Poor Weldability Research Articles

A New Paradigm for Designing High-Fracture-Energy Steels

The steels used for structural and other applications ideally should have both high strength and high toughness. Most high-strength steels contain substantial carbon content that gives poor weldability and toughness. A theoretical study is presented that was inspired by the early work of Weertman on the effect that single or clusters of solute atoms with slightly different atom sizes have on dislocation configurations in metals. This is of particular interest for metals with high Peierls stress. Misfit centers that are coherent and coplanar in body-centered cubic (bcc) metals can provide sufficient twisting of nearby screw dislocations to reduce the Peierls stress locally and to give improved dislocation mobility and hence better toughness at low temperatures. Therefore, the theory predicts that such nanoscale misfit centers in low-carbon steels can give both precipitation hardening and improved ductility and fracture toughness. To explore the validity of this theory, we measured the Charpy impact fracture energy as a function of temperature for a series of low-carbon Cu-precipitation-strengthened steels. Results show that an addition of 0.94 to 1.49 wt pct Cu and other accompanying elements results in steels with high Charpy impact energies down to cryogenic temperatures (198 K [–75 °C]) with no distinct ductile-to-brittle transition. The addition of 0.1 wt pct Ti results in an additional increase in impact toughness, with Charpy impact fracture energies ranging from 358 J (machine limit) at 248 K (–25 °C) to almost 200 J at 198 K (–75 °C). Extending this concept of using coherent and coplanar misfit centers to decrease the Peierls stress locally to other than bcc iron-based systems suggests an intriguing possibility of developing ductile hexagonal close-packed alloys and intermetallics.

Effect of Zn content on the microstructures and mechanical properties of laser beam-welded ZK series magnesium alloys

The effect of Zn content on the microstructures and mechanical properties of laser beam welded ZK series magnesium alloys (ZK21, ZK40, and ZK60) has been studied. Owing to the lower heat input, laser beam welding can successfully be employed to weld ZK series magnesium alloys having Zn content up to 4 wt%, which are difficult to weld by means of conventional arc welding. However, ZK60 is susceptible to solidification cracking and presents a poor weldability, which may originate from the net-like distribution of more Mg51Zn20 precipitates along grain boundaries (GBs) in the fusion zone (FZ). With increasing Zn content, the amount and size of precipitates along GBs in the FZ increase, and the morphology of grains in the FZ adjacent to fusion boundary changes from cellular to equiaxed dendritic. The grains in the FZ of ZK40 alloy are the finest among the three alloys, whose size is only about 4.8 μm, and the ZK40-welded joint achieves the highest ultimate tensile strength of 312 MPa, which is up to 90.4% of the base metal.

예비전류를 이용한 자동차용 프리실드 강판의 저항 점 용접성 향상에 관한 연구

In the present day, new performances are demanded for automobile body steel sheet for example, light weight, durability, resistance corrosion and environmental protection. For an improvement of resistance corrosion, European automobile companies developed pre-sealed steel in 1990's. Pre-sealed steel have organic coated layer upper zinc coated layer. Evaluating a resistance spot weldability of pre-sealed steel plate is important because resistance spot welding is used mainly in automobile body joining process. In this study, evaluating a weldability of pre-sealed steel plate was carried out using lobe diagram, dynamic resistance and high-speed image. Usually, Pre-sealed steel has poor weldability than GA steel's. To improve a weldability of pre-sealed steel sheet, pre-current was applied to welding process. Pre-current is applied prior to base-current. In conclusion, the weldability of pre-sealed steel has been improved than GA steel's by applying pre-current.

Electron scanning microscopy analysis of fracture surfaces in laser welded samples of hardmetals

AbstractHardmetals of tungsten carbides in a cobalt matrix have been attempted to weld in similar joints by Nd/YAG laser in continuous wave mode. The combination of mechanical and thermo-physical properties of these materials poses a significant difficulty in welding these materials, especially for grades with low Co content, as 6 to 12% in volume. However, grades with higher Co content also showed a poor weldability. In fact, when welding hardmetals with a Co content of 25% in volume, a large amount of pores was observed on the fracture surface. Analysis under scanning electron microscopy showed these pores were coated with a thin film.Observations with energy dispersive spectroscopy (SEM-EDS) revealed this film is rich in cobalt and is highly unstable. Figure 1 shows the characteristics of the film around a pore evidencing its corrugated aspect originated during rapid cooling conditions verified during laser welding.

