A two-dimensional finite-element nonlinear transient heat conduction model was developed and used to simulate deep-penetration keyhole laser welds in aluminum alloys. The weld thermal profiles were calculated in an arbitrary reference plane as the laser beam approached and passed the plane. From the calculated thermal profiles, three-dimensional quasi-steady-state shapes of the weld pools were determined. The predicted weld bead shape and dimensions were in good agreement with the experimental results. The experimental laser welds in aluminum alloys contained large amounts of porosity. The model predicted large mushy zones for aluminum laser welds during solidification, which in turn increase the probability of porosity formation by increased bubble entrapment.

Finite-element modeling of heat flow in deep-penetration laser welds in aluminum alloys

Study of surface temperature on laser cutting and welding power absorption

Lasers date back to 1960(1) which earmarked the initial operation of a ruby laser. For the following years, average laser output power remained low and, as a result, the development of materials processing applications progressed slowly. An important commercial application evolved in 1965(2) when the Western Electric Company adapted a pulsed system for production drilling of diamond wire drawing dies. In this application the processing time for drilling of a die was reduced from 24 hours for conventional techniques to less than one hour with lasers. A variety of commercial drilling applications have since been developed; one of the most successful being laser drilling of cooling passages in gas turbine parts (2,3). The success of these initial production applications hinged on the laser’s ability to accurately deliver a precisely controlled amount of energy to a highly localized region of a workpiece.

Welding with High Power Lasers

Multi-kilowatt C0 2 lasers are increasingly being used for welding applications due to their capability of producing deep penetration welds at high welding speeds. Although most metals are being successfully welded, laser welding of aluminum alloys is proving to be very difficult. Combination of high thermal diffusivity and optical reflectivity to C0 2 laser radiation causes difficulty in coupling the laser energy to the material. Much higher laser intensities are required to ensure coupling and to form the keyhole, than to maintain the keyhole and obtain smooth fusion, as absorption of laser energy improves dramatically with the keyhole formation. Laser welds in aluminum alloys contain extensive porosity, and excessive vaporization of alloying elements occurs from the fusion zone. The welds have severe undercuts and large discontinuous cavities on the bead surface. A more precise control of the weld heat input and a better understanding of the influence of process parameters and the alloying content on the weld characteristics is required to improve the laser weldability of aluminum alloys. High power laser welds in aluminum alloys were characterized in terms of the weld shape and dimensions, the nature and amount of porosity, loss of elements from the fusion zone, and the microstructures in and around the welds. A two-dimensional finite element heat conduction model was developed 5 to numerically simulate the melting and solidification phenomena in keyhole laser welds. Significant variations in weld dimensions were observed among the different aluminum alloys with marked difference in the welding process efficiencies. The magnesium containing 5xxx series alloys showed the highest weld penetrations, as well az periodic variation in the penetration along the weld length, evidently caused by an unstable and oscillating keyhole. Alloy 2219 contained large shrinkage S' . , , ... . ' b.' % '% '% % "" ' " " P' " " i l iv cavities in the welds, but showed the minimum amount of voids among the alloys investigated. Laser welds in the 5xxx series alloys had excessive amount of spherical porosity and large irregular shaped cavities. Significant vaporization of magnesium and aluminum from the weld, and the entrapment of parts of the " unstable cavity in the solidifying metal, are believed to cause the excessive amount of voids observed in the 5xxx series alloys. AceesoIn For INTIS GFA&I D2 TAV ~ [

Influence of Process Parameters on Laser Weld Characteristics in Aluminum Alloys

The literature relating to the laser welding of a number of alloys was studied, this revealed
gaps in the current state of knowledge. It was found that no indepth parametric or
inicrostructural investigations had been carried out into the laser welding of non-ferrous
alloys. Although problems associated with the laser welding of aluminium alloys had been
identified, there was little in the literature to suggest an indepth study into the effect of
surface modifications in enhancing the laser weldability of this material had been carried out.
The laser welding of aluminium based metal matrix composites has also yet to be studied in
detail. A programme was therefore set up to carry out an indepth investigation into the laser
welding of a number of alloys namely; Nimonics C263, and PE11, aluminium 6061, and
aluminium 6061 based boron fibre reinforced, and silicon carbide particulate reinforced metal
matrix composites.
The study was aimed at developing an understanding of the microstructural effects of the
laser welding process on the alloys, and assessing the structural integrity of the resultant
welds. The effect of laser processing parameters such as laser power, laser beam traverse
speed, lens focal length, and the manipulation of these parameters on the welding efficiency
and weld area integrity was also investigated. Other tasks within the project included a
study on the possibility of using an anodic film to enhance the laser weldability of Al 6061.
Finally attempts were made to identify novel phases observed in the weld area of the
composite materials.
Nimonics C263, and PEll exhibited laser welds free of cracks and porosity. The difference
in composition between the two alloys did not result in any significant dissimilarities in their
response to the laser welding process. The welds in both alloys exhibited a fine columnar
dendritic microstructure, and while carbides were observed in the interdendritic regions of
the welds, electron optical analysis did not reveal any y precipitates in this region. It was
concluded that for the welding of thin gauge materials above a threshold laser power the
resultant welding efficiency shows a greater dependence on laser beam mode, and laser spot
size, than on laser power, and beam traverse speed. Aluminium 6061 was not easily welded
with a laser in its as received form, and the welds showed some degree of porosity.
Anodising was found to improve the welding efficiency in this material. While the presence
of an anodic film on the metal surface increased the welding efficiency of the alloy, no
relationship was found between the thickness of the anodic film and welding efficiency in the
range of film thicknesses investigated in this work. Weld regions were observed to be
cellular dendritic in structure, with narrow heat affected zones. No precipitates or low
melting point phases could be identified in the weld region. Melt zones were successfully
produced in the composite materials, with the main problem encountered being that of
porosity adjacent to the weld bead. It was shown that manipulation of the laser welding
parameters resulted in a decrease in this porosity. In the weld beads, a number of novel
phases were observed. These were identified with the aid of TEM, and SIMS analysis
techniques.

Laser welding of selected aerospace alloys

: This report contains the results of the first part of a fundamental study of the deep-penetration laser welding process The study was conducted during the year 1974-1975 with the objective of generating information relevant to the application of laser welding in manufacturing, with particular emphasis on materials of interest to the Navy The program has concentrated on (1) a study of the influence of laser parameters on welding performance, and (2) a study of the effect of the physical properties of metals on their laser welding response In the course of study 1, speed/power relationships were determined, along with the effects of laser operating mode and focusing optics on weld penetration and bead shape and cross-sectional area As a result of study 2, it was determined that within the range where operation could be maintained in the deep-penetration mode, a linear relationship exists on a log-log plot between a dimensionless speed parameter, and a power parameter which is also dimensionless It was noted that this represents a direct relationship between speed and penetration under conditions of constant power and focusing optics This correlation can thus be used to select initial deep-penetration laser welding parameters for any metal providing its properties are known Energy absorption and melting efficiencies were also determined for a range of elemental metals and a new relationship was determined for the limiting melting efficiency

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B A Jacob

Papers

Evaluation of Basic Laser Welding Capabilities.