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- Solidification Cracking (4)
- Crack Initiation (2)
- Critical Strain Rate (2)
- Dilution (2)
- Hot Cracking (2)
- Solidification cracking (2)
- 4043 Filler Alloy (1)
- 4043 Filler Dilution (1)
- 6060 Aluminium (1)
- 6060 Aluminium Alloy (1)
- Al-Mg-Si Alloy System (1)
- Al-Mg-Si Alloys (1)
- Aluminium Alloys (1)
- Aluminium alloy (1)
- Aluminium alloys (1)
- Aluminum Weldability (1)
- Analysis techniques (1)
- Arc Welding (1)
- Cast Microstructure (1)
- Cooling (1)
- Cooling Rate (1)
- Cooling rate (1)
- Crack Growth (1)
- Crack Initiation-Growth (1)
- Crack Prediction (1)
- Crack growth (1)
- Cracking (1)
- Critical Strain (1)
- Critical Strain Rate-Dilution (1)
- Defects (1)
- Hot cracking (1)
- Light metals (1)
- Mechanism Modeling (1)
- Microstructure (1)
- Minor Elements (1)
- Parameter Effects (1)
- Phase Constituents (1)
- Practical investigations (1)
- Reference lists (1)
- Silicon Content (1)
- Solidification Path (1)
- Strain Rate (1)
- Strain rate model (1)
- Temperature (1)
- Weld Defect (1)
- Weld Metal Microstructure (1)
- Weldability Testing (1)
The objective of the present thesis is to make advancements in understanding solidification crack formation in aluminum welds, by investigating in particular the aluminium 6060/4043 system. Alloy 6060 is typical of a family of Al-Mg-Si extrusion alloys, which are considered weldable only when using an appropriate filler alloy such as 4043 (Al-5Si). The effect of 4043 filler dilution (i.e. weld metal silicon content) on cracking sensitivity and solidification path of Alloy 6060 welds are investigated. Afterwards, cracking models are developed to propose mechanisms for solidification crack initiation and growth. Cracking Sensitivity. Building upon the concept that silicon improves weldability and that weldability can be defined by a critical strain rate, strain rate-composition combinations required for solidification crack formation in the Al- 6060/4043 system were determined using the newly developed Controlled Tensile Weldability (CTW) test utilizing local strain extensometer measurements. Results, presented in a critical strain rate – dilution map, show a crack – no crack boundary which reveals that higher local strain rates require higher 4043 filler dilution to avoid solidification cracking when arc welding Alloy 6060. Using the established crack - no crack boundary as a line of reference, additional parameters were examined and their influence on cracking characterized. These parameter influences have included studies of weld travel speed, weld pool contaminants (Fe, O, and H), and grain refiner additions (TiAl3 + Boron). Each parameter has been independently varied and its effect on cracking susceptibility quantified in terms of strain rate – composition combinations. Solidification Path. Solidification path of the Al-6060/4043 system was characterized using thermal analysis and phase identification. Increasing 4043 filler dilution from 0 to 16% in Alloy 6060 arc welds resulted in little effect on thermal arrests and microstructure, no effect on solidification range, refinement in grain size from 63 to 51 μm, centerline columnar grains disappearance, and decreased cooling rate from 113 to 89 °C/s. Moreover, in order to make direct comparison with literature, castings of controlled mixtures of alloys 6060 and 4043 were also investigated, thereby simulating weld metal composition under controlled cooling conditions. Castings showed a different trend than welds with small increases in silicon content (i.e. increase in 4043 filler dilution) resulting in huge effect on microstructure, no effect on liquidus temperature, drop in solidus temperature from 577°C to 509°C, increase in quantity of interdendritic constituent from 2% to 14%, and different phase formation. Binary β-Al5FeSi, Mg2Si, and Si phases are replaced with ternary β-Al5FeSi, π−Al8FeMg3Si6, and a low melting quaternary eutectic involving Mg2Si, π, and Si. Also, variation of the cooling conditions in castings revealed the existence of a critical cooling rate, above which the solidification path and microstructure undergo a major change. Cracking Model. Implementing the critical conditions for cracking into the Rappaz-Drezet-Gremaud (RDG) model revealed a pressure drop in the interdendritic liquid on the order of 10-1 atm, originating primarily from straining conditions. Since, according to literature, a minimum of 1,760 atm is required to fracture pure aluminum liquid (theoretical), this demonstrates that cavitation as a liquid fracture mechanism is not likely to occur, even when accounting for dissolved hydrogen gas. Instead, a porosity-based crack initiation model has been developed based upon pore stability criteria, assuming that gas pores expand from pre-existing nuclei. Crack initiation is taken to occur when stable pores form within the coherent dendrite region, critical to crack initiation being weld metal hydrogen content. Following initiation, a mass-balance approach developed by Braccini et al. (2000) revealed that crack growth is controlled by local strain rate conditions. Finally, a simplified strain partition model provides a link between critical strain rates measured across the weld and predicted at grain boundaries within the mushy zone. Although based on simplified assumptions, predicted and measured critical strain rate values are of the same order of magnitude. However, because of a longer mushy zone experienced at higher 4043 filler dilution related to a reduction in cooling rate, these models predict a lower weldability with increasing filler dilution, in contradiction with experimental observations. Combining the crack initiation and growth models suggests that hydrogen and strain rate, respectively, determine crack formation. An hypothetical hydrogen – strain rate map defines conceptually the conditions for cracking, suggesting better weldability at low weld metal hydrogen content. With the aid of the modified varestraint test (MVT) and a controlled hydrogen contamination system, results, presented in the form of ram speed – hydrogen map, revealed that hydrogen has little effect on crack growth, providing support to the proposed cracking models. However, a drop in weldability corresponding to the peak in weld metal hydrogen supersaturation suggests a different solidification cracking mechanism, where cavitation supports crack growth.
