Laser Metal Deposition(LMD) for the first time in the last century in 90s by the U.S. Sandia National Laboratory is put forward, then in many places around the world have been developed by many universities and institutions are studied independently, so the name of this technology are.

How to Laser Metal Deposition Works

For example, near net forming technology of Laser Engineered Net Shaping, direct metal deposition(DMD) American Michigan University, the University of Birmingham in the UK, the direct laser Fabrication(DLF) etc..

Although the name is not the same, but they are basically the same principle, molding process, the powder to the working plane through the nozzle, the laser beam is gathered to the point, the coincidence of powder optical action points, through the table or the nozzle moving, get cladding entity accumulation.

LMD laser takes shape quickly

LENS technology is the use of laser kilowatt laser beam, due to the large, generally above 1mm, it can get dense metal metallurgical bonding but its solid, dimensional accuracy and surface finish are not too good, the need for further processing to the machine.

Laser cladding is a complex process of physics, chemistry and metallurgy. The parameters in cladding process have great influence on the quality of cladding. The process parameters of laser cladding in the main laser power, spot diameter, defocusing amount, powder feeding rate and scanning speed, the temperature of the molten pool, their dilution of the cladding rate, crack, surface roughness and cladding parts density have a great impact.

At the same time, each parameter also affects each other, and it is a very complicated process. Proper control methods must be adopted to control various influencing factors within the allowable range of the coating process.

Difference Between Coaxial Feeding and Side Feeding

In the process of laser cladding metal powder, there are two ways of coaxial feeding powder and side feeding powder. The design of side feeding mode is simple and easy to adjust, but there are many shortcomings.

First of all, because the laser beam along any plane curve shape curve scanning, curve angle of each point and the direction of movement of the powder laser beam scanning speed direction is inconsistent, leading the cladding powder packing shape changes, directly affect the precision of surface cladding layer and uniform, resulting in cladding track the rough and the cladding thickness and width of the uneven, it is difficult to ensure the size and shape of the final parts meet the requirements.

Secondly, the location of powder feeding is difficult to align with the laser spot center. This alignment is very important, and a small amount of deviation will lead to the deterioration of powder utilization and the deterioration of cladding quality.

Again, the lateral powder, laser powder preheating and bundle to pre melting, laser energy can not be fully utilized, prone to sticky powder and under cladding, non metallurgical bonding defects.

In addition, the side feed mode is only suitable for linear cladding trajectories, such as moving only along the X or Y directions, and not suitable for complex trajectory motions.

In addition, the side feeding is only suitable for the manufacture of some wall thickness parts, because the powder ejected by the side feeding nozzle is divergent rather than convergent, which is not conducive to guarantee the accuracy of the formed thin-walled parts. When the powder transport direction is the same as the direction of the substrate movement, the coating shape is obviously affected by the powder transport direction and the direction of the substrate movement.

In addition, if the direction of powder transport is perpendicular to the direction of movement of the substrate, the shape of the cladding will vary substantially in parallel with both directions.

Therefore, the lateral feeding powder has obvious directionality, and the geometry of the cladding layer varies with the direction of movement.

The coaxial powder overcomes the above shortcomings, the laser beam and the center line of the nozzle on the same axis, so that even if the scanning speed to change the direction, but the spatial distribution of powder flow relative to the workpiece is always consistent, can obtain the anisotropy of cladding layer is consistent, also due to the powder feeding and the laser beam is coaxial therefore, the change can be well adapted to the scanning direction, eliminate the influence of powder conveying direction of the cladding layer shape, to ensure the accuracy of parts manufacturing, and powder spraying is convergent shape, it is possible to manufacture some thin-walled specimens, solve the cladding molded parts size precision, this advantage in the cladding process of thin wall parts of the very obvious.

Thus, the coaxial feeding mode is beneficial to improve the stability and uniformity of the powder flux and cladding shape, thus improving the accuracy and quality of the metal forming parts.

Development of Laser Cladding Rapid Prototyping Technology

Study on American manufacturing technology of laser cladding is studied earlier in 1980s, to the end of 1990s has established a series of laser cladding manufacturing technology and application in die areas such as functional parts manufacturing.

And EBSM similar manufacturing technology of laser cladding can directly manufacture metal functional compact structure, good mechanical properties, but by the factors of laser spot size and worktable movement accuracy limit, the direct fabrication of functional parts size precision and surface roughness is poor, often need to subsequent machining to meet the use requirements.

The development of laser melting deposition (LMD) is a little late, and the United States military has paid great attention and support to this technology. With the support of the United States, the United States has taken the lead in the study of the practical application of the technology.

LMD Metal Functional Object

In 1997, the United States MTS company specializing in titanium alloy aircraft structure laser melting deposition technology development and application of AeroMet company, in support of the army and air force, Department of defense research programs in the United States, the research of laser melting deposition of titanium alloy aircraft structure parts;

In 2000, the static load test on the wing of titanium alloy was completed.