Application of electrode force change in single sided resistance spot welding using servo gun

Single sided resistance spot welding (SSRSW) is considered as a feasible method to join hydroformed or closed section parts to others in vehicle productions. Unfortunately, it is difficult to guarantee the weld quality utilising conventional air gun. Because of a lack of support inside the closed section parts, the impact of electrode driven by pneumatic gun will cause large deformation of the workpieces at the welding stage and will lead to a crack around weld region after welding completion. In addition, poor weldability is another pressing problem for welding operations. Servo gun with new gun driven method has some merits such as realising the soft touch between electrode and workpieces and changing electrode force during the welding process which are greatly suitable to the sheet to tube joining. Based on the characteristics of servo gun, the present paper investigates a new method to increase the weld quality of sheet to tube joining with SSRSW method. By adjusting the electrode force during the welding process, weld strength would be increased, weld deformation be decreased and weldability lobe curve be widened. The results verify that servo guns can provide high assurance for welding quality of sheet to tube joining and have broad prospect in SSRSW.

Microstructure and mechanical properties of Sc modified Al–Cu alloy (AA2219) electron beam welds

Al alloy AA2219 is a high strength alloy belonging to the 2000 series Al alloys widely used for aerospace application. One of the drawbacks of most of the high strength Al alloys is that they suffer from poor weldability. However AA2219 is an exception due to the presence of more Cu that helps in healing cracks by providing extra eutectics. Although AA2219 has excellent weldability the strength of a welded joint is only 35–45% of the base metal. The loss of strength is due to the dissolution of the strengthening precipitates during melting. Therefore, there is a need to improve the fusion zone strength of AA2219 welds. In this study an attempt was made experimentally to use electron beam welding and modify the base metal chemistry with addition of Sc and Zr in order to improve the mechanical properties of the AA2219 weld joints. Fine equiaxed microstructure was obtained in the fusion zone of electron beam welds with improved mechanical properties. Eutectic was found to be distributed discontinuously which resulted in improved yield strength, tensile strength and ductility compared to welds made without Sc and Zr.

A Comparison of Weld Properties with or without Filler Wire on Laser Welding of Magnesium Alloy for Car Industry

Magnesium alloys are becoming important material for light weight car body, due to their low specific density but high specific strength. However they have a poor weldability, caused by high oxidization tendency and low vapour temperature. In this study, the welding performance of magnesium alloy was investigated for automobile application. The material was rolled magnesium alloy sheet contains 3wt%Al, 1wt%Zn and Mg balance. The effects of filler wire addition was investigated on 2kW Nd:YAG laser welding. For the results, the mechanical properties of welded specimen were similar with base metal in laser welding with and without filler wire. The bridging ability was improved with filler wire without weld properties deterioration on laser welding of magnesium alloy.

Improvement of mechanical properties of aluminium–copper alloy (AA2219) GTA welds by Sc addition

Aluminium alloy AA2219 is a high strength alloy widely used for aerospace application. One of the drawbacks of most of the high strength aluminium alloys is that they suffer from poor weldability. However, AA2219 is an exception due to the presence of more copper that helps in healing cracks by providing extra eutectics. Although AA2219 has excellent weldability, the strength of a welded joint is only 35–45% of the base metal. The loss of strength is due to the dissolution of the strengthening precipitates during melting. Therefore, there is a need to improve the fusion zone strength of AA2219 welds. In this study, an attempt was made experimentally to use different gas tungsten arc welding (GTAW) techniques and modify the filler chemistry with addition of scandium (Sc) and zirconium (Zr) in order to improve the mechanical properties of the AA2219 weld joints. The application of the GTAW DCEN with Sc and Zr to the filler wire proved to be the most economical and perhaps the optimal choice for welding AA2219 aluminium alloys. Addition of Sc and Zr to the fusion zone during welding enhanced grain refinement in the GTAW welds leading to better mechanical properties when compared to welding with conventional AA2319 filler.

