Weld initiation time

In spin welding, the weld initiation time (WIT), consists of stage I and stage II and decreases with axial pressure. The WIT is found to be 1.5 sec at 2.0 MPa, 0.75 sec at 4.3 MPa, and less than 0.25 sec at 6.8 MPa. All of these effects may be attributed to the increased temperature rise, hence, melting rates, that occur at higher pressures. 

One more relationship is to be mentioned. In dry fatigue of smooth components, the initiation time makes the major part of the lifetime (e.g. 90%). For components with welds (not first class), with sharp notches or exposed to a corrosive environment, this relationship is turned upside down, so that the initiation period is... 

As the name implies, corrosion fatigue is affected by both the severity of corrosive conditions and mechanical, cyclical stress factors. Stress raisers such as notches, holes, weld defects, or corrosion pits can initiate fatigue cracks and a corrosive environment can reduce crack initiation time. For many materials, the stress range required to cause fatigue failm-e diminishes progressively with increasing time and with the number of cycles of applied stress. 

Although caustic cracking occurs over a wide range of temperatures, these appears to he no correlation between temperature and time to failure. Because few failures have been reported at nearambient temperatures, it appears that crack initiation times are inordinately long unless precracking, for example, in the form of weld defects, has occurred. Caustic cracking of carbon steel has been found to occur over a narrow range of potentials near the active current peak of potential/log current curves. 

Ultrasonic Welding. Ultrasonic welding has been appHed to Tefzel with weld strength up to 80% of the strength of the base resin. Typical conditions include a contact pressure of 172 kPa (25 psi) and 1—2 s cycle time. The two basic designs, the shear and butt joints, employ a small initial contact area to concentrate and direct the high frequency vibrational energy. 

Immediately following the initial assembly (and at any other time leaks are suspected), the box and purification train should be tested for leaks. A quick test for leaks is to pressurize the dry box until the gloves stick straight out. The gloves should remain in this position for several hours if no leaks are present. If leaks are indicated, testing is most easily accomplished while the box is pressurized. If the inert atmosphere is helium, the preferred method is the helium sniff test. All joints, welds, and connections should be checked. In the absences of a helium-sensitive probe or if the inert atmosphere is other than helium, the bubble method may be used a small amount of soapy water is placed on leak-prone welds and joints and the appearance of any bubbles is noted. 

The feasibility of FSW of titanium was first demonstrated prior to 1997 at which time a TWI group sponsored project (GSP 5689) was instituted to further the development of FSW in titanium alloys [1]. Proof- of-concept and initial development was performed mainly on Ti-6A1-4V. While some reports of FSW of titanium are available in conference proceedings or have been presented at conferences [2-6] no papers in the archival literature have been published on the subject. Conceptually, FSW of titanium is attractive because it may mitigate some problems associated with fusion welding of titanium alloys. For example, because peak temperatures in FSW are necessarily lower than those encountered in fusion welds, problems such as grain growth in the HAZ and embrittlement due to contamination by interstitial elements (0, N, C) uptake may be reduced. 

The initial aluminum FSW studies conducted at The Welding Institute (TWI) used a cylindrical threaded pin and concave shoulder tool machined from tool steel (Ref 2). Since that time, tools have advanced to complex asymmetric geometries and exotic tool materials to friction stir higher-temperature materials. This... 

As shown, the weld zone continues to harden even after 6 years of natural aging. Similar hardness results are reported by Merati et al., where minimum hardness in the HAZ of friction stir welded 7050-T7651 increased by 9% after natural aging for 10 months (Ref 74). Time between FSW and the initial hardness measurements was not reported but is believed to be more than 2 months. 

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