Repairing worn or damaged molds and dies typically represents a huge cost advantage over fabricating new tooling, and laser wire welding has become a well-established technique for this purpose. However, the characteristics of wire welding have limited its use primarily to making small area, high-precision repairs.
Now, a complementary technology, direct metal deposition (DMD), extends the capabilities of laser welding, making it a fast, cost-effective alternative method to laser wire and TIG welding for a much wider range of repair applications. Learn how the DMD process works and how mold making and repair shop B&J Specialty Inc. in Wawaka, Ind., effectively utilizes the technology in its operations.
Rise of the wire
Laser wire welding rapidly gained in popularity after it was first introduced because it provides more dimensionally precise and mechanically stronger welds than TIG, MIG or other traditional welding methods. Another key advantage of laser wire welding is reduced heat input into the part.
The high heat imparted during traditional welding processes can alter the metallurgical structure around the weld and cause shrinkage or warpage of the part. Metallurgical changes may degrade mechanical properties of the part leading to improper operation or reduced lifetime. Dimensional changes may necessitate significant post-processing – with an associated time and cost – to restore the part to its nominal dimensions.
Another important advantage of laser wire welding is that it is generally faster and requires less operator skill and training that traditional methods. Both of these factors reduce cost.
Because of these reasons, laser wire welding has become the go-to method for precision repair and modification of relatively small parts or small areas on larger parts. For example, it’s an ideal way to repair a worn or damaged edge on an extrusion tool. But, for building up material over a large area, traditional welding is still the dominant method despite the drawbacks related to post-weld rework. Now, however, the introduction of laser DMD, also known as laser powder cladding, offers an alternative and highly advantageous method for processing large areas.
Laser DMD defined
Figure 1 illustrates the basic elements of laser DMD. Fine metal powder is forced through a conical channel in the process head, which is concentric with the laser beam. Both the laser and the powder stream are focused at the same point on the work surface to a dimension of about 1 mm in diameter. The high laser power causes the powder and a small layer of the substrate material to be melted almost immediately. The laser and powder supply is moved relative to the work surface to trace out the desired pattern, and the melted powder and substrate solidify quickly to form a new layer which is metallurgically bonded.
Metal powders available and compatible with laser DMD cover a wide range of alloys, including steels, nickel, cobalt, titanium and aluminum.
A key differentiator between laser DMD and wire welding is that the former naturally lends itself to rapid coverage of large areas. Additionally, laser DMD is performed without operator intervention and under CNC control, once job parameters have been entered. This makes the technique fast and essentially independent of operator welding skill.
Typically, the deposited clad track is on the order of 1 mm wide and has a dimensional accuracy of 0.010 in. to 0.020 in. Thus, depending on job tolerances, some final machining of the cladded area is often required to produce the precise desired dimensions.
Like laser wire welding, laser DMD does not introduce a large amount of heat into the part and thus produces a small heat-affected zone and generally avoids part distortion. Therefore, rework to correct for part distortion is usually not required. Additionally, the welds themselves exhibit low porosity, fine microstructures and low dilution. Furthermore, the bulk mechanical strength of the deposited material is excellent and equal or better to that achieved though TIG welding. Together, these characteristics make laser DMD a cost-effective and practical alternative to TIG welding for large-scale mold and die repair or modification.
The Coherent EVO Mobile represents the type of laser DMD equipment that is now being employed for mold and die repair. The system is actually a highly flexible, Class 4 laser machine that can be used for welding and wire welding as well as laser DMD. In fact, it can be configured with various different laser sources and processing heads to optimize operation for numerous different tasks and materials.
The EVO Mobile is an open laser workstation mounted on casters, enabling even large, heavy components or difficult-to-move parts to be processed. Once the machine is rolled into place, the beam delivery optics can be swiveled horizontally by 360 degrees, enabling more than 6 ft. of reach. This is accomplished through joystick control, which delivers a position accuracy of less than 0.1 mm. An integrated microscope allows the operator to view the work surface and achieve precise positioning.
The EVO Mobile can be operator controlled using the joystick to trace out a specific weld path. For laser DMD, it is virtually always utilized in automated, CNC mode. Here, the cladding head follows a previously defined weld path and usually repeats that path several times in order to build up the required thickness of new material.
System operation is controlled through the Coherent App Suite, a CAM environment for planning virtually all aspects of laser processing. Pre-programmed moves and on-the-fly adjustments are then operator initiated and controlled through the 10-in. touchscreen display.
Typically, a repair operation consists of four main steps. First, 3-D (CAM) part data is imported into the system, or the shape of the area to be processed is digitized by the operator. Next, the system calculates the necessary CNC moves required to restore the damaged area to the desired dimensions. Third, the system automatically executes the programmed laser DMD operation. Finally, any machining necessary to bring the part to final dimensions is performed.
B&J Specialty Inc. provides build-to-print and repair services for molds, dies and tooling, mainly for customers located in Michigan, Ohio and Indiana. There are two imperatives for the company’s continuing success in the repair business: product quality and quick turnaround. Austin Shoppell, welding specialist at B&J Specialty, credits laser DMD, implemented using the EVO Mobile, as a key component in accomplishing these musts for much of the company’s large area mold repair activities.
At B&J Specialty, the EVO Mobile is positioned right underneath a crane, which allows parts weighing as much as 30,000 lbs. to be brought directly up to the machine. Typically, the customer has already marked up the damaged areas on the part. In some cases, the original CAD file for the part is loaded into the EVO Mobile, but most of the time, Shoppell creates the weld path himself by simply “drawing” it using the machine joystick and inputting it into the software.
The need for surface preparation in a repair operation depends up the nature of the part and the damage.
“If there’s a height variation in the damaged area, we’ll probably mill it flat before repair,” Shoppell explains. “Or, we might machine a broken part before laser power cladding it. But, if the surface is evenly worn, we can usually just rebuild directly on to it with no preparation.”
In terms of benefits, Shoppell identifies several key areas where laser DMD delivers advantages over other methods the shop uses.
“It’s delivered us maybe a 300 percent increase in repair speed over TIG or laser wire welding, and that is really important for our business,” he says. “Our customers are always in a hurry because their own machine downtime costs money. Laser DMD has allowed us to obtain the speed increase because the welding process itself it quicker, and less post-processing is required.
“However, we’re not sacrificing anything in terms of quality with laser DMD,” he continues. “Quite the opposite. We get great cohesion between the weld and substrate. This has been a concern in the past when doing laser wire welding with some of the alloys we come across. For example, we’ve had trouble with the welds not adhering when we had a die made from a tool steel, like D2 or A2, and repaired it using another steel, such as H13.”
Another aspect of improved process speed over traditional welding is reduced setup time. Shoppell says that now that he’s developed a fair amount of experience with the EVO Mobile, he can set up and teach the machine the weld pattern he wants, “literally within minutes.”
“Laser powder cladding isn’t the solution for every problem, though,” he concludes. “If it’s just a broken corner on a tool, laser wire welding is faster, and it won’t put as much heat into the part. But, the EVO Mobile does wire welding, too. In fact, its ability to laser weld, wire weld and laser DMD is another reason it’s become our main machine for repair.”