Lecture 7 Heat Affected Zone

1. Lecture 7 Heat Affected Zone 助理教授：王惠森 huei@mail.isu.edu.tw 您可以在以下的網址 Down Load 本檔案 http://140.127.180.27/www/index.htm

2. Heat-affected zone • The heat-affected zone (HAZ) is the area of base material, either a metal or a thermoplastic, which has had its microstructure and properties altered by welding. The heat from the welding process and subsequent re-cooling causes this change in the area surrounding the weld. The extent and magnitude of property change depends primarily on the base material, the weld filler metal, and the amount and concentration of heat input by the welding process.

3. Strengthened Matel • Metals can be strengthened in many ways, such as • work hardening, • solution hardening, • precipitation hardening, • transformations hardening, • dispersion hardening. • The heat-affected zone (HAZ) can be significantly affected by welding in materials strengthened by • work hardening, • precipitation hardening, • transformation hardening.

4. Work Hardening Materials-1 • When a work-hardened material is annealed, the deformed grains in the material tend to recrystallize by forming fresh, strain-free grains. • The stored strain energy is the driving force for the recrystallization of a work-hardened material • When a metal is cold-worked and plastically deformed, numerous dislocations are generated. These dislocations can interact with each other and form dislocation tangles. Such dislocation tangles hinder the movement of newly generated dislocations and, hence, further plastic deformation of the metal. In this way, a metal can be strengthened or hardened by cold-working. Such a strengthening mechanism is often called work hardening

5. RECRYSTALLIZATION AND GRAIN GROWTH of W.H.M in HAZ • The effect of work hardening is completely lost in the fusion zone owing to melting and solidification and is partially lost in the HAZ owing to recrystallization and grain growth.

7. Precipitation Hardening • Precipitation hardening, also called Age hardening, is a heat treatment technique used to strengthen malleable materials, especially non-ferrous alloys including most structural alloys of aluminium and titanium. • It relies on changes in solid solubility with temperature to produce fine particles of an impurity phase, • Since dislocations are often the dominant carriers of plasticity, this serves to harden the material. • precipitation in solids can produce many different sizes of particles, which have radically different effects. Unlike ordinary tempering, alloys must be kept at elevated temperature for hours to allow precipitation to take place. This time delay is called ageing.

8. Precipitation Hardening Materials-I Aluminum Alloys • Dr. Alfred Wilm started work to improve the strength of Al-alloys which then left much to be desired.

10. Al-Cu Diagram-1 Solid Solution • The first step in the precipitation hardening of an aluminum-copper alloy, say A1-4% Cu, is to heat-treat the alloy in the a phase temperature range until it becomes a solid solution (the  phase). This is called "solution heat treating." • The second step is to quench this solutionized alloy to room temperature or below so that it becomes supersaturated in copper. • The third step is to heat-treat the supersaturated solid solution at an elevated temperature (about I90。C) for an optimum period of time-artificial aging. • substantial precipitation of the strengthening phase can also be obtained naturally at room temperature, if sufficient time is allowed. This aging process is called natural aging.

11. ' phase Al-Cu Diagram-2 Artificial Ageing Five sequential structures can be identified during the artificial aging of aluminum copper alloys: • (AI2Cu) • The GP zones (Guinier-Preston) • ' phase • " phases

12. Correlation of thestructures with hardness • Maximum hardness (and strength) occurs when the amount of " (or GP2) is at a maximum in this alloy, although some contribution may also be provided by  ' (6). • As  ' grows in size and increases in amount, the coherent strains decrease, and the alloy becomes overaged. • As aging continues even further, the incoherent  phase forms,and the alloy is softened far beyond its maximum strength condition.

13. Structure During Precipitation • As shown schematically in Figure, the lattice strains are much more severe around a coherent precipitate than around an incoherent one. The severe lattice strains associated with the coherent precipitate make the movement of dislocations more difficult and, therefore, strengthen the material to a greater extent.

14. HAZ Microstructure of Al alloy

15. Hardness in HAZ • During postweld natural aging (PWNA), the GP zones form in the area around position "e," resulting in an increase in hardness. • On the other hand, during postweld artificial aging (PWAA), a significant amount of the metastable phase precipitates, and the hardness of the HAZ increases significantly. However, as mentioned previously, because of the presence of coarse metastable-phase particles, position "d" does not respond effectively to postweld artificial aging. • aluminum alloy artificially aged to contain the metastable phase of ‘ • position "a" is unaffected by the welding heat, whereas position “e" is fully reverted (or solutionized) during welding. The continuous decrease in hardness from position "a" to position "e"

16. Transformation Hardening-Steel • the maximum temperature • the heating rate is very high

17. Microhardness Variation • The high heating rate, together with the brief high-temperature retention time, can also result in the formation of nonhomogeneous austenite during welding. Such a nonhomogeneous austenite, upon subsequent rapid cooling, can cause the formation of localized high-carbon-martensite colonies. Consequently, the microhardness of the HAZ tends to scatter over a wide range

18. HAZ of Mild Steel