Laser Welding Dissimilar Metals – Ti-6Al-4V to Inconel 718

Capability to laser weld dissimilar metals is of increasing interest in a number of industries, including aerospace, automotive, medical devices, and energy. The flexibility to create components in which the materials are optimized for reduced cost and/or superior performance (e.g. mechanical, corrosion resistance, wear resistance) is one reason for the interest in dissimilar metal welding.

The purpose of this article is to describe the process that Prima Power Laserdyne Application Engineers followed to develop a sound weld between these two alloys.

Planning for Dissimilar Metal Welding

The ideal dissimilar metal combination for welding by laser involves metals that are soluble in each other. An example is copper (Cu) and nickel (Ni).

For more complex alloys (Ti-6Al-4V and Inconel 718 fall into this category), differences in physical and chemical properties must be considered.

First, the formation of brittle intermetallic phases such as Ti2Ni and TiNi3 produced during welding of these alloys can lead to the failure of the weld joint at relatively low stress. The challenge is, therefore, to define a process that minimizes the volume of these brittle components.

The characteristics of laser welding are important in meeting this challenge. With its narrow weld fusion zone and rapid heating and cooling, laser welding has been proven to yield a smaller, and many times an acceptable, volume of intermetallic phases in dissimilar metal welds.

Secondly, differences in the thermophysical properties, such as the melting and boiling points, thermal conductivity, density and coefficient of expansion, between these alloys can lead to stresses that cause cracking within the fusion zone during cooling of the weld.

Controlled heating and cooling through the choice of laser parameters is often used to manage stresses in the weld. Therefore, laser parameters that have the greatest effect on heating and cooling rates and on stresses in the weld are identified and optimized.

Laser Weld Process Development

Laser and process conditions that favored (1) controlled mixing of the two alloys, (2) rapid cooling of the weld and (3) full penetration were selected for this dissimilar alloy combination. Choosing conditions that controlled mixing of the materials and gave rapid cooling was aimed primarily at suppressing formation of brittle Ti-Ni intermetallic phases.

Parameters varied in developing the final process were:

• Average power (continuous wave and modulated output).
• Power density.
• Pulse shape (sine and square wave modulation).
• Modulation parameters (peak power, pulse energy, pulse frequency, pulse duration) – see Figure 1 for an example of parameters that produced a full penetration weld.
  

  Figure 1: Square wave modulation at 800 Hz using 1.6 kW average power and 4.5 m/min speed produced full penetration welds. Modulated power is between 1.2 kW and 2.0 kW.

• Focus position with respect to the workpiece surface.
• Focus position with respect to the weld joint.
• Weld speed.

For this application, only Argon shield gas was used. Titanium and its alloys have a strong affinity for oxygen and nitrogen at high temperature (>800⁰F or 400⁰C). Using shield gases that include oxygen or nitrogen leads to embrittlement of the welded joint.

Results in Laser Welding Ti-6Al-4V to Inconel 718

Crack free, full penetration butt welds were produced by offsetting the laser beam approximately 200 μm from the interface toward the Inconel 718 side and using a combination of a modulated laser output power (square wave) and the highest welding speed that gave full penetration (Figure 2c).

a) Laser beam centered on the weld joint.	b) Laser spot shifted 200μm towards Ti alloy plate.	c) Laser spot shifted 200μm towards Inconel 718 plate.
Figure 2: Crossections of full penetration butt-welds between 1 mm thick Ti-6A-4V (dark) and Inconel 718 (light). The weld with the laser beam focus directly on the weld joint (a) cracked in the fusion zone after it was removed from the fixture whereas the weld with laser beam focus shifted 200 µm toward the Ti-6Al-4V plate (b) cracked in the fixture upon cooling. The weld with laser beam focus point 200 µm toward the Inconel 718 plate (c) is sound.

Want to Add Laser Welding Capability to Your LASERDYNE System or Improve the Current Process with Better Shielding and Optics Protection?

A quick and economical way to take advantage of the SmartShield™ welding nozzle is to install Kit Number 652473-200. This kit was specifically created for LASERDYNE systems on which the welding gas line option was not installed during its original manufacture.

This kit (pictured in Figure 3), includes:

• SmartShield welding nozzle designed for use with a 200mm focal length lens.
• Flow control for the weld shield gas.
• All hose and plumbing connections to install the kit within a few minutes.

BeamDirector BD3Y showing installation of shield gas flow meter and SmartShield™ laser welding nozzle	Flow meter and gas piping for SmartShield™ retrofit kit.
Figure 3: Pictures of BeamDirector BD3Y with SmartShield laser welding nozzle and shield gas plumbing kit designed for customer installation.

This kit is designed for quick and easy installation by the user. It uses the same gas (60 psi [4 bar]) both for shielding the weld and for protecting the optics during welding. This differs from the kit installed in manufacturing in that the latter allows use of compressed air for the air knife and a second shield gas, such as nitrogen or argon.