High Strength Stainless Austenitic CrMnCN Steels – Part I: Alloy Design and Properties

Generally the strength of stainless austenitic steels does not live up to their good corrosion resistance. Solid solution hardening by interstitial elements is a means of raising the strength, but is used only moderately because of poor weldability, which, however, is not required in various applications. The solubility of nitrogen is high in stainless austenite of steels with 18 mass% of Cr and Mn each, but low in the melt. Carbon reveals the opposite behaviour. Instead of producing high nitrogen steels by pressure metallurgy, about 1 mass% of C+N is dissolved in the melt at ambient pressure. The new cost‐effective C+N steel reaches a yield strength of 600 MPa, a true fracture strength above 2500 MPa and an elongation above 70 %. Conduction electron spin resonance revealed a high concentration of free electrons. Thus, the ductile metallic character of the C+N steel is enhanced, explaining the high product of strength times toughness. The high interstitial content requires rapid quenching to avoid an embrittling precipitation and respective intercrystalline corrosion.

Study on Property and Microstructure in HAZ of High-Strength Abrasion-Resistant Steel

In this paper, a type of the high-strength abrasion-resistant steel-NM360 has been studied with the aid of the modern physical simulation technology to solve the problem of its poor weldability. In the experiment, the welding thermal cycles under different cooling conditions with the peak heating temperature of 1320oC were simulated via the Gleeble-1500D thermal/mechanical simulator. The SH-CCT diagram of NM360 has been established by investigating the microstructure transformation course and the hardness of the welded heat-affected zone (HAZ) near fusion line during different cooling process. Moreover, the impact toughness and fracture in HAZ with different cooling rate have been studied. The established SH-CCT diagram and impact toughness in HAZ provide the referential experimental basis for selecting proper welding parameters. At last, the welding parameters for NM360 steel are determined with the aid of nomography of carbon dioxide gas shielded arc welding and empirical formula.

Weldability and Microstructure of Nickel-Silicon Based Alloys

Abstract The NiSi x based alloy typically has poor weldability due to its lower ductility. A limited amount of work has been performed on the weldability of NiSi x based alloys. Therefore, the effect of heat treatment and welding parameters on weldability of the alloys, and the relationship between the weldability and microstructure were studied. The results show that the as-cast Ni-Si-Nb-B alloy (Ni 76.5%, Si 20%, Nb 3%, and B 0.5%) could be successfully welded after preheating at 600 °C. The welding procedure should be performed on the alloys before any heat treatment and a preheating at 600 °C should be used. The fusion zone is harder than the matrix due to a large amount of γ phase and a finer microstructure. The cracks are predominantly intergranular in heat affected zone and associated with the needle-like γ phase. The heat treatment before welding increases the tendency of cracking in the fusion zone.

Friction welding of Cr–Zr copper alloy

Summary Copper and copper alloys have a higher thermal conductivity than even aluminium, itself regarded as having good heat conduction. When materials with high thermal conductivity are joined, the heat rapidly diffuses in the base metal in a way that generally results in poor weldability due to the difficulty of concentrating heat in welds. To obtain sound welds with satisfactory penetration during fusion welding of copper alloys, it is necessary to ensure high-temperature preheating and high interpass temperatures. Copper alloys are also reported to face problems such as solidification cracking and blowhole formation.1 To solve these problems, it is necessary to perform welding at low heat input for a short time, which are conflicting requirements because of the high heat conduction.

Weldability and high temperature performance of welded joints of heat-resisting alloys

The applications of various heat resisting alloys applications depend upon the service temperature and the environment in which they are employed; austenitic stainless steels are employed mainly in the temperature region of approximately 850–950 K and Fe-base, Co-base and Ni-base superalloys are employed for high-temperature use at more than approximately 950 K. Problems concerning the integrity of weld procedures and high-temperature performance of weld zones should be considered for the application of these heat-resisting alloys to welded structures. In fact, most heat resisting alloys have poor weldability compared with carbon and low-alloy steels and are subject to problems such as cracking during welding. Furthermore, there are many instances of deteriorating high-temperature performance of the weld zone due to changes in the microstructure which accompany welding. On the other hand, heat-resisting alloys are employed in severe high-temperature environments and it is highly possible that there will be an occurrence of fissures and deterioration due to deformation, creep damage, oxidation and carburising. There are many cases where materials which have been in service for extended periods have increased weld crack susceptibility. Repair welding of these requires further attention to be given to the possibility of weld cracks.

