joints. This type of weld is either a conventional weld with approximately 50% penetration in the bottom sheet or an edge weld. Welding is performed in the same way as for mild steels, but the clamping forces needed for a good joint fit-up are often higher with AHSS than for mild steels. To achieve good laser-welded overlap joints for Zn-coated AHSS, a small intermittent gap (0.1-0.2 mm) between the sheets is recommended, which is identical to Zn-coated mild steels. In this way, the Zn does not get trapped in the melt, avoiding pores and other imperfections. An excessive gap can create an undesirable underfill on the topside of the weld. Some solutions for lap joint laser welding Zn- coated material are shown in Figure 6.
Figure 6: Laser welding of Zn-coated steels to tubular hydroformed parts.L-3
StudiesL-59 have shown welding Zn-coated steels can be done without using a gap between the overlapped sheets. This is accomplished using dual laser beams. While the first beam is used to heat and evaporate the Zn coating, the second beam performs the welding. The dual laser beam configuration combines two laser-focusing heads using custom-designed fixtures.
Remote Laser Welding
Remote scanner welding is used for many automotive applications, including seating (recliners, frames, tracks, and panels), BIW (trunks, rear panels, doors I hang on parts, side walls, and pillars) and interior (IP beams, rear shelfIhat rack) (Figure 7). Compared to conventional laser welding, remote scanner welding has several advantages. Those include a reduced cycle time (via reduction of index time), programmable weld shapes (ability to customize weld shape to optimize component strength), large stand-off (longer protection glass life), and reduced number of clamping fixtures (via reduced number of stations).
Remote laser welding, or “welding on the fly”, combines a robot and scanner optics to position the focused laser beam on the workpiece on the fly. The robot arm guides the scanner optics along a smooth path about half a meter over the workpiece. Extremely nimble scanning mirrors direct the focal point in fractions of a second from weld seam to weld seam. A fiber-delivered, solid-state laser is the source of the joining power far away from the processing station. The scanning optic or Programmable Focusing Optic (PFO) at the end of the laser’s fiber-optic cable is the central element for precise positioning of the laser’s focus point on the component to be welded. Inside the PFO, two scanner mirrors direct the beam through a “flat field” optic, which focuses the beam onto a common focus plane no matter where it is in the work envelope of the PFO. The PFO is also equipped with a motorized lens that allows the focus plane to be moved up and down in the Z-axis. The repositioning of the focused laser beam from one end of the entire work envelope to the other takes about 30 ms.T-9
Figure 7: Remote laser welding of automotive applications.T-9
There are three basic preconditions for welding on the fly. First, a solid-state laser is needed as the beam source. Solid-state lasers enable delivery of the laser beam through a highly flexible fiber optic cable, which is required when joining components in 3D space with a multi-axis robot. Second, a laser with excellent beam quality and the appropriate power is required. Beam quality is the measure of focus-ability of a laser, and the long focal lengths required for remote welding necessitate superior beam quality (i.e., 4 to 8 mm-mrad) to achieve the appropriate focused spot size (i.e., about 0.6 mm) at the workpiece. For remote welding in automotive body production, typically about 4 to 6 kW of laser power is used. The third essential precondition is precise positioning of the weld seams, which requires axis synchronization between the robot and the scanner control. This allows the weld shape programmed in the scanner control for a specific shape weld to have proper shape with the robot moving at various speeds over the part to be welded. Some control architectures use “time” synchronization. The problem here is that if the robot speed is changed for any reason, the weld shape will also change because the axes are not synchronized.T-9
Body-in-White (BIW) Joining
Laser-based solutions can offer a high- and cost-effective improvement potential for steel-based BIW joining. The laser joining design’s stiffness increases in direct relation to the laser weld length. Also, at low process time, there is up to a +14% torsional stiffness increase without any additional joining technique, shown in Table 1.
Table 1: Stiffness performances comparison for several joining designs.A-16
Laser weld shape optimization can help to homogenize performances and increasing the laser weld shape factor leads to a signification reduction of IF fracture risk (Figure 8).