Al-Mg-Si alloys are known to be highly susceptible to solidification cracking except when using an appropriate filler metal (e.g., 4343 or 5356), although the amount of dilution required to avoid cracking has never been a well-defined quantity. The aim of the present study is to determine a relationship between filler metal dilution, local strain rate conditions, and cracking susceptibility. Making use of the controlled tensile weldability (CTW) test and local strain extensometer measurements, a boundary has been established between crack and non-crack conditions for different local strain rates and dilution amounts, holding all other welding parameters constant. This boundary, presented in the form of critical strain rate-dilution map, defines the critical amount of 4043 filler metal required to avoid solidification cracking when arc welding 6060 aluminium, depending upon local strain rate.
Earlier work has established that a critical amount of 4043 filler is required to avoid solidification cracking when arc-welding 6060 aluminium, depending upon local strain conditions. For example, when the mushy zone behind the weld pool experiences a tensile strain from combined thermal and shrinkage stresses, the possibility exists for crack initiation. For a greater rate of strain, it has been determined that a greater 4043 dilution (i.e. higher weld metal silicon content) is required to avoid crack initiation. Making use of the Controlled Tensile Weldability (CTW) test and local strain extensometer measurements, a boundary has been established between crack and no-crack conditions for different local strain rates and filler dilutions, holding all other welding parameters constant. Using this established boundary as a line of reference, additional parameters have now been examined and their influence on cracking has been characterized. These parameter influences have included studies of weld travel speed, weld pool contaminants (Fe, O, and H), and grain refiner additions (TiAl3 + Boron). Each parameter has been independently varied and its effect on cracking susceptibility quantified in terms of a critical strain rate required to initiate cracking for a given 4043 filler dilution.
A perspective will be given that outlines important considerations in evaluating and predicting weldability. An examination will be made of the local conditions necessary for solidifications crack initiation and growth in a weld. This will be done in light of two prominent thermo-mechanical approaches involving critical strain and critical strain rate. Critical conditions will be identified based upon values available in the literature. Methods used to measure strain and strain rate will be compared. The interpretation of crack lenght measurements commonly used to quantify weldability will be questioned, based upon our current understanding of the problem. Complications and problem areas needing better definition will be identified and discussed, including strain distribution in the mushy zone, segregation at grain boundaries, effect of impurities, and effect of cooling rate on solidification path. Finally, a suggestion will be made for a new approach to weld development using in-situ strain rate measurements and new composition-strain rate maps that define the boundary between crack an no-crack conditions.
Aluminium alloy 6060 is typical of a family of Al-Mg-Si extrusion alloys, which are considered weldable only
when using an appropriate filler alloy such as 4043 (Al-5Si) or 5356 (Al-5Mg). This study concerns the
thermal analysis and solidification path determination of aluminium alloy 6060 diluted with variable amounts
of 4043. Casting of controlled mixtures of alloys 6060 and 4043 was used to simulate different weld dilutions
in order to facilitate thermal analysis. Thermal analysis and metallography were applied in order to reveal
solidification reactions and phases, and allow calculation of solid fraction versus temperature curves.
Although the liquidus temperature was little affected, different phases formed with increased 4043 dilution,
with a trend toward lower temperature reactions and a larger fraction interdendritic constituent.
The occurrence of solidification cracking during welding remains a little understood phenomenon, in spite of extensive studies and tests performed to evaluate and compare the relative weldability of many different alloys. From an approach often adopted in the welding community attributed to Prokhorov, solidification cracks are believed to form when a critical tensile strain is exceeded, specific to the alloy, assuming that the mushy-zone has limited ductility. Tensile stresses and strains normally form behind a moving weld pool as a result of solidification shrinkage and thermal contraction, as influenced by welding parameters and the degree of restraint.