Ａｌ‐Ｍｇ合金の超音波接合 １ Ａｌ‐Ｍｇ合金の超音波接合性におよぼすマグネシウムの影響

The behavior of magnesium (Mg) in the ultrasonic welding process of aluminum (Al) alloy containing Mg was investigated to understand the reason for the poor weldability. Commercially pure Al (A1050), Al-2.6 mass% Mg alloy (A5052), and Al-4.1 mass% Mg alloy (A5086), 1.2 mm thick each, were lap-welded and the fracture surfaces were examined by SEM observation and AES analysis. For all of these materials, the tensile load of the weld increased with an increase in the clamping load or in the welding time. In the case of Al-Mg alloys, higher Mg concentration required higher clamping load or longer welding time to obtain the tensile load equivalent to A1050. SEM observation of the fracture surface of the weld indicated that dimple pattern occupied the almost whole fracture surface of A1050. On the other hand, upon Al-Mg alloys, scraped fracture pattern, dimple pattern and brittle fracture pattern were observed and the scraped pattern occupied the largest area in the fracture surface. AES analysis of each fracture surface showed that Mg was segregated at the surface presenting the scraped and the brittle pattern. Especially at the scraped pattern surface, welding for short time of 0.5 s induced the segregation of Mg about twice as high as the concentration before welding. When the specimen having the surface segregation of Mg formed by the pre-heat treatment was welded, the scraped pattern was observed on the almost whole fracture surface and the tensile load of the weld decreased. Conclusively, it was found out that the surface segregation of Mg induced during the process of the welding caused the ultrasonic weldability of Al-Mg alloy to deteriorate.

PRODUCTION OF A QUENCHED AND TEMPERED ROLLED PRODUCT FOR THE REINFORCEMENT OF CONCRETE

Until recently, metallurgical plants in the nations of the CIS (Commonwealth of Independent States) ‐ including Russia and Ukraine ‐ produced weldable rolled products for the reinforcement of concrete by using hot-rolled killed low-alloy steels 35GS and 25G2S. These steels belong to strength class A400S (A-III) under GOST 5781 (mimimum yield point 400 MPa). The production of these steels consumed 28‐40 kg of ferroalloys per ton of steel, and the consumption coefficient realized in obtaining the rolled products from the ingots was 1.24‐1.30 kg/ton. The carbon content of steel 25G2S was limited to the standard 0.31%, while the carbon content of steel 35GS was 0.40%. Attempts were made to replace the hot-rolled killed low-alloy steel with hot-rolled semikilled steel 32G2Rps or quenched and tempered semikilled carbon steel St5ps, which also contains up to 0.40% C and correspond to strength class A400S (under GOST 10884, quenched and tempered steel St5ps corresponds to class At-IIIS). At such carbon contents, the reinforcement steel is characterized by poor cold shortness and limited weldability and fails to meet existing international standards. International standard YSO 9655/2, European standard EN 10080, and the national standards of different European nations specify a maximum carbon content of 0.24% for finished rolled products of the given designation. However, these norms stipulate only the carbon equivalent, not the methods used to make or cast the steel. In addition, the construction industry worldwide has instituted the widespread use of a new class of reinforcement steel, now using grade A500S instead of grade A400S. The export-oriented strategy being adopted for the growth of ferrous metallurgy in the CIS nations due to stagnation of the domestic construction industry makes it necessary to improve the quality of their rolled products and bring those products’ chemical composition and properties in line with world standards, to ensure that the products will be competitive in the world market. A reduction in production costs also improves their competitive standing. In 1993, a group of metallurgical researchers based at the West Siberian Metallurgical Combine organized the mass

各種アルミニウム合金管とＳＵＳ３０４ステンレス鋼管の摩擦圧接性

Friction welding of various aluminum alloy pipes to SUS304 stainless steel pipe was carried out making clear weldability at dissimilar material pipe. A1050/SUS304, A5052/SUS304, A5056/SUS304, A6061/SUS304 and A6063/SUS304 joints showed good weldability and the maximum joint efficiencies of these joints in tensile test were about 91%, 71%, 60%, 79% and 97%, respectively. In these cases, the formation of Al13Fe4 intermetallic compound was induced by the interdiffusion through the weld interface. On the other hand, other joints had poor weldability. In the cases of A2011/SUS304, A2017/SUS304 and A2024/SUS304 joints, CuAl2 and Al7Cu2Fe were observed on the friction surface, and the maximum joint efficiencies of these joints were about 26%, 39% and 32%, respectively. For the case of A7075/SUS304 joint, Al23CuFe4 was observed on the friction surface, and the maximum joint efficiency was about 37%.

