Mitigation Strategies for Delayed Cracking in Laser Welding of Advanced High-Strength Steel

Cracking susceptibility control of 7075 aluminum alloy in pulsed laser welding with filler strip

The root cracking in the root-pass welds was studied in the Report 1. It may be too conservative to employ the critical preheating temperature obtained from root-pass cracking tests for the necessary preheating temperature.In this study, the critical preheating temperatures for preventing cracking in the root-pass welds and multi-run welds have been compared for steels including mild steel, pressure vessel steel and high strength steel. The cracks investigated in these cracking tests include root crack, toe crack and under-head crack.The findings in this study are as follows:1) The cad cracking susceptibility of carbon manganese steels and low alloy steels with carbon more than 0.16 % may be better evaluated by Ceq(WES) than PCM. For steels with carbon less than 0.16 %, PCM may be a proper parameter rather than Ceq.2) The critical preheating temperature for preventing root cracking in multi-run welds is less than that for root-pass welds by 50 to 75°C, mainly because of a decrease in hydrogen accumulation at the root by a post heating effect of subsequent passes.3) The critical preheating temperature for preventing toe and under-bead cracking in restrained multi-run butt weldments is lower than that for multi-run root cracking, especially in a lower Ceq range.4) It may be practical to select the preheating and inter-pass temperature based on the toe cracking prevention preheating temperature rather than that for root cracking in root-pass welds.

Microstructure and mechanical properties of welded joints between nickel base alloy and 10Ni5CrMoV steel by MIG welding

ABSTRACTWe have developed a technique for observing the weld cracking behaviour in laser welds of high strength steel sheets, using an X-ray transmission in-situ observation system. Weld cracking in the lap joints of this type of steel could be clearly seen, demonstrating that this was, in fact, delayed cracking initiated from the solidification crack at the weld end. It was also discovered that the delayed cracking behaviour such as cracking modes, incubation periods and crack growth rates, varied according to the carbon content in the steel and welding length. The former influenced the hardness of the weld bead, while the latter affected solidification cracking sizes and residual stress. The results of this crack propagation are useful not only for developing laser welding methods of high strength steel but also for additive manufacturing, aimed at mass production.

Dissimilar metal sheets of Ni47Ti44Nb9 (at.%, short for NiTiNb) shape memory alloy and Ti6Al4V (TC4) alloy with the same thickness of 0.2 mm are joined by using micro laser welding technology. The microstructure and crack in the weld are studied by optical microscope (OM) and scanning electron microscope (SEM). The results show that the cracks are easy to generate during the laser lap welding of NiTiNb alloy and TC4 alloy due to plenty of brittle intermetallic compound of Ti2Ni. However, the welding cracks can significantly reduce or disappear by adding filler metal of Ni foil. No-defect joint is obtained when NiTiNb alloy is located on the upper and 50 μm thick Ni foil is added during the lap welding process. The crack sensitivity can be decreased because of finer grains and elliptic boundaries. The shear load of the joint can reach 162 N.

Advanced High Strength Steels or AHSS (Advanced High Strength Steels) have superior properties to conventional steel are widely used in the automotive industry, they provide weight reduction and at the same time, increase vehicle safety in the event of collisions.Flangeability is an essential element for forming automotive components such as chassis and suspension components.Research is ongoing into the development of forming techniques that can improve the performance of AHSS during industrial operations, particularly flange drawing.Therefore, there is great interest in investigating the extensibility of the pre-cut cutting edge and determining the factors that significantly influence the formability of the product.The test used to evaluate flangeability is the Hole Expansion Ratio (HER).In this research, the objective was to evaluate the influence of the conditions of four edge cutting methods on the hole expansion test of advanced multiphase high-strength steel class 780 MPa through four methods: Machining, LASER and punching with expansion of the hole in the direction of the punch (burr downwards) and in the opposite direction (burr upwards).The results of this study corroborate the literature that indicates that the state of the edge after cutting directly influences the expansion capacity of the hole.