Figure 8: Impact of laser weld design optimization on fracture type.A-16
DP 800 (with additional retained austenite and associated bainite) has the advantage of weight reduction and equally good properties when laser welding as the DP 800. The absolute strength of DP 800 is slightly higher, but the ductility for the DP 800 is greater, shown in Figure 9.
Figure 9: Absolute strength and ductility of DP 800 and DP 800.T-10
Figure 10 shows a cross-tension test in which both materials fail outside the weld zone, DP 800 failing entirely in the HAZ and DP 800 failing partly in the HAZ and partly in the BM.
Figure 10: Cross-tension testing of DP 800 and DP 800.T-10
Figure 11 is the microhardness profile of 1.6-mm Q&P 980’s laser weld joint. Microhardness of both welded seam and HAZ are all higher than BM, and there is no obvious softened zone in HAZ.
Figure 11: Microhardness profile of 1.6-mm Q&P 980’s laser weld joint.B-4
Figure 12 is Erichsen test result for the BM and weld seam of 1.6-mm Q&P 980, showing good stretchability.
Figure 12: Erichsen test result of 1.6-mm Q&P 980, laser welded.B-4
Hybrid Laser and GMAW Welding
In hybrid welding process parameters such as stick out and torch angle are very important to decide overall joint performance. A model has been developed to predict the penetration and toe length under similar heat input conditions, shown in Figure 13. The gap, stickout and angle shows synergic agreement with penetration and toe length but the interactions among them can show disagreement.
Figure 13: Effects of toe length and penetration.T-10
The weld joint strength increases with the increase in wire feed rate for a given laser power shown in Figure 14.
Figure 14: Wire feed rate versus tensile strength of hybrid laser and MIG welds.T-10
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Adhesive Joining
As with mild steels, AHSS-hybrid joints can be made by combining adhesive bonding with RSW, clinching, or self-piercing riveting. These hybrid joints result in higher strength values (static, fatigue, and crash) than the spot welding alone (Figure 1). If local deformation and buckling can be avoided during in-service applications of weld bonding/adhesive hybrid joining, the potential for component performance is enhanced.
Figure 1: Comparison of bearing capacity for single and hybrid Joints.B-3
Hybrid RSW and Adhesives
Many automotive joining BIW applications are using the combination of RSW together with adhesives to obtain superior joint performance. This combination is referred to as WB. Figure 2 shows a tensile shear test for resistance spot welds, adhesives, and weld bonds. There were trends of fracture strength increasing slightly with material strength with the spot welds. Also, the fracture strength was very high, increasing significantly with increasing material strength for adhesive bonds. The fracture strength was the highest for welded bonds and increased significantly with increasing material strength.
Figure 2: TSS failure strength and energy absorption for all joints and materials.T-10
Figure 3 represents a peel test for the same samples. Trends observed were fracture strength increasing slightly with material strength but is lower than the other joining methods for spot welds. Fracture strength for adhesive bonding is greater than Weld Bonding (WB) and spot welding. Energy absorption was not very sensitive to material strength. The bond using adhesive alone had poor failure strength as well as poor energy absorption.
Figure 3: Peel strength data for all joint types.T-10
Figure 4 shows cross tensile results for the three samples. Spot welds showed fracture strength reaching a maximum for DP 600. Adhesive fracture strength increased slightly with increasing material strength. The welded bond’s fracture strength was greater than the other joint types and increased significantly with increasing material strength.
Figure 4: CTS fracture strength and energy absorption for all joints and materials.T-10
Figure 5 shows the fatigue testing performed in tensile shear mode on DP 800 [LINK TO DP steel page in Metallurgy] material (1.2-mm gage). Spot welds showed the lowest fatigue properties of the test samples. While the weld-bonded samples performed much better than conventional spot welds, they were still weaker than the adhesive joints. Adhesive had the best fatigue performance. The parent material properties had little influence upon the fatigue properties of spot welds because the spot weld itself acts as a fatigue crack initiator.
Figure 5: DP 800 fatigue results.T-10