Effect of Structural Changes on Tensile Properties in an Electron Beam Welded and Heat-Treated Ti-Al-Mn Alloy

The high strength-to-weight ratio of titanium alloys has contributed to their extensive use in the aerospace sector. More recently, however, their excellent resistance to corrosion and generally superior mechanical properties have resulted in their increasing use in the chemical industry as well as by the medical profession. Many of these applications involve welded joints and from this point of view the alpha-beta titanium alloys are considerably more difficult to join than the alpha or the beta alloys. Within the alpha-beta family, though many highstrength alloys have been developed to have greater fracture toughness than the standard alloy Ti-6AI-4V, their use has been limited because of poor weldability [1]. This is because of invariably low ductility in the as- welded condition and there is some evidence that, with the addition of increasing amounts of beta stabilizers, the ductility deteriorates further [2]. This has led to the design philosophy in the erstwhile Soviet Union involving the development of titanium alloys with moderate strength but appreciably greater ductility for welded fabrications subjected to complex stresses. While heavier cross-sections can be used for maintaining load-carrying capacity, the ductility would ensure satisfactory weldability [3].

Hydrogen cracking behavior in an iron aluminide alloy weldment

Significant interest exists in developing polycrystalline, long-range ordered iron-aluminide alloys for high temperature applications because of their unique properties such as superior oxidation resistance. However, detrimental properties such as low room temperature ductility and poor weldability restrict the use of iron-aluminides in structural applications. This paper describes the results of hot dutility tests conducted on iron-aluminide alloy FA-129 to determine the effect of test temperature, grain size and atmosphere on the ductility. Also, a preliminary study was performed to determine the hydrogen cold cracking sensitivity of weldments produced in varying atmospheres of water vapor.

The effect of oxidizing environments on high temperature fatigue failure of 3CR12 alloy: Aghion, E. and Ferreira, J. Can. Metall. Q. (Oct.-Dec. 1993) 32 (4), 369–374

The influence of the plastic behaviour of two aluminium alloys, very popular in welding construction, on friction stir weldability, is analysed in this work. The two base materials, a non-heat-treatable (AA5083-H111) and a heat-treatable aluminium (AA6082-T6) alloy, are characterised by markedly different strengthening mechanisms and microstructural evolution at increasing temperatures. Their plastic behaviour, under different testing conditions, was analysed and compared. The two base materials were also welded under varied friction stir welding (FSW) conditions in order to characterise their weldability. The relation between weldability, material flow during FSW and the plastic behaviour of the base materials, at different temperatures, was analysed. It was found that the AA6082 alloy, which displays intense flow softening during tensile loading at high temperatures, and is sensitive to dynamic precipitation and overageing under intense non-uniform deformation, displays good weldability in FSW. Under the same welding conditions, the AA5083 alloy, which in quasi-static conditions displays steady flow behaviour at increasing temperatures, and is sensitive to moderate hardening at high strain rates, displays poor weldability.

Weld solidification cracking in cast Ni 3Al alloys

Most of the cast Ni[sub 3]Al alloys currently under study have marginal resistance to solidification cracking, and therefore, they have poor weldability. Crack-free welds can be made on the alloys only with difficulty, and with poor reproducibility. Based on metallographic analysis of solidification cracks in welds of one such alloy, identified as IC221M, modest composition modifications were made to the base alloy to improve solidification-cracking resistance. The tensile properties and fabricability of the modified alloy, IC221W to denote improved weldability, were evaluated and found to be comparable to those of the base alloy and used to weld plates of cast IC221M. A crack-free weldment was obtained, which had good tensile properties up to 900 C.