In this paper are characterized problems of high strength steel weldability in underwater wet welding conditions. Water as a welding environment intensifies action of unfavourable factors which influence susceptibility to cold cracking of welded steel joints. The susceptibility to cold cracking of S355J2G3 steel and S500M steel in wet conditions was experimentally estimated (by using Tekken test). It was concluded that the steels in question are characterized by a high susceptibility to formation of cracks in welds. Usefulness of the proposed Temper Bead Welding technique (TBW) was experimentally verified as a method for improving weldability of the steels in the analyzed conditions.

Structural changes and the main features of the fracture of the base metal and the coarse-grained region of the heat-affected zone of the welded joints of high-strength steels have been studied by simulating the thermal cycle of welding and post-welding heat treatment. The effects of the simultaneous action of heating for high-temperature tempering and of deformation allowing the estimation of the impact of residual welding stresses have been studied. The probable reasons of the formation of cracks in welds upon the postwelding tempering have been determined.

High-strength structural steels are used in machine, steel, and crane construction with yield strength up to 960 MPa. However, welding of these steels requires profound knowledge of three factors in terms of avoidance of hydrogen-assisted cracking (HAC): the interaction of microstructure, local stress/strain, and local hydrogen concentration. In addition to the three main factors, the used arc process is also important for the performance of the welded joint. In the past, the conventional transitional arc process (Conv. A) was mainly used for welding of high-strength steel grades. In the past decade, the so-called modified spray arc process (Mod. SA) has been increasingly used for welding production. This modified process enables reduced seam opening angles with increased deposition rates compared with the Conv. A. Economic benefits of using this arc type are a reduction of necessary weld beads and required filler material. In the present study, the susceptibility to HAC in the heat-affected zone (HAZ) of the high-strength structural steel S960QL was investigated with the externally loaded implant test. For that purpose, both Conv. A and Mod. SA were used with same heat input at different deposition rates. Both conducted test series showed same embrittlement index “EI” of 0.21 at diffusible hydrogen concentrations of 1.3 to 1.6 ml/100 g of arc weld metal. The fracture occurred in the HAZ or in the weld metal (WM). However, the test series with Mod. SA showed a significant extension of the time to failure of several hours compared with tests carried out with Conv. A.

For the purpose of clarifying the effect of alloy compositions on cracking in laser welds of dental Ni-Cr alloys, 12 commercial and 11 experimental Ni-Cr alloys for crown and bridges were subjected to pulsed YAG laser spot welding, and their welds were investigated by optical and scanning electron microscopy, and EDX and thermal analysis methods. Main conclusions are as follows. 1. Cracks in laser welds were solidification cracks at grain boundaries in weld fusion zones. 2. In the case of commercial dental Ni-Cr alloys, a considerable number of eutectics enriched in Si, A1 and/or S were formed. Greater cracking susceptibility was interpreted by considering that these eutectics solidified at solidification temperatures far lower than the nominal solidus. 3. In the case of experimental Ni-Cr alloys with a small amount of eutectics, S and Si enhanced cracking sensitivity, but Mn reduced cracking. 4. The above results suggest that it is beneficial to the prevention or reduction of cracking to decrease harmful elements such as Si and S forming low solidification temperature eutectics or to add some elements such as Mn forming higher solidification temperature eutectics instead of lower ones.

Welding repair of irradiated nuclear reactor materials such as austenitic stainless steels used for the reactor internals has been challenging due to irradiation generated helium in the material matrix over an extended period of time. In this study, improved laser welding techniques, including the novel Auxiliary Beam Stress Improved (ABSI) technique that proactively manages the stresses during laser repair welding is investigated to substantially reduce or avoid the occurrence of intergranular helium-induced cracking (HeIC). Here, laser weld cladding, with and without ABSI technique, was applied to deposit weld metal on irradiated stainless steel (Type 304 L) blocks with various helium concentrations up to approximately 20 atomic parts per million (appm). Welded samples were cross-sectioned and analyzed using optical and electron microscopes. It was found that, with the improved laser welding techniques, no macroscopic (millimeter-sized) cracks were observed in any of the samples. Microscopic (less than 50 μm) cracks were present in the heat affected zone. Statistical analysis on the sample with the highest helium concentration (~20 appm) reveals that the ABSI technique could effectively reduce the size and amounts of microscopic cracks.