In 2001, the production of three titanium alloy bearing structures were allowed to use the time on the plane, its performance is superior to the traditional manufacturing process, at the same time because of saving material and machining, the manufacturing cost is reduced from 20 to 40%, the production cycle is shortened by 80%. However, because the fatigue performance of the main structure of titanium alloy did not exceed the standard of forgings, the application of this technology on large components of aircraft was not realized. The company was closed in 2005.

Nevertheless, the laser material manufacturing technology with low cost, short cycle and high performance is still playing an important role in the aerospace industry and the national defense industry in the United states.

The problems of Laser Cladding Rapid Prototyping Technology

However, due to the addition of layers of LMD, the deposited material undergoes complex thermal cycling in different regions. The LMD thermal cycling process involves melting and a large number of reheating cycles at lower temperatures. This complex thermal behavior leads to complex phase transitions and microstructural changes. Therefore, it is difficult to control the composition and structure of forming parts.

On the other hand, the rapid formation of molten pool with a small laser beam leads to higher solidification rate and instability of the melt. Because of the transient change of heat in the process of solidification, the complex residual stress is easy to produce. The existence of residual stress inevitably results in deformation, and even cracks in LMD forming parts. The main difficulty of LMD technology is the uncontrollable microstructure and the formation of residual stress.

Advantages of Laser Cladding Rapid Prototyping Technology

The essence of DMD / LENS is the three-dimensional accumulation of metal melt under computer control. Unlike DMLS and SLM / SLRM, the metal powder is already in the heating and melting state in the nozzle, so it is especially suitable for laser rapid forming of high melting point metal.

Advantages of LMD rapid prototyping technology

In fact, the U.S. Sandia National Laboratory, funded by the U. Department of energy, LENS in the early stages of development, will be positioned in direct precision manufacturing of aerospace and military equipment in the field of complex shape and high melting point metal parts; and on this basis, the molding material system to expand for tool steel, stainless steel, titanium alloy, nickel based high temperature alloy etc..

Complex parts LENS Sandia National Laboratory in the United States forming research work, forming comprehensive mechanical performance close to or related parts is superior to the traditional preparation process; but limited to the national defense security, at present there are few reports related technical details.

In particular, by adjusting the feeding device, gradually changing the powder composition in different position, to achieve the same parts of the continuous change of the component, so LENS in the processing of heterogeneous materials (such as FGM) has unique advantages.

Rapid prototyping technology of wire feed laser cladding

Due to the inevitable defects in powder laser manufacturing, such as the low utilization rate of powder (20 ~ 30%), the pollution of powder, the relatively expensive powder and so on. The feeding of wire fed laser not only has a high utilization ratio of material (almost 100%), but also has no dust pollution, and the equipment requirements are lower and more economical.

Therefore, in recent years, some organizations have begun to shift their eyes to the research of wire feed manufacturing technology. At present, the most used is the TIG arc fuse, while the laser is very few.

To sum up, at present, the global research on wire feed laser incremental manufacturing is relatively small, but the research of using TIG arc fuse is more.

Generally, the tensile strength and yield strength of TIG arc welding method are lower than those of laser reinforced material, and the elongation is higher than that of laser. The organization, preparation of titanium alloy prepared by TIG arc method to fuse lamellar structure, and laser material manufactured by widmanstatten structure, which is due to the energy characteristics and the different input in two ways.

Comparison of rapid prototyping technology of powder feeding and wire feeding laser cladding

Powder feeding mode is superior to wire feed in process window and internal defect, and the laser feeding with powder feeding greatly reduces the required laser power threshold.

When the wire feeding type is adopted, the height of the deposition layer decreases linearly with the increase of laser power, which affects the forming efficiency. At the same time, the wire feeding speed must be increased, but the wire feeding speed increases with the stability of wire feeding.

Therefore, the parameter matching between the three parameters, such as laser power, wire feeding speed and scanning speed, is very important. While feeding power, the height is basically unchanged when the power is increased.

In terms of dimensional accuracy, the powder feeding in the thickness direction except the bottom is narrower, the other parts are even in thickness and the side wall is very straight; in the length direction, the molten pool is not down and the shape is straight.

The wire type thickness in the thickness direction is uniform, but because of the rigid wire wire and the light disturbance and the neutrality of more stringent requirements, and therefore prone to minimal deviation silk and light, so that the side wall shape is not very straight, the bending; in the length direction, where the laser begins and ends, the inclined deposit and flows down, this is because the laser power is large and because the powder feeding laser light, stop before stop feeding, so in effect the ending position of laser in a simple sedimentary layer, caused by the ice layer deposition pool.

To sum up, in the process window, internal defects, dimensional accuracy and surface accuracy, the powder feeding is better than the wire feed; in terms of efficiency and economy, the wire feeding has outstanding advantages.