In the case of underwater wet welding, weld crack often occures in heat affected zone.The purpose of the study was to decrease or prevent this crack by using Ni, Fe-Ni and stainless steel electrodes.SS41 steel plate of 12 mm thickness, SM50C steel plate of 12 mm thickness and 80 kg/mm2 grade high strength steel plate (WT80C) of 16 mm thickness were used as base metals.The results were summarized as follows, (1) Using Ni, Fe-Ni, D309 and D310 electrode in welding SM50C and WT80C were effective to prevent under bead crack and crack free weld was able to be obtained. Furthermore there were not found any craking in weld metals of SS41, SM50C and WT80C steel.(2) There were found underbead crack and weld metal crack in welded parts of SS41, SM50C and WT80C steel with D430 electrode.(3) The microscopic segregation of Ni and Cr in weld metals of SS41, SM50C and WT80C steel with austenitic stainless steel electrodes were a little.

This study was carried out to know the laser weldability of Ni-base superalloy, Inconel 718. Inconel 718 whose thickness and grain size were 2 mm and 84 μm were prepared for a bead on plate test using CO2 laser welding with a power of 1 to 3 kW and with a traveling speed of 0.5 to 5 m/min. The testing plate was solution heat-treated at the temperature of 1340 K for 3.6 ks before welding.The shape of penetration depended on traveling speed and was typically classified into two types of Type C and Type N by specialized shape. Type C had low depth to width ratio like a champagne glass and Type N had high depth to width ratio like a nail head. Type C and Type N weld beads are seen at a side of low welding speed and at a high welding speed side, respectively and the critical welding speed between Type C and Type N is 1.5 m/min in the present study.Liquation cracks easily occurred at HAZ in the case of nail head like penetration. We used a radius of curvature of fusion boundary at neck zone of penetration to represent a feature of penetration. It can be seen that a critical radius of curvature exists by which whether cracks occur or not in HAZ can be judged. No weld cracks were found when the radius of curvature exceeded the critical value of ρcr. Furthermore, as a countermeasure welding cracks can be prevented by increasing the laser heat input over the valus of 100 kJ/m in the present study. According to observation of grain boundary liquation at HAZ, liquation crack was related to grain boundary liquation.

Aluminium alloys are widely used in aeronautic industry to produce light weight structures. Laser welding is used in modern applications and has many advantages over conventional welding techniques to join Aluminium alloys. Selection of process parameters is crucial in obtaining a good weld joint. This selection is specific to application and material. Aluminium alloys need special attention while welding with laser due to their physical properties. The objective of this study is to realize fine weld width, deep penetration joint with excellent mechanical properties. Hence, the influence of different laser parameters on the weld width in butt joint configuration is investigated using pulsed ND: YAG laser welding machine. The experiments are designed in Taguchi in such a way that the weld bead grows from conduction mode to keyhole mode. Process parameters (peak power, scanning speed, frequency, pulse width) are varied while keeping variables like focusing distance, shielding gas ad weld strategies are common for all the experiments. The material used is AA 2024. For all the investigated parameters, the weld width, cracks and pores on the weld bead are observed.

<div class="htmlview paragraph">One of the most popular weight reduction strategies in auto bodies is usage of tailor welded blanks (TWB). TWBs with high-strength low-alloy steels and advanced high-strength steels (AHSS) offer possibility of significant weight reduction. In the present work laser welding studies were carried out to fabricate TWB's involving 1.6 mm thickness AHSS DP 590 (dual phase steel) and 0.8 mm thickness conventional IF (interstitial free) steel in different combinations and characterized for their metallurgical and formability characteristics. Fusion zone microhardness was varying with change in material combination. While DP590-DP590 and IF-IF have shown fusion hardness of 300-350HV and 150-180HV, respectively, the dissimilar DP590-IF showed a fusion zone hardness of 250-330HV.Beam offsetting towards the thinner side has reduced the peak hardness. Microstructural analysis revealed mixture of martensite and refined ferrite in the fusion zone of dissimilar welds. The erichsen cup height for TWBs (11 mm) was between cup heights of the welded coupons of the two base metals DP590 (9.5-10 mm) and IF (13.5-14 mm), respectively. Limiting Dome height tests also revealed a similar observation that the limit strains of the TWBs are between the limit strains of the two base materials.</div>